Does electric thermal storage make sense as a replacement for a furnace in a studio apartment?
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Hey GBA, I recently moved into a new house that was built in 2002. This house has 2 buildings one is the main house and the other is a garage with a MIL suite. Both furnaces and A/C units are original and the main house has not worked 2 days in the month I have been here. The MIL suite A/C has been running for 1 to 2 hours straight and only off for like 15 minutes while the main house is the opposite. So I am looking at getting both replaced.
I am going to get heat pumps for both houses and I was thinking gas furnaces, but I saw in my electric company’s rebates they had one for a electric thermal storage. Main question is would it make sense to not get a furnace and just use the heat pump and ETS system? It is about 1,000 sqft, but the ceilings are vaulted, and can go as high as 15ft. This is in Denver Colorado hardiness zone 5b. I am renting the MIL suite out so it will need to be something simple to operate also.
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what are your off-peak electric rates vs gas, and what does the utility generation mix look like during off-peak times for your service area?
I assume heat pumps alone simply won't cover the loads?
And I assume you've also already ruled out envelope improvements for some reason (insulate, air-seal)?
I just moved in so I am not quite sure what my rates are yet and my power company has 4 different ways to charge for electricity depending on how a house uses it (5 if you include net metering for solar). The heat pumps I have seen only work in 40F+ and my area gets below that regularly. The only real envelope improvements I can do would be replacing the windows or taking the drywall out in the garage and insulated the that. The MIL suite is completely above the 2 car garage expect for the stairs. The furnace and A/C is dying so they do need to be replaced ASAP as I just turned on the MIL suite A/C a few days ago as someone is moving in to rent it and my electricity usage has more than doubled. The MIL suite A/C is a 1.25 ton unit and the main house is a 3 ton.
>"The heat pumps I have seen only work in 40F+ and my area gets below that regularly."
I'm not familiar with your area (economic nor climate) but there are certainly heat pumps that work well below freezing (below 0 F actually) that are pretty widely available as far as I know.
As far as 'envelope' efficiency: without knowing what you're working with, I would at least consider doing a basic assessment on air-tightness, attic insulation levels, basement insulation levels, etc. This site has tons of information on these topics. Replacing windows is likely not your lowest hanging fruit (though I could be wrong, perhaps it is in your case).
If you were expecting to carry most of the winter load with what is essentially electric resistance heat, I'm thinking the gas would likely be more efficient (total systems approach, given electric generation and transmission losses). I'm not that familiar with the ETS systems though. (They appear to be resistance heaters with a storage component?) The reason I asked the question I did is because this answer might depend on specifics of your grid/rates, etc. One could perhaps argue moving away from gas and towards electric is inherently good from a paradigm shift perspective, but with resistance heat the efficiency is not amazing.
>"The heat pumps I have seen only work in 40F+ and my area gets below that regularly."
You're just not looking hard enough.
The 99% outside design temperature for Denver is in the ~ +5F range, give or take 2F depending on location:
https://higherlogicdownload.s3.amazonaws.com/ACCA/c6b38bda-2e04-4f93-bd51-7a80525ad936/UploadedImages/Outdoor-Design-Conditions-1.pdf
There are MANY heat pumps ducted & ductless that have capacities rated at +5F and lower. The Northeast Energy Efficiency Partnerships (NEEP) maintains a searchable list of heat pumps of various types, all of which have a specified capacity at +5F or lower suitable for cooler climates such as Denver's: https://ashp.neep.org/#!/product_list/
If you run a Manual-J or IBR heat load on the space, or analyze the fuel use to come up with the load at +5F, and pick a model that has between 1.2x and 1.5x that capacity @ +5F it should be pretty easy find something that works. Most 1000' studio apartments will have a heat load of less than 12,000 BTU/hr @ +5F, and almost anybody's 3/4 ton or 1-ton cold climate mini-split would have sufficient capacity, eg:
https://ashp.neep.org/#!/product/31913
https://ashp.neep.org/#!/product/26102
The 3/4 ton Fujitsu puts out more than most:
https://ashp.neep.org/#!/product/25333
There are ducted heat pumps on that list that work well into negative digits F too. Measure the duty cycle on the existing 3 ton AC on afternoons when it's roughly your 1% outside design temp to estimate the cooling load, and run a fuel-use load calculation on the furnace to get a good handle on the 99% design heating load using this method:
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
With bigger air-handler non-modulating ducted units be careful to NOT oversize the heat pump by more than 1.2x for the 99% load. Even undersizing it a bit for the 99% load and using the auxiliary strip heat to cover the shortfall for those deep cold snap hours is usually more comfortable, and burns less power overall than a 1.5x+ oversized heat pump that has limited (or no) modulation range.
Do you have a solar system? They work awesome in Denver with all the sun, and I see a LOT of installations when I visit family out there. I would try to avoid any form of battery storage unless you have no choice. It takes a LOT of battery storage to do much. Do you have the option for net metering out there?
Xcel used to offer rebates for evaporative (swamp) coolers too, which also work pretty well in Denver due to the low humidity levels. The downside is increased water consumption which may be a cost to think about when comparing them to heat pumps. It’s a trade off of pay for water but less for electricity with an evaporative cooler.
Bill
I do not have solar yet, but I am planning on getting it before the year is over. United Power does have net metering as I think it is colorado law. I did also talk to Steffes about getting one, do you recommend any other brands or installers? It has been kinda hard to find any info about them.
Generally where city gas is available it is the least costly way to heat a house and water. If you want to cut the gas line on principal that is fine but it is likely to cost more and be less comfortable.
On the warmest days a correctly sized AC unit should run continuously. If the unit in the MIL is able the keep the space cool on the hottest days it is perfect. The main unit sounds like it is too large. Convincing a contractor to install a smaller unit can be a tough sell.
Do you have a link for more info on the electric thermal storage you are considering? Are we talking about a big water heater?
Am I right that you can go for weeks and not get above 0? 20 below is very tough duty for a heat pump.
Walta
Denver rarely sees -20F and 0F is uncommon. A heat pump will certainly work there. What is the electric thermal storage system you mention?
Thermal storage systems typically require a large variation in electric rates to be cost effective. Despite being increasingly logical, this is rare.