Which gas furnaces are most efficient in their use of electricity?
richmass62
| Posted in Energy Efficiency and Durability on
I am about to replace a 13 year old 120,000BTU furnace with a much smaller unit that will be less noisy.
I understand that the fans of these units are a significant power draw. The state of MA will give the same rebate to all 95%AFUE furnaces. Is there a difference, where some brands of 95% AFUE furnace will actually use a lot less power?
FYI my furnace is in the (conditioned) attic so I will end up putting in a horizontal downflow unit.
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The annual electricity use of a new 95%+ furnace CAN be high, but it doesn't need to be so. Any Energy Star certified furnace (https://www.energystar.gov/productfinder/product/certified-furnaces/results) will have an efficient ECM blower.
Much more important for energy efficiency, though, is the design of the duct system. https://www.greenbuildingadvisor.com/articles/dept/building-science/hidden-flaw-some-high-efficiency-furnaces
Rich,
I can't advise you on brands but suggest that you have an independent HVAC engineer run an aggressive Manual J on your home to size the system. Based on comments posted here, it seems contractors often oversize systems, which increases costs for customers while reducing indoor comfort.
Here is a post to help you find a consultant for your project: https://www.greenbuildingadvisor.com/articles/dept/musings/who-can-perform-my-load-calculations
I have often read that a variable speed fan with a high-quality motor is much more efficient than standard single or dual-speed fans. But a lot of variables go into specifying an HVAC system.
There's a helpful link within the Allison Bailes article that John linked above:
https://www.ahridirectory.org/ahridirectory/pages/home.aspx
The AHRI website has a searchable database, including estimated electricity usage for gas furnaces. Click Furnace (under residential) and enter some model information.
FYI, that AHRI website database is rough for searching. Get a cup of coffee and a stack of patience before you go digging in there.
Just an FYI.
If you have a heating history on this place the load can be calculated based on wintertime fuel use, and a Manual-J would not really be necessary to spec a replacement unit.
But MassSave may still require a Manual-J, especially if you're taking the 0% loan.
When the furnace & ducts are up in the attic above the insulation (a bad idea in a MA climate) the potential for error in a Manual-J is large, but a fuel-use based load calculation automatically has the duct losses factored in, since it's impossible to factor them out.
Running a fuel use heat load can be a critical sanity check on the quality / accuracy of a Manual-J, and well worth it. It's pretty common for Manual-J's to come in 10% or even 15% higher than a fuel-use calculation, for a variety of reasons. But if the Manual-J comes up with a heat load more than 20% greater than the fuel use calculation it's likely to have serious errors, and resolving those errors until they harmonize with the fuel use numbers (within 10%) would be good, since they up-size from there. (ASHRAE recommends a 1.4x oversizing factor from the 99% heat load for maximum comfort & efficiency.)
For details on how to perform a fuel-use based load calculation, see:
https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new
I also noticed from a few articles here and elsewhere that it is possible to have a contractor remove the stock motor from my 13 year old gas furnace blower and replace it with a more efficient "evergreen-im" motor. Reading this it appears I might be able to save 200 kwH per month during mid-December to mid-March when electricity prices are most expensive.
This might be the way to go, as I can use the $ savings for a mini split to create a second heating zone in the house.
Rich: To save 200kwh in a month seems unlikely. It would take a 28% high duty cycle for a 1kw single speed blower motor to even use 200kwh TOTAL in a month, and a 55% duty cycle for a 500w blower motor.
A typical condensing furnace with a permanent split capacitor motor uses about 9-10 kwh per million BTU (MMBTU), and high efficiency permanent magnet blower replacement uses a bit more than half that, call it 5 kwh/MMBTU. Non-condensing furnaces typically use 6-7kwh/MMBTU.
In order to save 200 kwh per month with the ~4-5 kwh/MMBTU difference means you would have to be using on the order of 50 MMBTU/month, (= 500 therms or ~500 ccf/month) which indicates a very large &/or leaky house in a cold/very-cold climate. The US average gas use for space heating is about 500 therms per YEAR.
After Googling it for a sanity check on my imperfect memory, NYSERDA's numbers are pretty close to my recollection on kwh/MMBTU:
http://www.taitem.com/wp-content/uploads/tt_furnace_electricity_use_20130507.pdf
[edited to add]
NYSERDA apparently did an in-situ monitoring program of retrofit high efficiency blower motors, covered in detail here:
https://www.nrel.gov/docs/fy14osti/60760.pdf
From the executive summary portion note:
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Not surprisingly, results indicate that BPM replacement motors will be most cost effective in homes with right-sized HVAC equipment (with longer run times) and proper ductwork (i.e., low static pressures). Additionally, more dramatic savings can be realized if the occupants use fan only operation to circulate air when there is no thermal load. There are millions of cold-climate, U.S. homes that meet these criteria, but the savings in most homes tested in this study were modest.
