Finally, a Right-Sized Furnace

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Finally, a Right-Sized Furnace

Dettson is selling a 15,000 Btu/h furnace that modulates down to 6,000 Btu/h

Posted on Mar 2 2018 by Martin Holladay

For years, builders of energy-efficient homes have been frustrated by the lack of low-load furnaces. An article I wrote in 2013 about this problem began with this question: “Why are the smallest available American furnaces rated at about 40,000 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. /h?”

A 40,000 Btu/h furnace is likely to be more than twice the size of what is needed to heat a small energy-efficient home. Many homes in this category have a design heat load of only 12,000 or 15,000 Btu/h.

I once asked John Straube, a professor of building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. science at the University of Waterloo in Ontario, “With so many builders frustrated by the problem of oversized furnaces, are equipment manufacturers paying any attention?” Straube answered, “Not really. We’ve talked to Trane, Carrier, Lennox, and we’ve told them that we want a smaller, cheaper alternative. Their response is always the same: ‘No, we’re not interested. We are not seeing any demand.’”

Finally, a Canadian company named Dettson Industries has come to the rescue. They have developed a modulating gas-fired furnace, the Dettson Chinook C15-M-V, rated at 15,000 Btu/h. At its low fire rate, the furnace has an output of 6,000 Btu/h.

Compared to an oversized furnace, a right-sized furnace has several advantages. It will have longer operational cycles, which should improve occupant comfort compared to a furnace with short cycling problems, and it should do a better job of keeping the interior temperature at the thermostat setpoint than a furnace that short cycles.

The Dettson Chinook furnace is available in several sizes ranging up to 120,000 Btu/h. Needless to say, lots of furnace manufacturers offer models in the 40,000 Btu/h to 120,000 Btu/h range, so the larger furnaces manufactured by Dettson are nothing special. This article will focus on the two smallest models, the C15-M-V (rated at 15,000 Btu/h) and the C30-M-V (rated at 30,000 Btu/h).

In addition to these two models, Dettson offers the Chinook Compact (model CC15-M-V). The Chinook Compact is similar to the smallest Dettson Chinook, but packaged in a smaller cabinet (to fit into a cramped mechanical closet).

The best models have a variable-speed fan and a modulating gas valve

Dettson sells its Chinook furnaces set up for either either natural gas or propane.

For residential furnaces, Dettson offers a limited lifetime warranty on the stainless-steel heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank. and a 10-year warranty on other furnace parts.

Dettson Chinook furnaces are available as:

  • A single-stage furnace with a PSC fan motor;
  • A two-stage furnace with a PSC fan motor;
  • A two-stage furnace with an ECM blower;
  • A top-of-the-line modulating furnace with a variable-speed fan and a modulating gas valve.

Customers interested in the smaller furnaces will want to purchase the modulating furnace, since the single-stage and two-stage furnaces aren’t available in any sizes smaller than 45,000 Btu/h.

Purchasers of a modulating Chinook furnace can also purchase an optional split-system air conditioner to match (the Alizé cooling unit). Since the Alizé is actually a heat pumpHeating and cooling system in which specialized refrigerant fluid in a sealed system is alternately evaporated and condensed, changing its state from liquid to vapor by altering its pressure; this phase change allows heat to be transferred into or out of the house. See air-source heat pump and ground-source heat pump., it transforms the HVAC system into a dual-fuel system.

The 15,000 Btu/h modulating Chinook (model number C15-M-V) will modulate down to 6,000 Btu/h, while the 30,000 Btu/h Chinook (model number C30-M-V) will modulate down to 12,000 Btu/h (see the specification table reproduced at Image #3, below). The Dettson C15-M-V has a temperature rise range of 25 F° to 55 F°. The airflow rate at high speed is 417 cfm.

According to Jacinthe Nichols, the R&D project leader at Dettson, the C15-M-V blower draws only 44 watts at full power and 41 watts at low power.

The Dettson Chinook Compact (CC15-M-V) has specifications that are similar to the C15-M-V. It is smaller, however, measuring only 10 inches by 23 inches. Like the Dettson C15-M-V, the Chinook Compact is Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. rated. For more information, see the Chinook Compact web page.

