"All Electric" Home with Instant Hot

[winnie]

Different heat pumps have different lower temperature limits. I am reading 'set up to run down to about zero F here' as 'selecting a heat pump that correctly operates at zero F' For a heat pump to operate at these low temperatures you have to consider things such as outdoor coil defrosting, and may need heat strips to ensure condensate drainage.

I am currently exploring low end minisplits for my own home. The first ones I was considering are specified to provide heat down to an 5F outdoor temperature. This covers most of my heating needs, and I was really only planning the heat pump for 'shoulder season' heating. But I figured I should look to see what I could get if I wanted something that would be capable of operating on even the coldest days.

I am now looking at ones rated to provide heat down to -30F outdoor temperature, with a 47F COP of 4.0 and a 5F COP of 3.36 :

https://www.shareddocs.com/hvac/docs/1009/Public/07/32809001401.pdf

-Jon

 

[kwired]

I’m kind of lost here..
”set up to run down to about zero F here”..

Mine runs, period.
Now admittedly, It’s only been below 5 F 17 times here since 1940.
Our average low is lower 30s
but…

my strip heat comes on when the temp inside gets 3 degrees below the set thermostat temp.
Believe me, my wife knows that magic “3” number. She hits that spot when she wants a quick warm.

Some guys install a "lockout thermostat" in the outdoor unit that won't let it run below it's setting and it actually transfers the control from compressor lead directly to backup heat lead of the controls. They are often setting these as low as zero degrees F, where several years ago they may have been setting them at 5 or 10 degrees. That is the point they have determined it cost same or more to operate heat pump as it cost to operate the backup heat. If heat pump can't keep up there is always second stage on the indoor thermostat that will kick in backup heat anyway once the temp drops about 2 degrees below the setpoint.

 

[kwired]

Different heat pumps have different lower temperature limits. I am reading 'set up to run down to about zero F here' as 'selecting a heat pump that correctly operates at zero F' For a heat pump to operate at these low temperatures you have to consider things such as outdoor coil defrosting, and may need heat strips to ensure condensate drainage.

I am currently exploring low end minisplits for my own home. The first ones I was considering are specified to provide heat down to an 5F outdoor temperature. This covers most of my heating needs, and I was really only planning the heat pump for 'shoulder season' heating. But I figured I should look to see what I could get if I wanted something that would be capable of operating on even the coldest days.

I am now looking at ones rated to provide heat down to -30F outdoor temperature, with a 47F COP of 4.0 and a 5F COP of 3.36 :

https://www.shareddocs.com/hvac/docs/1009/Public/07/32809001401.pdf

-Jon

Never ran into condensate drainage heat strips, these are outdoors and all that matters is defrosting the coils so air can flow through them, though that will leave a big icy mess below the unit which is usually blocked up above the ground by at least a foot or so vs AC only units that often sit pretty much on the ground. There will be *crankcase heaters* to keep oil warmed inside the compressor when not in use. Those may be optional to guys purchasing them down south but is pretty much automatic they will have them here.

*technically maybe not the right term anymore since they are all scroll compressors anymore* plus I haven't paid that close attention but I think they still have such heaters on scroll compressors

 

[kwired]

Also seeing minisplit heat pump units that don't have any aux heat, they must work in pretty cold temps or else they would need to have backup heat.

 

[gadfly56]

Well to most users bottom line is what is going to cost them less to operate, I get the fact that there are losses even before your meter.

A heat pump is still cost effective well below 50 degrees vs most any backup heat source that supplements it, it just that 50 degrees may very well be the point where the differences start to become less.

Therms of gas are much cheaper than therms of electricity.

 

[gadfly56]

Not sure where that number came from…
we are about 3%, and the EIA average is 5%.
just sayin’…

Well, I can't find the source I used, so lets go with 5%. That raises the energy at the heat pump to 38 units and lowers the minimum COP to 2.37, assuming that the gas and electricity costs per therm are equal.

