Include natural gas backup

[dfrankow]

Thanks for your work on this calculator.

In Minnesota, it sometimes gets very cold. A lot of the time we can run a heat pump efficiently, but 4 installers I talked to all recommended having natural gas backup for the coldest days.

It would be interesting if your calculator had some way of accounting for that natural gas backup. Essentially that would just take some days out of the comparison. It might fit into your "Heat loss" tab (which I didn't see until this moment).

[Siecje]

You can compare with natural gas and then use natural gas when the COP of the heat pump is less than the COP shown.

For example

The heat pump is cheaper as long as the COP is larger than 1.41.

4 installers I talked to all recommended having natural gas backup for the coldest days.

You can also have an electric resistance heater in the air handler then you don't need natural gas hookup and don't need to pay the monthly hookup fee.

[dfrankow]

I appreciate the info.

Yep, the COP threshold is valuable.

All 4 installers recommended not using electric resistance heat, because a large fraction of the heating expense will come from the coldest days, and electric resistance will be much more expensive (they all said). Even the installers who work with mncee.org, that promotes energy efficiency, all said natural gas backup was the practical move for the coldest days in Minnesota.

[Siecje]

What do you pay for electricity and natural gas? What is your monthly gas connection fee?

[dfrankow]

I like your passion.

Rough numbers:

Gas price is ~$1/therm Furnace efficiency: 96% Electricity price is ~$0.116 / kWh

Amount of gas used for heating (12 months): ~800 therms

Basic charge $9.50 per month, "city franchise fee" $6.50 per month.

Two more issues with all electric:

1. We've already gone through bids and accepted one, so it's pretty late in the process to switch.
2. To get rid of gas, we'd also need to replace our water heater and stove.

I was looking into moving all-electric, but then when every installer said I should have a gas backup, I gave in.

[Siecje]

To get rid of gas, we'd also need to replace our water heater and stove.

Sounds like an excuse to get a heat pump water heater and an induction stove :)

but then when every installer said I should have a gas backup, I gave in.

Maybe there is not enough room in your distribution board. It doesn't sound like it if you don't have an electric water heater or stove.

Last winter I had two days where it was -30C so the 15 kW resistance heater ran for about 48 hours but let's say it runs for 100 hours a heating season.

150 kWh * 0.116 = $17.4

Which is less than 2 months of the connection fee.

[dfrankow]

Again, I like your passion. I was on that road to all-electric.

It's hard to find really good data, but we had a lot more days than 2 at really cold temperatures.

Minneapolis had 53 days below zero in 2013-2014: https://www.dnr.state.mn.us/climate/journal/at_or_below_zero_13_14.html.

I'm not sure where you live, but I've been reading on Reddit's r/heatpump that people outside of Minnesota and those inside Minnesota view cold temperatures differently.

[dfrankow]

More data: https://www.dnr.state.mn.us/climate/journal/zero_and_below_streaks.html

"The stretch in 2021 lasted 116 hours from 4pm February 11, to 11am February 16th."

That's just the streaks, not the total.

[Siecje]

How many days at or below -30C? My heat pump was able to keep up until it was -30C for two days. I know because the auxiliary electric heater wasn't configured correctly and it was only a problem when it was -30C for two days.

I have a 3.5 ton Daikin Sky Air.

[dfrankow]

It's hard for me to find data below -10F, but that is also real. There are stretches.

-30C is -22F. That does happen, but it's not common. Only a couple of days.

I have been bid a 2-ton Lennox. I think it won't do that well below 5F (-15C). COP@5F is 2.22.

Also, our house is 115 years old, not super well sealed. I've read https://www.energyvanguard.com/blog/can-a-heat-pump-work-in-minnesota/, and it can work, but they sealed their house very well.

[Siecje]

not super well sealed

To estimate the heat load correctly the installer should have done a blower door test to know how well sealed the house is.

What is your estimated heat loss?

[dfrankow]

Which heat pump do you have that you are worried the heat output at 0F (-17C)?

We don't have one. We've been bid the Lennox EL18XPV-024. SEER2 14 EER2 11.5 HSPF2 8.2 COP@5F 2.22

not super well sealed

To estimate the heat load correctly the installer should have done a blower door test to know how well sealed the house is.

What is your estimated heat loss?

I don't know.

All I know is approximate gas therms used for a heating season: 800.

[Siecje]

All I know is approximate gas therms used for a heating season: 800.

How many heating hours?

[dfrankow]

Unfortunately I don't even know how to estimate that.

Does the below help?

=====

https://portfoliomanager.energystar.gov/pm/degreeDaysCalculator says (relative to a base of 65F)

Weather Station ID: 726580 Weather Station Name: MINNEAPOLIS/ST.PAUL

Time Period: February 01, 2022 - January 31, 2023 Annual HDD ("Heating Degree Days") : 4202 °C Annual CDD ("Cooling Degree Days"): 553 °C

[dfrankow]

FYI here is what the installer says in response to the possibility of all-electric heat:

Your house has enough ductwork for 1,000CFM of air flow. To heat your house with electric strip back up or just a heat pump we would need about 2,000CFM of duct work. The only real option that works with heating loads as high as Mn has is an electric boiler, fan coil, and heat pump. So you’re looking at about $25k in equipment installs or even more to re duct the entire house, to raise your annual heating budget by 50-100%. Heating your house all electric resistance heat in January is going to cost $600-$1,000 a month.

Now you can say "mini-splits don't require the ducts", but it might easily require multiple mini-splits, I don't know. That might be where the "$25k" comes from. At some point, perhaps unfortunately, I just have to trust the installer.

I am just passing along this information because I think it's valuable for advocates of energy efficiency to understand the different environments where we are trying to make this work. I would like all-electric, I just am overwhelmed by all the factors that go into it.

[dfrankow]

His comment reminded me of what multiple installers said: with a high heating load (i.e. a very cold day), you need a lot of airflow to get the heat pump to work, because it's not as hot as something like a furnace. With existing ducts, it's often not enough. We didn't deeply explore mini-splits, but unfortunately we're now tired of bidding and evaluating, it's confusing and tiring. :/

Thanks again for your passion.

[Siecje]

I disagree that you need natural gas. According to a quick search my city is slightly colder than Minneapolis and the heat pump worked fine except for 2 days when it was less than -30C and I needed to use resistance heat to supplement the heat pump. Part of the problem was that the heat pump was defrosting when it didn't need to. I went outside and there was no snow/ice on the outdoor unit. This is because when it is really cold the air can't hold a lot of water. If the heat pump was smarter I wouldn't need as much resistance heat.

Now about your ducts not being large enough. Since the air temperature from the heat pump is not as high you need more air to heat the building. That is true and that is true all the time. Now when it is really cold the resistance heat in the air handler is bringing the supply air to at least the same temperature the heat pump would normally get it, if not higher. By normal I mean when it is not as cold outside.

So unless you are getting a non ducted install your ducts need to work with a heat pump.

Are you only using the heat pump for cooling and spring/fall? It sounds like that's what the installer is thinking.

[Siecje]

At some point, perhaps unfortunately, I just have to trust the installer.

The problem is your incentives do not align.

The installer wants to charge you for more equipment that you will have to pay them to maintain to keep your warranty with them.

They also don't want a no heat call on the coldest day of the year.

[Siecje]

Feel free to continue commenting here but there is also the Heat pump discord https://discord.gg/dFjCrFuYN5