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 Btu/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 envelope 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…
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Goodman seems to have gotten the memo
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.
Response to 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.
No Nightly Setbacks
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.
Response to Doug McEvers
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.
That's What I Said
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.
As with so many things, a
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.
Jon R You Are Correct
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.
It's still fossil fuel
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.
Dettson Smart Duct System (HVAC in a box)
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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