Modern Hydronics Spring 2022

MODERN HYDRONICS 2022 SPRING

CONTROLLING ATW HEAT PUMPS SNOW MELT OPTIMIZATION PELLET BOILERS & FORCED AIR

MONTREAL HEALTH NETWORK ADDS HEAT PUMPS Part of an $18.8 million energy efficiency project

a publication of

CONTENTS

Modern Hydronics

MH14 SNOW MELT Close That Door

Understanding and controlling the inefficiencies of snow melting.

By Curtis Bennett

MH4 HEAT PUMPS

Which is Better?

Control options when combining DHW, space heating and cooling using an air-to-water heat pump.

By John Siegenthaler

MH18 PRODUCT SHOWCASE

MH10 PROJECT

Optimizing Operations

MH20 DESIGN

A Montreal west-end health network undertakes an $18.8 million energy efficiency program.

By Doug Picklyk

Pel-Air Systems Considering design options for adding a pellet boiler to a forced-air heating system.

By John Siegenthaler

MODERN HYDRONICS

a supplement of Heating Plumbing Air Conditioning Magazine

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MODERN HYDRONICS

SPRING 2022

MH3

HEAT PUMPS

Modern Hydronics

WHICH IS BETTER? Considering control options when combining domestic hot water plus space heating using an air-to-water heat pump.

BY JOHN SIEGENTHALER

O

ne of the advantages of air-towater heat pumps is their ability to provide space heating, cooling, and domestic water heating. The system shown belown in Figure 1 is a “template” for such a system.

When operating in heating mode the heat pump maintains the water in the buffer tank at an elevated temperature

suitable for space heating and at least a portion of the domestic hot water (DHW) heating load. The reverse indirect tank provides buffering between the heat pump’s heat output rate and the variable space heating load created by five independently controlled zones of heat emitters. Domestic water absorbs heat as it passes through the copper or stainless-steel coils suspended within the buffer tank.

TRADEOFFS The combination of space heating and DHW sets up a bit of a “quandary.” To optimize the heat pump’s coefficient of performance (COP) the water in the buffer tank should only be heated to the

minimum temperature necessary to maintain comfort in the building. That’s easily accomplished using a properly set up, and relatively inexpensive, outdoor reset controller. Some modern airto-water (ATW) heat pumps even have this control logic included within their internal controls.

However, to optimize the use of heat pump energy for DHW the tank temperature should be maintained at a temperature of at least 120F (49C) at all times.

This is based on a nominal five-degree temperature loss across the heat exchanger coils and assumes that DHW delivered from the tank at 115F (46C) is acceptable.

Continued on MH6

Figure 1. Example of an air-to-water heat pump providing space heating/cooling and DHW. MH4

SPRING 2022

MODERN HYDRONICS

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HEAT PUMPS

Modern Hydronics

Syracuse is between 5F and 10F for 79 hours per year.

Bin temperature tabulations are available from multiple sources including ASHRAE and ACCA Manual J. The bin temperature data drives the space heating load model as well as the performance model for the heat pump heating capacity and COP.

Lower tank temperatures will reduce the percentage of heat supplied to domestic water from the heat pump, and thus increase the amount of heat the needed from a supplemental heat source.

Here’s the “quandary”: Is it better to use outdoor reset to control the tank water temperature and take the “penalty” in the form of increased supplemental heating for domestic water heating, or should the heat pump maintain the tank temperature high enough to meet the DHW load 24/7, and take the “hit” of a lower seasonal COP?

COMPLEX SOLUTION That’s a complex question to answer. It involves specifics for the DHW load relative to space heating load, the COP and heating capacity performance of the heat pump, and the regional climate the system operates in. It also involves the relative cost of installing a dual function buffer tank versus a single function buffer, and the cost of installing a means of providing supplemental heat for boosting DHW to a desired delivery temperature.

THE MODEL To get a quantitative assessment of these factors I set up a spreadsheet simulation that melds much of this information together. It produced results that seem reasonable given the relatively simple modeling methods used. I’ll share those results with you shortly. First let’s put the “givens” out there along with the “assumptions.”