 

[wwhitney]

Therms of gas are much cheaper than therms of electricity.

But with a heat pump you have to take your kWh price of electricity and divide it by the COP (and the proper change of units) to get the cost of "therms delivered."

Your previous computation showed that when COP is 2.4 or higher, it's more energy efficient to burn the gas to make electricity to drive the heat pump. My understanding is that most heat pump applications will average a COP of at least 2.4, so heat pumps are at least break even on that front.

The cost comparison will vary by regional pricing, but for a rough comparison it would be reasonable to assume a COP of 3.

Cheers, Wayne

 

[winnie]

I am in Massachusetts and am paying $0.29 per kWh.

I have an 85% efficient oil fueled boiler for steam radiators.

Heat content of heating oil is 139000 BTU per gallon. So with losses 1 gallon gives 118200 BTU to the home

With a COP of 3, 1 kWh provides 10200 BTU of heat to the home. So I need 11.6 kWh to equal 1 gallon of oil (with these assumptions).

With these assumptions, operating a heat pump is at break even with oil at $3.35 per gallon.

I was lucky. I bought my oil in February at $3.39; it is now at $4.60 and was north of $5 for a bit.

So I am very much in the range where using a heat pump for electric heating is an economic win. Not a no-brainer because there are significant up-front costs.

-Jon

 

[electrofelon]

If you burn gas directly for heat, the process is typically 90-95% efficient at delivering the heat value of the fuel to the home. If you burn that gas to produce electricity, not so much. The EIA says that the US generating fleet is not quite 40% efficient turning fossil fuels into electricity. So, for 100 units of thermal energy, you get (generously) 40 units of electricity out. The distribution losses are about 9%, so now we are down to 37.4 unit of energy delivered to the heat pump. To be as efficient as direct heating, the heat pump can't operate at a COP below (90/37.4) or 2.4. And please, no nattering about how we can just increase the efficiency of the grid. We're talking about today. So, if you are in a climate zone with many more heating degree days than cooling, the barrier is a COP of 2.4.

Your efficiency at burning gas is way too high I believe. Isn't it more like 70% typically? I am looking at a Lopi Northfield gas stove and it is the most efficient one I can find and I believe is around 70%. I don't really care too much about arguing about the carbon footprint or overall efficiency of electricity running heat pumps etc,. But electricity has many advantages such as easier infrastructure in the house, no venting, split type systems also being able to do air conditioning, for those of us with solar systems being able to take advantage of our "free" electricity, etc.

 

[suemarkp]

The other problem with heat pumps is the cost of having to deal with any failures in the refrigerant lines. Lose a TXV or reversing valve, and you're doing evac, purge, refill. That cost me about $800 both times it happened. I'm tempted to get a quickie EPA license so I can buy the refrigerant at $10/pound instead of the $50/pound the HVAC service people charge. My time to evac a system is free. Last time, they sat around for a few hours waiting to pull enough vacuum. I had a better guy before who would start it evacuating in the morning and come back in the afternoon after doing another job to finish. He retired though...

 

[retirede]

But with a heat pump you have to take your kWh price of electricity and divide it by the COP (and the proper change of units) to get the cost of "therms delivered."

Your previous computation showed that when COP is 2.4 or higher, it's more energy efficient to burn the gas to make electricity to drive the heat pump. My understanding is that most heat pump applications will average a COP of at least 2.4, so heat pumps are at least break even on that front.

The cost comparison will vary by regional pricing, but for a rough comparison it would be reasonable to assume a COP of 3.

Cheers, Wayne

With my highest-ever gas prices this winter, my electricity is 3 times more expensive for the same energy. So I’d break even with a COP of 3.

Last year, my electricity was almost 10 times the price of gas.

 

[electrofelon]

Another thing to consider, is that many places do not have natural gas in which case it would be propane which I believe is quite a bit more expensive per thermal unit.

 

[wwhitney]

Your efficiency at burning gas is way too high I believe. Isn't it more like 70% typically? I am looking at a Lopi Northfield gas stove and it is the most efficient one I can find and I believe is around 70%.

