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How a heat pump works

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Re: How a heat pump works

#131

Post by elil17 »

asow92 wrote: post #49

As someone who lives in the northeast and has a 3600sqft home, I'm wondering how many heat pumps it would take to replace my gas steam boiler system. I'm not sure if heat pumps would be cost efficient given those parameters.

One* heat pump will do fine, but you might need a low temperature heat pump depending on where you are. After you factor in that gas is cheaper than electricity per kWh, the system should save you money as long as it is running at an efficiency higher than ~130%.

A contractor should be able to select an appropriate system for you, but if you want to do it yourself, look for yourself, get the 2021 ASHRAE climatic design conditions for your area and choose a heat pump with a minimum operating temperature which is lower than the coldest month 99.6% heating drybulb temperature (DB) (should be the top row, left most temperature).

*You would also need a fairly large heat pump, perhaps 7 tons (tons here means tons of refrigeration, or how many tons of ice the system can melt per day) depending on how well insulated you are.

Re: How a heat pump works

#132

Post by Nihilartikel »

nashashmi wrote: post #111
gabrielhidasy wrote: post #65

Earlier quoted context omitted.

They literally are more efficient than burning gas, a good heat pump can do over 300% efficiency (KWs of heat in your house vs KWs of electricity used) while a furnace can only hope to get 100%. Even if the heat pump is fed from gas power plants (which burn gas with about 50-60% efficiency, more if the waste heat is reused), they are still using less gas for the same amount of heat than directly burning it.

400% efficiency? Is that even possible? (Did AI write this?)

If I can spend 1 joule to transfer two joules of enthalpy from outside to inside, then that is 200% efficiency.

Burning 1 joule of gas would give you 1 joule of heat in a perfect world. That is what a non pump does.

Re: How a heat pump works

#133

Post by aaronax »

nashashmi wrote: post #111
gabrielhidasy wrote: post #65

Earlier quoted context omitted.

They literally are more efficient than burning gas, a good heat pump can do over 300% efficiency (KWs of heat in your house vs KWs of electricity used) while a furnace can only hope to get 100%. Even if the heat pump is fed from gas power plants (which burn gas with about 50-60% efficiency, more if the waste heat is reused), they are still using less gas for the same amount of heat than directly burning it.

400% efficiency? Is that even possible? (Did AI write this?)

More technically it would be a coefficient of performance of 4, which in some frames of reference is like 400% efficiency.

Re: How a heat pump works

#134

Post by boringg »

elil17 wrote: post #117
boringg wrote: post #7

What is going on? Is there a global push to get everyone to switch to heat pumps. It seems that there is heat pump content everywhere. Clearly there is a push. I understand that its for climate benefits but it feels very coordinated. edit: didn't realize the attention I would get. Let me add someone answered my question -- a good product market fit and people talking is probably where the push came from. That said HV…

Well of course it is being pushed, though I would not necessarily call it coordinated. Companies that sell heat pumps are advertising hard. It's an easy sell for them with rising gas prices. The US government is "pushing them" by offering tax credits for heat pump systems. This is part of a plan to reduce pollution, improve energy independence/energy security, and save citizens money on their heating bills. No secret…

I think this is probably the right answer. It keeps popping up everywhere.

I'm always a skeptic on when something gets super hyped. In this case it's more use-case specific that makes me skeptical - majority of people its 100% fit.

Re: How a heat pump works

#135

Post by sbradford26 »

snuxoll wrote: post #104
SigmundA wrote: post #68

Earlier quoted context omitted.

They can be, natural gas is about 1000 BTU per cubic foot. An expensive high efficiency gas furnace might be 95% efficient (most normal furnaces are in the 80's) 1000 x 0.95 = 950 BTU/cuft A high efficiency natural gas power plant is 60% efficient and a modern high efficiency low temp heat pump can have a COP up to around 2.7 at 17F 1000 x 0.6 x 2.7 = 1620 BTU/cuft A 40% power pant is still more efficient than a 95%…

> this is a no brainer in milder climates which is why I have a heat pump and no gas furnace here in FL even though I have gas to the house for other uses. They're a no-brainer in every climate. Newer heat pumps can handle fairly cold temperatures (-13°F) pretty easily, there may be a loss in efficiency but in the majority of the world it doesn't stay that cold. A few days of lower than normal efficiency, and at wors…

So heat pumps do lose efficiency at lower temperature such as -13F but the much bigger issues is loss of capacity. If you look at the curve of available heating capacity of a heat pump versus the heating needs for your house that tend to be a bit of a mirror image. When it is warmer out your house needs less heat but heat pumps have the highest capacity they will have. As temps drop the house will require more heat while the capacity of the heat pump is dropping.