The spreadsheet simulation is based on a specific heat pump, in this case a nominal 5-ton (@60,000 Btu/h) unit with “low ambient” refrigeration system enabling it to operate at outdoor air temperatures as low as -22F (-30C). The heating capacity and COP of this heat pump, like any ATW heat pump, are highly dependent on operating conditions, specifically outdoor air temperature and the water temperature leaving the heat pump’s condenser. That dependency was modeled using data provided by the manufacturer to create “curve fit” equations that were implemented in the spreadsheet.

The building modeled was a single-family home with a design heat loss of 36,000 Btu/h based on 70F (21C) inside and -10F (-23C) outside temperatures. This home’s hydronic distribution system serves a combination of panel radiators and radiant floor circuits. The heat emitters were sized for a design load using 110F (43C) supply water temperature.

The system’s assumed location was Syracuse, NY (about a 3-hour drive east of Niagara Falls). The simulation used long term “bin” temperature data, as shown in Figure 2.

Bin temperature data organizes the hourly average outdoor temperature, at a given location, into groups that, in this case, are five-degrees Fahrenheit “wide.” For example, Figure 2 shows that the long-term average outdoor temperature in MH6

SPRING 2022

SPACE HEATING ONLY I modeled several scenarios involving different average buffer tank temperatures. I wanted to see how these temperatures affected the seasonal average COP of the heat pump. The results (next page, Fig. 3) show that the Figure 2. Long term “bin” seasonal average COP of this temperature data for northern particular heat pump, as exNew York state. pected, will vary depending on the water temperature of the space heating distribution system. Low temperature systems definitely have the advantage.

For example, a radiant panel system that could supply the building’s design heat load using water at an average temperature of 90F (32C), and without outdoor reset, would allow the heat pump to attain a seasonal average COP of about 3.3. The seasonal COP would decrease to about 2.3 if the system required a sustained average supply temperature of 120F (49C).

That’s a significant difference.

In Syracuse (6720 ºF•days “degree days”), where electricity currently costs $0.117/kWh, the savings associated with the higher seasonal average COP would be $213 per year.

Since the heat pump’s performance improves at lower water temperatures it makes sense to keep this temperature as low as possible. That’s easily done using an outdoor reset controller to regulate the buffer tank temperature. I modeled this strategy and found the heat pump’s seasonal COP increased to 3.47. That, in my opinion, is excellent performance for an ATW heat pump in a cold climate application. It rivals the seasonal performance attainable by geothermal water-towater heat pumps in the same application, and at a fraction of the (unsubsidized) installation cost.

BUT WHAT ABOUT DHW? I also integrated a daily domestic hot water load of 60 gallons

MODERN HYDRONICS

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HEAT PUMPS per day heated from 50 to 120F into the spreadsheet, and ran it for several assumed buffer tank temperatures. Any supplemental energy for DHW was assumed to be provided by electric resistance heating (an electric tankless or a tank-type heater). The seasonal average COP was based on the total energy used for space heating plus DHW. The results are shown in Figure 4.

There’s a significant drop in the system’s seasonal average COP as the water temperature maintained in the buffer tank decreases as more electric energy is required to bring DHW up to a consistent temperature of 120F. The use of full outdoor reset control of tank temperature, along with electric supplemental DHW heating, and assuming that the heat pump provides all domestic water heating during the months with no space heating (May through September), yielded an estimated annual COP of 2.82.

ANOTHER POSSIBILITY How about operating the heat pump using outdoor reset of the tank temperature, and handling the DHW load with a separate electric resistance heater?

I modeled this scenario and found that the effective seasonal COP of the system (e.g., total energy output of the system divided by total electrical energy input) was 2.57. This reflects that 11% of the total load (space heating + DHW) would now be provided by electric resistance heating at a COP of 1.0.

In the interest of full disclosure here’s what wasn’t included in the computer simulations this article is based on: standby buffer tank heat loss; energy used for heat pump defrost; and energy to operate the system circulator.

