Heat pump upgrade path with old furnace and new AC
graygreen
| Posted in General Questions on
If the AC is new but the furnace is old and likely needs replacement soon, or at least an expensive repair (the inducer is whiny). Is there an economical path towards a heat pump?
This is energy climate 5A. Heat pumps struggle on the coldest days and people get dual-fuel setups to be able to have the efficiency of a heat pump along with the effectiveness of natural gas. However, all other appliances are being electrified at which point the cost of maintaining the natural gas connection year round negates any cost savings of having natural gas.
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Replies
The bad news is the heat pump is going to replace both the furnace and the AC, no way around it.
The good news is the newest heat pumps do a lot better at cold temperatures. What you need to figure out is your heating load and design temperature and then look in the Neep.org database to see what equipment is available to meet your load at design temperature.
This article explains how to calculate your actual heating load based upon historical fuel usage:
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
Look up your county in this document:
https://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/County%20Level%20Design%20Temperature%20Reference%20Guide%20-%202015-06-24.pdf
That will give your heating design temperature.
What you probably want is something like the Mitsubish M Series, which still cranks out quite a bit of heat at -13F: https://ashp.neep.org/#!/product/34584/7/25000/95/7500/0///0
Get those numbers and come back to us.
Doing my calculations for the largest energy use period on my bill for the last 2 years ends up with 66k BTU/hr. My existing furnace is 95k BTU (92% efficiency).
I am still actively working on some significant energy improvements (replacing a really bad back door, installing an ERV). It will be great if the furnace can last one more year and I can do this calculation again. I am also thinking about putting in a mini-split in the finished open attic space so the furnace doesn't need to do any duties there and to help with temperature consistency issues. That might get me down to the BTU you are linking to.
I don't see a price on NEEP- how do I find the price of these units?
What's your design temperture?
1% Cooling 89
99% Heating -3
HDD/CDD 2.7
This is the 3-ton version with air handler:
https://hvacdirect.com/mitsubishi-svz-ka36na2-36-000-btu-16-seer-ductless-mini-split-heat-pump.html
Sounds like you need about 5 tons.
Although, "5 tons is never the answer."
https://www.greenbuildingadvisor.com/article/sizing-a-heat-pump-system-for-cooling
The price of the unit isn’t that useful since you need the unit installed and that’s usually 3x the unit cost. You can google them, most will pop up.
I appreciate the advice about calculating what size unit to install. The problem still remains though that throwing away the existing air conditioner seems like an expensive proposition and buying a natural gas furnace now should be less expensive. In an ideal world I would get a heat pump but keep using the existing AC in the summer until the AC breaks down- saving wear and tear on the heat pump in the summer. Then when the AC dies switch to using the heat pump for summer as well as winter.
What is your cooling load? One possibility is if your cooling load is much lower than your heating load, you could get a heat pump that is sized for your heating load and use it only for heating. I don't know if this would be more economical - it depends on the cost of new furnace, cost of gas and electricity.
That's what I was getting at in my last comment. Is there a setup that makes sense to use the heat pump for heating but not for cooling (until the AC breaks down)? At least it is possible if a new furnaces is installed next to the old one. I don't have space for that though without moving the existing furnace.
nobody mentioned what seems obvious. Put in a high efficiency gas furnace as it will be way cheaper to run until at least the time it takes for the ac to wear out. The new furnace will be used by the future heat pump to move air so get a good one and worry about cooling load and other details in3-8 years when you need too.
just my 2 cents.
yeah, I need to compare the total financial outlay of this scenario versus a heat pump. At the moment it is cheaper to operate, but it won't be once the rest of the appliances are electrified- at that point the monthly fee for maintaining the gas connection for the furnace is a significant expense that will wipe out the savings. This comes down to figuring out when the water heater likely needs to be replaced (will do a heat pump water heater at that point).
Consider paying for the repairs. It may be the least worst option.
There's a whiny inducer that is > $1k repair. There is a possible path forward here to try to pre-emptively fix the inducer motor now before failure- I will look into that. If I can lubricate the motor or replace just the motor then this is a possible path forward.
I have already experienced that when something electronic goes out the HVAC companies refuse to repair it and instead do large replacements. I replaced a blown $5 ECM controller inrush current limiter myself but all the HVAC companies tell me they would have me replace the entire blower ($2k). They will spray WD-40 on the inducer motor, but I would probably be on my own for otherwise doing a simple fix rather than a full replacement.
The furnace is a Temp Star 92% efficient furnace and I was told that the parts are more proprietary than universal. The HVAC companies will only by them new at whatever price is charged. If I do the fix myself in some cases I can buy a used part on Ebay for very little. I would really rather pay someone else to do this work, but when the difference ends up being thousands of dollars DIY starts to make a lot of sense!