There were certainly substantial electric power reductions with the new motors. Average fan power reductions were approximately 126 Watts during heating and 220 Watts during cooling operation. Over the course of entire heating and cooling seasons, these translate into modest average electric energy savings of 163 kWh. Average cost savings were $20/year. Homes where the fan was used outside of heating and cooling modes saved an additional $42/year on average.
-------------------------------- end copy & paste
SOOoooo......
That's 163 kwh ANNUAL savings typical (not 200kwh/month), but more if:
A: Your 13 year old gas furnace is right-sized rather than over sized. A 120kbtu/hr it's almost certainly oversized, but you can run a fuel-use heat load calculation to determine just how much.
B: Your furnace is condensing
C: You also use the air handler for cooling or ventilation air distribution.
The executive summary concludes with:
---------------------------begin copy & paste
This replacement motor measure, therefore, should be used selectively in homes where several of the following conditions apply:
• The furnace likely has at least 10 years of useful life remaining.
• Heating and cooling equipment is right-sized (thus operates longer and more frequently).
• Operating pressures are relatively low (no constrictive ducts).
• The furnace fan is frequently used outside of heating and cooling operation
-------------------------------- end copy & paste
But don't plan on being able to buy a mini-split with those savings, even if "all of the above" applied.
Wow what an extensive answer. I think that the retrofit makes sense only if it's a lot quieter. In other words, we could live with our oversized furnace for 5 more years if it was not making so much of a racket and if we had a minisplit for daytime use to make the downstairs more comfortable. I do see now that the power savings will be a lot smaller. So if our furnace ran for 160 hours in January and the blower used 1000 watts, we would be able to cut the 160KwH usage to 115Kwh.
Also, even though our furnace is 13 years old it sat idle for 2 winters during the "Great Recession" and it's still performing quite well.
I will see how much a retrofit would cost and go from there.
With a fully functioning but oversized furnace there's no payback on a motor swap in dollar terms, and probably not in other terms either.
The noise from the air handler won't change with the motor swap- it's still the same cfm. Sealing all the metal ducts & register boots with duct mastic on every seam & joint can reduce the noise of ducted system quite a bit. Taping the seams of the air handler with aluminum tape can also make a difference.
It's probably best to the motor swap money for something with better payback. You can probably seal all of the accessible ducts & register boots with less than $50 in duct mastic, and it will still have been "worth it" when it comes time to right-size a replacement furnace (or perhaps a right-sized air handler running off the water heater, depending on your actual load.)
In the mean time, have you measured the load (including duct losses) yet?
https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new
HVAC guy got busy so wasn't able to come out this week but I might be able to answer those questions next week. Remember we are talking about a 13 year old motor; hard to say if that motor is unusually noisy, perhaps the noise level is greater than it was when the furnace was only a year old.
We did invest in properly sized ductwork (in fact, replaced 100% of it). The ducts are completely sealed. The noise is coming from a main air return that might be too large. I mean the amount of air is correct but 90% of the air is drawn from the kitchen area and the size of the duct will convey the furnace noise. We could add a branch in the main return to draw some air from other parts of the house (like the attic which has no heating vents, but is part of the conditioned space).
Here is a followup on what we did to reduce the noise. I did not replace the original furnace yet.
1) I opened up a new air return (8 X 12) on the 2nd floor in a bedroom. This means there is a shorter path back to the air handler. Then below that return I installed 2x4s on the sides of the air duct so I could make a damper for both air flow and sound using a piece of 3" thick EPS foam. My rectangle of foam reduces the amount of air the handler can pull from downstairs.
I have attached photos, one showing the view from inside my air duct and the larger photo showing a clearer view of new opening and the smaller "old opening."
This is much better because the oversized duct was pulling a lot of air from rooms with relatively leaky walls. This is not a part of the house where you want a lot of "negative pressure."
2) I also added a return a year ago in the attic so we have 3 points of return for the air handler system. That was done when attic was finished.
3) insulation was added below the first floor (including spray foam, rock wool, poly) to reduce the heat load and avoid pulling basement air into the 1st floor.
4) We installed a mini split downstairs (Mitsubishi 18000btu hyper heat model) so now we are able to have a "gas zone" upstairs and a PARTIALLY electric zone downstairs, using separate remote thermostats.
summary:
* less noise, more comfort (used to be 1 zone for entire house)
* should be a bit lower cost once we get a full winter of data
* gas usage down 33% already
* once we get a full winter of data, gas usage should be down 40%, electricity usage up 20%
* looks like we will achieve 50% reduction in gas usage with 10% increase in power use by the time we are done updating our furnace...