Assuming a well-insulated tank, and intelligent defrost controls, I estimate these effects combined would lower the seasonal COP by 5 to 10%.

The higher end of this derating would be for climates with higher relative huMH8

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Modern Hydronics

Figure 3. How buffer tank temperatures affect the average COP of the heat pump.

midity in winter, and more hours where outside temperatures are in the range of freezing, where the highest amounts of defrosting are necessary.

TAKE AWAYS From an overall performance standpoint, the combination of outdoor reset to regulate buffer tank, along with supplemental electric resistance heating to “top off” DHW, had a projected seasonal system COP of 2.85. This exceeds the estimated COP of 2.57 based on full reset for space heating and shifting all DHW load to electric resistance heating.

For this project, in this location, the annual savings associated with the higher seasonal COP would be about $119/year. That saving has to be weighed against the cost of detailing the system to provide DHW versus using a separate electric water heater.

FINAL THOUGHTS The heat pump that was modeled does not have a desuperheater, an option available on several water-to-water heat pumps used in geothermal applications.

It allows the hot refrigerant gas leaving the heat pump’s compressor to cool from a superheated vapour to a saturated vapour before entering the condenser. That heat is transferred to a stream of domestic water circulating between the heat pump and a tank type water heater. Desuperheaters are posMODERN HYDRONICS

Figure 4. How the seasonal average COP is affected when the heat pump is used for space heating and DHW.

sible with split system ATW heat pumps that house the compressor indoors.

An ATW heat pump equipped with a desuperheater could provide much of the energy required for DHW without the need of heat exchanger coils in the buffer tank. That energy is essentially “free” when the heat pump operates in cooling mode (since the heat would otherwise be dissipated outside).

The availability of such a unit would likely swing the design toward a single function buffer tank with the temperature regulated by outdoor reset. A separate heater would be used to provide any small temperature boost needed for consistent DHW delivery.

Another possibility would be controls that operate the heat pump based on outdoor reset control of buffer tank temperature during the heating season, but also allow the heat pump to elevate the buffer tank temperature to 120-130F during the “non-heating” season.

Stay tuned for more unique ways to configure ATW heat pumps systems in future articles.<> John Siegenthaler, P.E., has more than 40 years of experience in designing modern hydronic heating systems. His latest book is Heating with Renewable Energy (www.hydronicpros.com).

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PROJECT

At the Jewish General Hospital in Montreal, 26 air-to-water heat pumps were installed on a custom platform on one of the buildings.

OPTIMIZING OPERATIONS Montreal west-end health network undertakes $18.8 million energy efficiency retrofit program.

BY DOUG PICKLYK

T

MH10

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MODERN HYDRONICS

tions into one project we can recover a lot of energy savings in total,” explains Georges Bendavid, Director of Technical Services for CIUSSS West-Central.

The CIUSSS invested close to $18.8 million for this initiative, of which $6.7 million came from a combination of grants offered through the Government of Quebec and utilities including HydroQuébec and Énergir.

“The strategies that were put into place will allow us to save close to $1.4 million a year in energy costs,” says Bendavid. And based on having to finance about $10 million of the project, the CUISSS is projecting roughly a seven-year payback.

While the project did include lighting upgrades throughout the nine buildings, the heavy lifting for this initiative involved mechanical system upgrades in buildings dating as far back as the 1920s.

NEW BOILERS All but one of the nine facilities are heated with hydronics (Miriam, a small 25-bed long-term care site, is heated WWW.HPACMAG.COM

PHOTO: PATRICE BERIAULT

he Integrated Health and Social Services University Network for West-Central Montreal (locally known as CIUSSS – Centre intégré univeritaire de santé et de services sociaux). The area covered by the CIUSSS is home to approximately 371,500 people who are served by more than 30 member facilities. Included are one of Canada's leading hospitals (the Jewish General Hospital) and an interlocking array of three specialized hospitals, five local community service centres, two rehab centers, six long-term care sites, two day centres and several affiliated research facilities.