If you’re interested in a 15,000 Btu/h furnace with an Energy Star rating, choose either the Dettson C15-M-V, the Dettson C15-MS, or the Dettson C15-MV.

The furnace is optimized for heating, not cooling

Gavin Healy, the co-founder of Balance PointBalance point is the outdoor temperature at which the amount of heating provided by an air source heat pump just equals the amount of heat lost from the house. Below this point, supplementary heat (typically inefficient electric resistance heat or “strip heat”) is required. Typical balance point temperatures are in the range of 27 - 35 degrees Fahrenheit. Home Performance in Grass Valley, California, is an energy consultant who is familiar with the Dettson Chinook furnace.

When I called him up to ask him about the Chinook furnace, he told me, “We were interested in distributing the furnace in California, so we were working with Dettson to see if the equipment could be optimized for the California climate. But we haven’t worked with Dettson for the past few years.”

According to Healy, the developers of the Chinook made a few compromises. “All the builders wanted the box to be super-small, so it was designed to have a very compact shape. That ends up being a challenge in terms of the unit’s air flow performance. Dettson didn’t want to invest a ton of money into the ARI coil matching process, so there are a limited number of cooling coils to pair with it.”

He continued, “I would say that the furnace was developed without air conditioning in mind. It was developed as a small gas furnace. From our perspective, the fan has a high watt draw. That’s less of a penalty in Canada, where electricity rates are low and the cooling season is short, than it is in California.”

I asked Healy a follow-up question about the watt draw of the unit’s fan. “Compared to what we are achieving with furnaces from other manufacturers, the ratio of cfm per watt wasn’t as high as we would like to see. We would want the cfm to watts ratio to be double what we were getting from the Dettson.”

Specifying a furnace that is sized correctly for air conditioning is tricky. Healy noted, “A mismatch issue arises between heating and cooling performance, and it can be hard to get the air flows to work out right. Say you have the smallest furnace, and heating air flow that allows you to be at 500 or 700 cfm of air. If you wanted a 2-ton cooling load, you might want 1,200 cfm for cooling in California. So to get that air flow, you might be forced to choose the next larger sized furnace in terms of Btu/h, just to get the desired air flow for cooling performance.”

Healy also mentioned difficulties that Dettson is having complying with the new California regulations for Ultra-Low NOx (nitrogen oxide) furnaces. “Passing the NOx test is hard, and it’s harder as the Btu/h rating gets lower,” Healy said. “The smaller the burner, the more difficult it is to get the mix right.”

Healy emphasized that he didn't want to convey a negative impression of the Dettson furnace. He said, “I have tons of respect for what Dettson is trying to accomplish.”

Designing a furnace always involves trade-offs

I asked Jacinthe Nichols, Dettson’s R&D project leader, about California’s ultra-low NOx standard. “The Chinook complies with the low-NOx standard, which is under 40 nanograms per joule,” she told me. “However, we haven’t yet reached the ultra-low NOx standard, which is under 14 nanograms per joule. We’re trying to reach it, but the solutions are still in the lab.”

Nichols noted that the Chinook modulating furnace complies with Energy Star requirements, which include criteria for gas efficiency as well as fan efficiency.

When I asked about air flow requirements for air conditioning, Nichols noted that California has a dry climate, so the optimal design of a cooling system for California is different from the optimal design of a cooling system for more humid climates (like those on the eastern half of the North American continent).

“It is all about humidity,” Nichols told me. “In California, you want higher cfm because you don’t need the humidity removal. Here in Quebec, when you do cooling, you want to remove the humidity. We didn’t think of having higher cfm for cooling. Our design is to keep the cfm low to help dehumidify. It’s hard to design for all climate zones.”

Price and availability

According to Jonathan William, Dettson's director of client development, the cost of the Dettson C15-M-V furnace to contractors is US$900.