That isn't to say they cannot work I personally have a heat pump in my house and it worked fine through the -15F weather we got. But it requires careful design work initially. Also you run into simply size constraints where residential heat pumps tend to max out at 5 tons (60k BTUs) typically when furnaces can easily get up into the 100k+ BTUs. Which might require multiple heat pumps and separating existing duct work. But definitely new construction should start designing around these constraints now.

Re: How a heat pump works

#136

Post by pkulak »

yreg wrote: post #6

Technology Connections has a good video on heat pumps. (Actually several of them, I link the newest one.) https://www.youtube.com/watch?v=MFEHFsO-XSI

I just found this guy too. He answered a lot of questions I had about efficiency and use below freezing.

https://youtu.be/fxEqVuiHhM0

Re: How a heat pump works

#137

Post by timerol »

slfnflctd wrote: post #79
fullstop wrote: post #45

Earlier quoted context omitted.

Your high end apartment must have had a low end heat pump installed. In the midwest a geothermal heat pump would work wonderfully. Also, they are designed to run constantly in order to maintain the set temperature -- it's not like a natural gas fired furnace where it quickly reaches the set point.

What's the payback time on something like that for a modest home in a place which can get significantly under 20 degrees (F) or over 95 degrees for weeks or longer? How effective is it at those extreme temperatures when it matters the most, and how does this affect maintenance costs? Additionally, there are multiple types of restrictions on where geothermal pumps can be installed, and navigating this can add to costs…

> over 95 degrees

Note that a heat pump cooling a building is essentially the same as an air conditioner. So it's more a matter of how efficient the air conditioner you're replacing is. Cooling a house is much easier than heating, because the temperature differences are lower. In your example, cooling 95 F to 70 F is a 25 F difference, while heating 20 F to 70 F is a 50 F difference. The world record for heat is about 135 F, which is the same difference from 70 F as 5 F, a completely normal winter temperature in much of the world.

Re: How a heat pump works

#138

Post by aaronax »

bilsbie wrote: post #110
endisneigh wrote: post #105

Earlier quoted context omitted.

A portable machine to provide heat and cooling via heat pump technology. For cooling this exists in many forms: there are the window air conditioners, and "portable" window air conditioners which have a duct that connects to the window. The problem with the latter is that they're loud and the ducts result in a lot of heat loss. For heating there are some portable air conditioners that use a heat pump but they're subj…

Cool idea. So a two piece portable with a just coolant line going through the window to the outdoor part?

Coolant supply and return, both fully insulated. And condensate drain line, and power wires, and control wires. Do-able, but more complex than one might initially think.

Re: How a heat pump works

#139

Post by sbradford26 »

snuxoll wrote: post #120
sbradford26 wrote: post #93

Earlier quoted context omitted.

Ground source/geothermal would significantly reduce your heating costs but it would be significantly more than 10k. For that large of house something like 40k+ wouldn't be out of the question, with the largest expense being drilling. I personally think geothermal is great, sadly New England's geography makes getting a ground loop installed more difficult. In the midwest lots of people would do a horizontal loop by tr…

My brother-in-law lives in NE and has installed mini-splits and has overall enjoyed them. A ground source system would be "the best" for that region, but if you're not doing a new build then air source works just fine there with the correct equipment.

I also live in NE and we recently installed a mixed system. We have mini split units in the upstairs and a ducted air handler in the basement all attached to a cold weather heat pump outside. It has be working great for us so far, but we have the advantage of having a smaller house so we only needed one 4 ton unit outside.

Our house was built in 1973 and originally had electric baseboards and then was converted to an oil boiler with hot water baseboard. So retrofitting a heat pump was an interesting task.

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