In August, 2019, the CIUSSS began working with Énergère, a Quebec-based energy efficiency consultancy and con-

tractor, on a massive energy-savings project involving nine buildings which included changes to boiler rooms and entire mechanical systems to reduce the network’s environmental footprint and increase operational efficiency.

Over the past two-and-a-half years, more than 40 measures were implemented in the nine facilities, including the Jewish General Hospital (JGH), a 637 bed acute-care and teaching hospital that serves as the hub of the CIUSSS West-Central network.

The other buildings optimized include: Mount Sinai Hospital, Richardson Hospital, Catherine Booth Hospital, Henri Bradet Residential Centre, Donald Berman Maimonides Geriatric Centre, Saint Margaret Residential Centre, Saint Andrew Residential Centre, Father Dowd Residential Centre and Miriam Home and Services.

“At some of these facilities we’re removing big boilers that are running at 20% efficiency and replacing them with modern boilers running at 85% or more, and by combining all of these modifica-

Modern Hydronics

with electric resistance). In three sites steam boilers were replaced with efficient hot water systems and/or heat pumps for space heating (a steam operation was retained at the JGH for sterilization, humidification and the kitchens).

Where the steam was replaced, they’re running hot water through the same radiators for the perimeter heating, but now they’re able to modulate the water temperature based on outdoor conditions to better control the environment in each room, says Emilia Fernandes, P.Eng., project manager on the CIUSSS technical services team.

AIR HANDLING In addition to the perimeter heating, all of the steam coils in the air handlers were replaced with hot water. They were re-sized, are being fed with lower temperatures and also have outdoor reset.

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In the JGH alone, 24 coils were swapped out from the 100% fresh air systems and systems with return and modulating fresh air. Six of those coils— those in the fresh air systems—use small glycol loops with heat exchangers.

“We optimized the amount of fresh air, and made a lot of the systems variable volume to save energy there as well,” says Fernandes. “We’re saving energy in the distribution system where before we were losing a lot of efficiency.”

HEAT PUMPS The project did not move facilities away from natural gas, but the addition of heat pumps was designed to optimize the energy efficiency in three buildings.

“We optimized the boiler systems we had, and at the same time we added the heat pumps to help with the generation of the hot water,” explains Fernandes.

MODERN HYDRONICS

Commercial air-to-water (ATW) heat pumps were installed at three sites: three units at Mount Sinai, two at Maimonides and 26 at the JGH, which also installed a six-section water-to-water Multistack heat pump chiller.

At the JGH, the 26 ATW heat pumps were installed on the roof of one building, and they were placed on a platform that serves as an outside plenum where exhaust from the building goes through the heat pump array tempering the air so the units can run with efficiency even during a Montreal winter.

Two buffer tanks (750 gallons each) were installed in the JGH as part of the loop from the ATW heat pumps to create mass for more stable temperature control and efficient hot water production.

The JGH still operates gas boilers and has water-to-steam heat exchangers, but Continued on MH12

SPRING 2022 MH11 2021-08-04 3:47 PM

PROJECT

Modern Hydronics

the system was optimized with the addition of the heat pumps which bring the system up to a certain water temperature, and then the water-to-water heat pumps/chillers boost the temperature.

“We still have a primary high temperature loop and a secondary low temperature loop,” explains Fernandes. “We use the higher temperature to inject into the low temperature.”

The whole distribution system was also re-piped to optimize efficiency and energy savings for the domestic hot water.

Cooling at the JGH is handled with existing chillers—a system optimized in a previous project. However they do have a circuit to unload the new Multistack chiller into its chilled water system.

CONTROL AUTOMATION “With this type of project, one of the things that is definitely very important is

the controls,” says Bendavid. “The capacity to be able to look at what you are doing in real-time and over a period of time, and to also be able to see the energy savings.”

If a facility had an existing control system they tried to keep operations as they were, otherwise they centralized with Delta controls that allows communications to a central room in JGH where operations can be monitored remotely.