Dettson has at least four U.S. distributors:

HVAC contractors who do business in regions of the U.S. that aren't presently served by a distributor can buy Dettson furnaces directly from the manufacturer. I spoke to several U.S. distributors by phone. One of them, Kurt Albershardt of Southwest Energy Integrators in New Mexico, told me, “The Dettson furnace is phenomenally solid. It’s good stuff — all White-Rodgers components inside. The tech support is good. The biggest challenge is the wholesale distribution system. The logistics are a challenge. We have to ship out of Quebec, and an order costs $250 or $300 to ship here from Canada.”

Martin Holladay’s previous blog: “Looking Back at Insulating Advice from 1951.”

Click here to follow Martin Holladay on Twitter.

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Image Credits:

  1. Dettson Industries

Mar 2, 2018 3:01 PM ET

Goodman seems to have gotten the memo
by thrifttrust

Since last I checked, about a month ago, AC Warehouse now sells a 30,000/21000 BTU 96% two stage gas furnace. It comes in 800 and 1200 maximum CFM variants. It is in a standard size package (18" W 29"D 35" H) so it accepts standard cooling coils. It sells for less than 1000 USD.

Mar 2, 2018 3:16 PM ET

Edited Mar 2, 2018 3:25 PM ET.

Response to Thrift Trust
by Martin Holladay

Thrift Trust,
OK, I found it. It's the Goodman GMEC960303ANA (otherwise known as the Goodman GMEC960302BN).

The specs are here:

See image below.

Needless to say, this furnace is rated at twice the capacity of the smallest Dettson furnace.


Goodman 30000 Btuh furnace specs.jpg

Mar 3, 2018 11:31 AM ET

No Nightly Setbacks
by Doug McEvers

If part of your heat bill reduction program involves nightly temperature setbacks this mini furnace would not fit. Uber efficient homes may find a need for this furnace as they maintain a steady state temperature during the day due to the minimal heat loss. In average homes, part of the energy use reduction strategy may be to setback the thermostat for a period of hours during the evening to provide a better sleeping climate and reduce the Delta T. For the average size home with moderate levels of insulation you should use the Design Temperature Heat Loss as a guide in sizing your furnace. Today's modulating gas furnaces with ECM fans correctly sized using the DTHL is the safe bet for winter comfort.

Mar 3, 2018 12:31 PM ET

Edited Mar 3, 2018 12:31 PM ET.

Response to Doug McEvers
by Martin Holladay

You wrote, "If part of your heat bill reduction program involves nightly temperature setbacks this mini furnace would not fit."

In general, thermostat setbacks make the most sense for poorly insulated, leaky houses. The better your thermal envelope, the less sense thermostat setbacks make. So owners of a very efficient new home will probably follow the recommended strategy for this type of house: Don't bother with a nighttime thermostat setback.

That said, a house with a design heat loss of 9,000 Btu/h might be quite happy with a 15,000 Btu/h Dettson furnace -- and such a furnace might accommodate a nighttime thermostat setback without any problems, even on a cold morning.

Remember, with a good thermal envelope, your house isn't going to cool off very much in 8 hours -- even if the furnace doesn't run for 8 hours.

Mar 3, 2018 1:07 PM ET

That's What I Said
by Doug McEvers

A nightly setback only makes sense for the majority of the existing housing stock in the USA because they are leaky, drafty and poorly insulated. Only highly efficient homes would utilize this furnace due to it's size. On a positive note, let's upgrade the existing housing stock so it is comfortable, durable and highly energy efficient. Then this small furnace will be the perfect fit.

Mar 3, 2018 4:47 PM ET

Edited Mar 4, 2018 11:13 AM ET.

As with so many things, a
by Jon R

As with so many things, a general statement is of limited use without numbers.

A quick calculation says perhaps 20,000 Btu are stored in a house's thermal mass with a 5F drop. Compare that to the current load to find a nighttime cool down time. 1-2 hrs might be plenty and setback will work.

Once you get down to some size heat source (perhaps 20K Btu/hr at low fire), even a 2x over-sized furnace will always have a plenty long run time - because run times are bounded by min output vs thermal mass (and thermostat hysteresis) - independent of load. You can have adequate reserve for morning warm-up without short run-times.