A stove will be non-condensing, so its maximum thermal efficiency will be 80%. And even that is just heat going into the flame, much less of the heat gets into the pan. I'd guess only half. Induction stoves are likely as efficient as a gas stove. [I.e. burn the gas to make electricity to run the induction burner, and 30% - 40% of the heat value of the gas ends up in the pan; or burn the gas on the stovetop, and 30%-40% of the heat value of the gas ends up in the pan.]

But a condensing appliance can get 90% - 95% of the thermal value out of the natural gas. And on a furnace/boiler/water heater, the lengthy heat exchanger between the combustion side and the air/water side will transfer almost all of that heat into the target.

Cheers, Wayne

 

[mtnelect]

Attached is my request to the CEC which is now requesting comments from stakeholders, for the new Title 24 code.

 

[petersonra]

Making an assertion without any supporting evidence doesn't seem to me to be a very good way to convince anybody of anything.

 

[Dennis Alwon]

Heat pumps are certainly efficient at temperature below 50 F. I believe they are now effective from 25-30 F. They have gotten very efficient since they were first brought out into the market

 

[electrofelon]

Attached is my request to the CEC which is now requesting comments from stakeholders, for the new Title 24 code.

I don't understand what you are proposing or requesting. What does "include ductless heat pumps in your HVAC reviews" mean?

 

[mtnelect]

Thank you for your question ... Ductless refers to "Mini Splits", which uses refrigerant lines to connect to air handlers which are mounted in each room. Conventional forced air systems us ducts to supply air to each room. This is where the most inefficiency accrues, running ducts in attics with outside air temperatures and duct connections that leak air.

The efficiency of mini splits is you only heat or cool each room, not the whole house. Manufacturers tried to compete with zoning systems which were outlawed by CEC because they didn't work, they had return air problems.

On a previous post I mention heat pumps struggle under 50 degrees, I was referring to forced air systems. Mini splits can work at a much lower outside temperature, without auxiliary heat. The only drawback is the condensing unit will go into "Defrost" mode when it senses ice buildup.

 

[electrofelon]

Thank you for your question ... Ductless refers to "Mini Splits", which uses refrigerant lines to connect to air handlers which are mounted in each room. Conventional forced air systems us ducts to supply air to each room. This is where the most inefficiency accrues, running ducts in attics with outside air temperatures and duct connections that leak air.

The efficiency of mini splits is you only heat or cool each room, not the whole house. Manufacturers tried to compete with zoning systems which were outlawed by CEC because they didn't work, they had return air problems.

On a previous post I mention heat pumps struggle under 50 degrees, I was referring to forced air systems. Mini splits can work at a much lower outside temperature, without auxiliary heat. The only drawback is the condensing unit will go into "Defrost" mode when it senses ice buildup.

I understand what a ductless system is, my question is about that proposal attachment in your previous post. I am not from California or familiar with title 24 so I just wasn't sure what you were requesting of them that ductless systems be included in their reviews. What does that mean?

 

[mtnelect]

The California Energy Commission is now working on the new Title 24 code and is requesting comments from stakeholders regarding HVAC heat pumps, ducted and non-ducted, so I am supporting min splits. The CEC has only done cursory testing on mini splits to date. I want them to include ductless as a way to reduce decarbonization.

The Building Energy Efficiency Standards were first adopted in 1976 and have been updated periodically since then as directed by statute. In 1975 the Department of Housing and Community Development adopted rudimentary energy conservation standards under their State Housing Law authority that were a precursor to the first generation of the Standards. However, the Warren- Alquist Act was passed one year earlier with explicit direction to the Energy Commission (formally titled the State Energy Resources Conservation and Development Commission) to adopt and implement the Standards. The Energy Commission’s statute created separate authority and specific direction regarding what the Standards are to address, what criteria are to be met in developing the Standards, and what implementation tools, aids, and technical assistance are to be provided.