The project included a complete review of the control sequences and optimization of the equipment operations to maximize energy savings, including a complete review of the schedules of operation and adjustment of setpoints for the temperature of air and water, the quantity of fresh air supply and exhaust.

“This was done for all major heating, cooling, ventilation and domestic hot water production equipment,” notes

Fernandes. “Not only were the sequences optimized, but other control points were added to better control and monitor the systems.”

In addition, a complete recommissioning was done which allowed all the facilities to discover and fix elements that were neglected over the years, and to better coordinate with the building automation systems.

The CIUSSS expects to reduce GHG emissions by 49% annually—with 2,200 tonnes of GHG savings from the ATW heat pumps alone. The annual energy consumption will also be reduced by 39.3 million kilowatt hours, the energy consumed by about 1,637 households.

According to Bendavid, at the end of the day, “This type of project is not looking at just saving energy, it’s really looking at one way to finance the modernization of our aging institutions.” <>

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CANADA’S HYDRONIC

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INSTALLATION CONTEST! sponsored by

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You are rightfully proud of your work and love to share your expertise, and artistry, with the community. So make sure your camera lenses are cleaned and focused because Sweet Heat ’22 is now underway!

Entries will be featured on our Instagram and displayed at the Summit in September. There, the winning installations, as selected by a judging panel including John Siegenthaler, will be announced and their installations featured.

Entry is simple – send us photos (before and after shots work best) of a completed installation and fill out the simple entry form at hpacmag.com/sweet-heat-2022. There will be winners in both the residential and commercial categories.

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QUESTIONS? CONTACT DPICKLYK@HPACMAG.COM WITH THE SUBJECT LINE “SWEET HEAT CONTEST”

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SPRING 2022 MH13 2022-02-08 3:14 PM

CONTROLS

CLOSE THAT DOOR PHOTO: LOST IN THE MIDWEST/ADOBE STOCK

When it comes to controlling snow melt systems, the goal is to optimize the inherent inefficiencies to the best of your abilities.

BY CURTIS BENNETT

I

can still hear my parents yelling at me in the winter: “Close the door, we are not trying to heat the whole world.” And I’m sure many of you have heard something similar at some point in your lives.

I spent my childhood in rural Alberta where we would have “No School Days” because it was so cold outside that the propane in the school bus would start to gel. So leaving the door to the house open in those temperatures would cool the the place down pretty fast, and it could take some time to heat it back up.

Now, as an adult myself, I totally get it, I’m not paying to heat the outside. But wait …

Snow melting is a very big part of our hydronics industry in Canada. I’m not going to sit here and tell you that snow melt systems are all roses and butterflies, because each person reading this (and I hope there are more of you than just my mom) will start laughing and go on to the next article.

Snow melt systems are the most inefficient systems we in the HVAC industry have at our disposal. Each one of those Btu’s that your boiler just made went straight outside, never to be used to heat your house. Poof, gone.

Now I am no mechanical engineer, but I know as a rule—and this will change from region to region—that it’s about 250 Btu’s per square foot to melt snow on a slab, but in actuality this number is not important. The important thing is MH14

SPRING 2022

Sensors in snow melt systems can detect snow or ice on the pavement and trigger the process.

that the pipes for said systems are in slabs of concrete THAT ARE OUTSIDE. I did use upper case for a reason. I have told many people in my circle of friends and family about snow melting.

None, as of yet, had ever heard of it. The first thing they say is: “Wow that must cost A LOT of money.” Then the second thing is always: “That would be awesome at my house.”

Now don’t get me wrong, melting snow is a useful thing to do, I am just pointing out the heating aspect of it, that’s all. Now let’s take a look at a few things that can make snow melt systems use fewer Btu’s overall.

Let’s quickly go over the basic parts of the snow melt operation (if I miss something I apologize, this is for simplicity).

We have the boiler, piping, pumps and controls. I’m not going to talk about pumps and pipes, although yes, you can gain efficiency there, and yes you “should” be trying for that, but, for now we will talk about the other two.