Mar 4, 2018 1:33 PM ET

Jon R You Are Correct
by Doug McEvers

Here is a real world example. Our house in Eden Prairie, MN has a heating efficiency of about 2.3 Btu/sf/hdd. The house was built by others in 1978. The walls are 2"x 4" with R-11 fiberglass with 1" Dow foam sheathing on the exterior. The ceiling has been air sealed by me and has R-100 fiberglass insulation. The foundation has no exterior insulation but has R-10 Thermax interior insulation added by me in 2006 when we upgraded the attic insulation and installed a 95% Lennox furnace with ECM motor. The ACH50 is just a shade over 3 which coincidentally is right near the current MN Code requirement of 3 ACH50 or better for new construction. We have metered gas and I monitored this house for gas and electricity usage for 6 years after the retrofit.

Our Honeywell digital thermostat came with a factory preset setback to 62F and that is where we left it. I have determined the heat loss for our house to be 360 Btu per degree F through a manual heat loss calculation along with RemRate and gas meter readings tied to HDD for the period.

I have determined the thermal storage capacity of our house to be approximately 20,000 Btu per degree F. This was determined by watching the nightly temperature drift when the thermostat set back at 10:00 PM. By knowing approximately our total heat loss in Btu's per degree F, I could use this number along with the indoor temperature loss after setback and the Delta T to arrive at some kind of number as to our interior thermal storage capacity (mainly surfaces) of our house.

With this in mind I could better understand why our very efficient gas furnace would take a long time in getting our house back up to temperature in the morning when the thermostat called for 70 F. It was not just heating the air back up to 70F, that is relatively easy at .018 Btu per cf, our house volume of 28,800 cubic feet. The furnace also had to account for the Delta T and that 360 Btu per degree F heating requirement. But the furnace also had to warm those interior surfaces which had cooled to 62 F overnight due to the thermostat setback. If my calculation of 20,000 Btu per degree F of thermal storage is anywhere accurate we have 160,000 Btu's additional to make up each cold morning when the interior temperature reached 62 F.

Our design temperature heat loss is 30,600 Btu, our dry bulb design temperature in Minneapolis/St. Paul is -15 F. Our furnace is 66,000 Btu input high fire and 45,000 Btu low fire. (The original we replaced was a GE 150,000 Btu gas) On the coldest mornings the furnace will run continuous on high fire for about 2 1/2 hours before reaching and the indoor air temp of 70 F. As the morning progresses the run times become shorter until the thermal mass reaches 70 F, at this point the furnace will run for a period of minutes on low fire 3 times per hour (3 cph) to satisfy the heating load.

How does our house rate in energy efficiency? Our neighbor's house built one year earlier uses 2 1/2 times the gas we use. The square footage is similar, no thermal improvements have been made since it was built. The Category 1 homes tested by the MN Department of Commerce in the early 2000's averaged 3.45 Btu/sf/hdd. My best effort with Superinsulation in the mid 1980's was .9 Btu/sf/hdd. My best guess is Passive House is running around .5 Btu/sf/hdd or less.

Mar 7, 2018 12:32 PM ET

It's still fossil fuel
by Jon Harrod

I think this is a product I would have been excited about 15 years ago, when I still felt like there was a role for fossil fuels in green building. But, given the urgency of the climate crisis, known issues with methane leakage and fracking, and the rapidly dropping cost of renewable energy, I think it's time to abandon fossil fuels altogether. No more gas meters, no more pipelines, no more stranded assets. The difference in equipment costs between a 15kBtuh ducted minisplit and a 15kBtuh gas furnace is pretty small and will be more than offset by the avoided first cost of gas lines (not to mention ongoing meter charges). Plus, the minisplit provides air conditioning.

Mar 13, 2018 1:09 PM ET

Dettson Smart Duct System (HVAC in a box)
by Eric Marchand

Has anybody taken a good look at Dettson's 2.5" flexible duct system? As a home builder (not an HVAC contractor) the modularity of the system is appealing. I would love to get the opinions of some hvac contractors or anyone else who has experience with the system.

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