The boiler and controls will determine your system efficiency more than anything else, all else being considered MODERN HYDRONICS

equal. I am also assuming that everything is installed correctly, which is a whole other topic.

Let’s start with the boiler. We have the necessary technology today, and that’s condensing boilers, to be even more specific modulating condensing boilers, but in the case of a snow melt system the condensing part is the most important aspect.

As we should all know, you get the most efficiency out of the boiler when it is condensing, and in the case of snow melting your return water temperature is usually the temperature of the slab, yes I said usually. So if that is the case the max temperature that you should have coming back is around 50F (10C).

You don’t need much more than that to melt snow, realistically you can melt snow at 33F (0.5C), but to hasten the snow melting time I have seen many people try to get the slab up to 40F (4.4C) to 45F (7.2C).

Ok, so all that being said, a condensing boiler is perfect for snow melting applications. It’s going to be your most efficient choice.

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Modern Hydronics

“It's a big deal to get the slab temperature reading right ... sensor placement really needs to be considered.”

The optical sensor works by seeing an amount of snow falling. As the snow hits the top of the sensor it is able to see how much snow is falling. It can see the instant there is snow and the instant the snow stops and every variation in between.

There is some efficiency that this sensor can provide, and we will hit that in a moment.

SLAB SENSOR Next, let’s look at what the control does. A snow melt control has three parts: the CPU that makes all the decisions; the slab sensor, which is usually built into the next part; the snow/ ice sensor—this sensor is what lets the control know that there is snow falling or ice forming on the slab.

SNOW SENSOR The snow sensor can work in a couple different ways. The first is a continuity style sensor, and the other is an optical style.

The continuity style sensor basically senses snow by a change in resistance between some metal fingers on the top of the sensor. As snow falls on the sensor it melts between the fingers and causes a “short” circuit in the fingers. This tells the control there is snow.

In a snow melt system the slab sensor is what runs the control.

It is a big deal to get this slab temperature reading right. Place the sensor too close to any piping and it will get too hot of a reading and the control could shut off the system prematurely.

Likewise, if the slab sensor is not placed anywhere near any piping it may think the slab is too cold and continue to push heat. Sensor placement is something that really needs to be considered.

CONTROL STRATEGY The control strategy for a snow melt is pretty simple. The main temperature we are controlling is the slab temperature, but we need to know the outdoor temperature as well, because Continued on MH16

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SPRING 2022 MH15 2022-01-26 11:15 AM

CONTROLS

FINDING EFFICIENCIES Since we are literally heating the outside, any improvements we can make to save energy and money should be looked at. But what can we do better?

System idling is the practice of keeping the slab just below melting point until the snow sensor sees snow, reducing the amount of time it takes to raise the temperature of the slab to start melting the snow. If you don’t idle the slab it can take hours for the slab to heat up to start melting the snow. However, the amount of time that it’s not snowing usually far outweighs the time that it is.

Idling the slab consumes the most energy and money in a snow melt system.

I am not saying that it should not be done, but can we be more efficient?

Idling even a couple degrees lower can save a huge amount of energy. Also, we now have the power of the internet at our fingertips and inside some controls.

With internet connectivity we can use weather forecasts to do some predictMH16

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PHOTO: JASON/ADOBE STOCK

the snow sensor does not actually see “snow” it sees moisture. We have to know it’s cold enough outside that the “moisture” the sensor is reading is snow and not rain. Making sure the outdoor temperature sensor is placed in a good position to get an accurate reading is a huge deal.

The other temperatures we need are supply and return. We need to know the delta T of the fluid we are putting into the slab so we don’t “Shock” it—which is what happens by putting too much heat to the slab. So when the snow sensor senses some moisture, and the temperature outside is cold enough, and the slab temperature is below the set target, then we start pushing heat to the slab and the snow starts to melt. Not instantly but eventually.

You may already know all this stuff, but it’s important. We need to remember all this information in order to know where we can do better.

Modern Hydronics

Correct positioning the sensors in the concrete will help system efficiency.

ing. Use the forecast to set the control into idle when we think snow is coming instead of idling all the time. We can also see if it is actually snowing in your area and use that along with the snow sensor to ramp up the slab temperature to melt. I won’t get too much into this, just know that it’s coming.

Another big energy hog is the amount of time we use to melt the snow after the snow has stopped. This time is usually called the melt time of the slab and is different for every system. It depends on slab thickness, slab size and the heat capacity of the boiler—things I’m not going to get into. Just know that they’re all different when designed, so being aware of these specifications helps you know your melt time.

If your melt time is too long, then you are wasting valuable energy and money, too little and you still have snow on the slab. Keeping these times to the proper amount can save a lot.

The last item to help save energy is the time at which we know there is snow, and the time we know the snow has stopped. Accurately knowing this can save 15 to 20 minutes at the start of the melt cycle and at the end. It may seem insignificant for one cycle, but add up 20 minutes for 50 cycles a year for 20 years. Couple that with a 2 million Btu/h boiler. I think you see where I am headMODERN HYDRONICS

ing. It’s a lot of energy and money that can be saved.

This can be done with proper snow melt sensor placement. Too many times I have seen the sensor placed where the snow drifts at the side of the driveway.

Take a look at the surrounding buildings and even vegetation to get a good idea of how the snow may fall in areas of the slab to be melted. Thinking of these things will give the most accurate reading for the snow melt control to use.

This also goes to my point above with the type of snowmelt sensor. The more information the snow sensor can give the control the better the control can make decisions.

Wow, I did not think I had this much to share about snow melting. Hopefully you will take a couple tips out of this article. Just remember, close the damn door, and keep those Btu’s inside. <> Curtis Bennett C.E.T is product development manager with HBX Control Systems in Calgary. He formed the company with Tom Hermann in 2002. Its control systems are designed, engineered and manufactured in Canada to accommodate a range of hydronic heating and cooling needs in residential, commercial and industrial design applications.

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HYDRONICS PRODUCTS

Modern Hydronics

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DESIGN

PEL-AIR SYSTEMS Design options for adding a pellet boiler to a forced-air heating system.

BY JOHN SIEGENTHALER

I

’ve described pellet-fueled boiler applications in several past issues of HPAC, and all of them have been situations where the boiler supplied a hydronic distribution system.

While such applications are certainly the prevailing way pellet boilers are used, they are not the only option. It’s possible to couple a pellet boiler to a forced air distribution system.

Most of these applications will be retrofits—perhaps a way to transition a central forced air system away from fossil fuel and onto renewable wood pellets. The economics of such a conversion are increasingly attractive as fossil fuel prices soar.

Figure 1 (right) shows a basic system layout. The details between the boiler and thermal storage tank are typical. In this case a “loading unit,” which combines a thermostatic mixing valve and circulator provides flow between the boiler and thermal storage tank. It also prevents sustained flue gas condensation within the boiler by elevating the boiler inlet water temperature above 130F (54.4C) whenever possible.

The thermal storage tank is piped in a “three-pipe” configuration. This allows hot water from the boiler to flow directly to the load if the pellet boiler is operating at the same time as the load.

It also ensures that the thermal mass of the storage tank is fully “engaged” in the energy transfer process. This is critically important for any hydronic system with a biomass heat source.

The heat emitter is a coil mounted in the discharge plenum of a furnace. An MH20

SPRING 2022

Figure 1. A basic system layout with a pellet boiler feeding a forced air distribution system.

Figure 2. Piping design incorporating a three-way mixing valve.

MODERN HYDRONICS

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Modern Hydronics

Photo 1. A radiant heated coil.

Figure 3. Connect a magnehelic gauge to measure the pressure drop after the coil is installed.

example of such a radiant coil is shown in Photo 1 (above, top).

Most installations will require sheet metal transitions to be fabricated to join the coil to the furnace plenum as well as to the downstream ducting.

Never mount the coil on the air inlet to a furnace. Doing so could heat the blower motor above its maximum rated operating temperature causing it to lockout or burnout. It’s also likely that such a situation would void the furnace’s warranty due to the potential for high entering air temperatures.

For the system in Figure 1, water flow through the coil is regulated by a variable speed circulator. A closed control loop is established by monitoring the air temperature downstream of the coil.

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Figure 4. Install a separate air handler in parallel if both heating and cooling are required.

If this temperature begins to drop below a preset value, such as 110F (43C), the circulator increases speed, which increases heat output from the coil, and vice versa.

There are circulators on the market that have self-contained logic for setpoint temperature control. The temperature sensor in the discharge duct can be wired directly to such a circulator.

Circulators without this functionality, but equipped with either an analog input, (4-20 ma, or 0-10 VDC), or a digital input, (BACnet, LONworks, or PWM), can be regulated by one of several currently available temperature controllers.

Controlling the discharge air temperature is important in this type of application. That’s because the temperature of MODERN HYDRONICS

the water supplied to the coil from the thermal storage tank could be as high as 190F (88C), and perhaps as low as five degrees above the desired leaving air temperature. Without flow regulation there would be times when scorching hot air is pushed through the supply ducting and into the conditioned space.

The low density of this air would immediately cause it to rise toward the ceiling, and thus set up excessive air temperature stratification. Overheated air would also cause rapid thermostat cycles, and guaranteed comfort complaints.

Another way to control the discharge air temperature is using a three-way mixing valve. The piping for this is shown in Figure 2 (left).

Continued on MH22 SPRING 2022

MH21

DESIGN

Modern Hydronics

Figure 5. This diagram shows domestic water heating using brazed plate heat exchanger as well as supply and return piping for another independently controlled hydronics heating zone.

AIR-SIDE DESIGN Another important consideration is the drop in static pressure as air flows through the plenum coil. Most furnaces have ratings for air flow rate versus the external static pressure of the distribution system they connect to.

In most systems the static pressure is created solely by the duct system.

However, when a coil is added to the plenum it will definitely increase the total static pressure the furnace’s blower operates against.

In applications where there’s an existing furnace it’s possible to measure the current static pressure drop by connecting a magnehelic gauge connected as shown in Figure 3 (previous page).

Coil manufacturers can supply data that lists the static pressure drop across their coils as a function of the air flow rate through the coil. Use this data to determine the added static pressure at the nominal air flow rate the system needs to provide.

If the existing forced-air distribution MH22

SPRING 2022

system already has a cooling coil installed, it’s very unlikely there will be sufficient static pressure capacity to accommodate a hot water coil. In this case it’s better to install a separate air handler in parallel with the furnace as shown in Figure 4 (previous page).

Motorized dampers should be installed as shown. They open only when their associated air handler is operating. When closed they prevent air flow from “short circuiting” through the inactive air handler (or furnace).

FUTURE EXPANSIONS A unique benefit of this approach is that it allows heat from the pellet boiler to be used for ancillary loads­— domestic water heating is one example. Adding a manifold station and using it to supply some panel radiators, towel warmers, or radiant panel circuits is another.

The system in Figure 5 (above) shows an “on demand” subassembly for heating domestic water using a stainlesssteel brazed plate heat exchanger, small MODERN HYDRONICS

circulator, and a domestic water flow switch. It also shows supply and return piping for another independently controlled heating zone. There are many possible variations. The key concept is the ability to expand the system for the future needs of the building without extensive modifications.

I’ll close by admitting that I prefer hydronic heating distribution systems whenever possible. But I’m also a realist. There are a lot of forced air systems out there that could potentially undergo a pellet boiler “make-over”. Doing so not only transitions the system to a renewable fuel, it also opens up a wide range of possibilities to suit the future needs of the building. <> John Siegenthaler, P.E., has more than 40 years of experience in designing modern hydronic heating systems. His latest book is Heating with Renewable Energy (visit: www.hydronicpros.com).

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