Woodstove backups, Heat Pumps?

[McKeznak]

K so I've been watching the forums and everyone is always talking about their heat pumps. To be honest until joining these forums I've never even heard the term used. There's a few Geo-thermal systems around but they are pretty costly and really only found in new builds with a lot of space. I can't imagine you're all talking about those.

So I did what any one would and Google'd them. So if I understand this right is it basically just a reverse A/C unit, sometimes just an A/C in reverse, that instead pushes the warm by-product of the compression into the house instead of expelling it outside?

So whats the deal with them, seems like it's just another way to use electricity to produce heat? Can you get more heat per kWh out of a heat pump than a traditional resister based electric heater?

Anyone know why nobody around here would have one?

Sorry if it's in the wrong subsection, just always mentioned as a backup to stoves.

 

[lopiliberty]

All I know about them is if I relied on mine for 24/7 heat all winter long I would freeze my a** off. Theres also a heating element in them that kicks on if its too cold outside and it can't keep up and boy does that make the ol electric meter fly

 

[begreen]

This is more green room topic where there are several threads on heat pumps. There is a huge difference in performance with these units ranging from old inefficient models to modern dual speed compressors or the very popular inverter compressors showing up in mini-split heat pumps.

 

[McKeznak]

Thanks for moving it begreen. Strange yup never really come across these before.

 

[begreen]

You are correct. The best heat pumps are much more efficient than resistance heating.

 

[DougA]

Since you are in NE ON, you may find a heat pump to be a totally worthless proposition. They are great in moderately cold climates but at a certain temperature, the amount of heat they produce is equal to the cost of power they consume and even in S. ON, all the ones I am familiar with have an electric furnace as a backup. That means $$$$$$. Might as well use baseboards as your backup and save the cost of the furnace and duct work.

Ground source is much more efficient because the ground water will be in the mid 50's (here at least) all year long and you can pull a lot of heat out at that temp. The problem is the HUGE cost of installation unless you are on a lake.

When I built my house 30 yrs. ago, I looked into all the options and at that time, heat pumps netted out at zero. By the time you had paid the high up front cost through electricity savings, the unit would be about ready to be replaced from mechanical failure. They have come a long way since then but I still much prefer passive solar heat on sunny days through my large windows and wood heat. Both of these are very comforting ways to heat.

 

[jdp1152]

Are we talking about a heat pump or a ground source heat pump here (geothermal)??? Two might different beasts with regard to efficiency.

 

[Circus]

I understand it basically just a reverse A/C unit that pushes the warm by-product into the house instead of expelling it outside?

Yes

Can you get more heat per kWh out of a heat pump than a traditional resister based electric heater?

If it's not to cold.

Anyone know why nobody around here would have one?

It's to cold for air source. Will work in temperate months. I never understood why those throat choking boilers around here were fired up when it's 45 or 50 degrees out side.

 

[woodgeek]

The ratio of how much heat a Heat Pump delivers compared to a resistance heater is called the coefficient of performance, or COP. the best ductless air source heat pumps, called minisplits get a COP of 3.5-4 during mild weather (>40°F outdoors) and more like 2.5 when it really cold (say 20-25°F). Below 0°F, COP might be more like 1.5, barely better than a resistance heater. For a recently made system that looks more like a central AC system, with ducts, called a 'conventional split' (because there is an outdoor and indoor unit connected by refrigerant lines) the COP might be 0.5 less than the numbers above.

Bottom line, in much of the lower 48 and coastal/maritime provinces of Canada, minisplits deliver BTUs on an average seasonal basis that cost about half that of heating oil or propane. When average winter temps are well below about 20°F, the economic case for them gets a lot worse, and you don't see them as much. Even there a mini would work great during the shoulder seasons, but the economic case for buying one might still be weaker.

 

[peakbagger]

I have been using a mini split as supplemental winter heating. It seem to be worth running above 10 degrees F. It still puts out some heat above 0 and is rated to down to -15 deg F but the heat output is limited and the efficiency is approaching an electric heater. The minisplit I use is optimized for winter heating. Most standard mini splits will not run much below 50 degrees F. Many large whole house heat pumps also turn on auxiliary electric heaters below 40 degrees so they also become electric heaters when cold. I installed additional solar panels on my home to offset the usage of my mini split on a year round basis but I have access to net metering which allows me to build up a summer surplus of power and use it in the winter. Thus my power costs to run the unit are just the capital cost of the additional panels.

Even a winter optimized mini split heat pump is not going to be backup to a woodstove and if you factor in a loss of power event they are useless. They can be a good supplemental unit for most of the winter thus reducing your wood usage and if your home needs AC during the summer they tend to be far more efficient than window mounted AC units or central heat pump systems that have ducts that run outside of conditioned spaces.

 

[McKeznak]

Brilliant! All of you, thanks!

Ya I was wondering if there were efficiency issues below a certain temperature. Looks like it'll still be Oil backup for me.

 

[sloeffle]

woodgeek nailed the explanation.

I live in Central Ohio and a lot of folks will use their air source heat pump down to 32F and then burn propane or NG below that. Our electric company actually gives you money back if you install a ground source heat pump or an air-air heat pump. I installed a GSHP a few years ago and the cost was actually about the same as an air-air heat pump after the government and utility rebates. The GSHP also makes free hot water in the summer time.

 

[Highbeam]

In my climate we use heat pumps almost entirely for heat. It's just a side benefit that you also get AC. The energy delivered to the space from a heat pump is much more than just the heat of compression. The systems steals heat from the outside air and brings it inside for the home. When it is really really cold outside there is just not much heat to be stolen but the new minisplits are much much better at this than the conventional heat pumps.

If all you know about heat pumps is based on what you see in current houses and on your research from more than 10 years ago then you likely have no idea how good heat pumps have become.

 

[McKeznak]

highbeam is yours one of the more efficient ones? What's the lowest useful outdoor temperature on it?

 

[Highbeam]

highbeam is yours one of the more efficient ones? What's the lowest useful outdoor temperature on it?

I grew up with heat pumps and my current house doesn't have one yet. I have chosen to install one of the low temp minisplits from Mitsubishi or FUjitsu that make their full rated output down to 5 degrees F. Below that temp they keep making heat, but at a lower and lower output and COP until they shutoff in the negative teens F. I don't believe that there is ever a point where the heat pumps actually make less heat per watt than regular resistance heat.

In my climate, single digit temperatures happen but the more typical low low is in the teens where the COP of the heat pumps are quite good.

I would not want to buy a traditional and relatively inefficient heat pump and waste all that energy plus have to use electric resistance heat at temps below freezing. That's silly.

The modern minisplits are perfectly capable of heating a house all winter in most inhabited places. Heck, anywhere where the temps are above zero F.

 

[velvetfoot]

But even if working, the mini split would be putting out less and less heat as the temp. drops, meanwhile the heat loss through the bldg envelope gets larger.

 

[Highbeam]

But even if working, the mini split would be putting out less and less heat as the temp. drops, meanwhile the heat loss through the bldg envelope gets larger.

Not until you get below 5 degrees above zero F. the good ones put out full rated output until 5 and then starts tapering gradually. If you get below 5 and must depend on heat pumps then you need to oversize the system to make sure have enough horsepower to keep up with the heating demand.

Old fashioned heat pumps, conventional units, were less able to extract heat from the air at such low temperatures. Those guys needed auxilliary heat sources (electric strip or gas) to maintain output.

 

[McKeznak]

Awesome points thanks guys.

0 F or below is quite the norm here (late dec- mid feb) at least through the nights, so I'm guessing that explains the lack of them. The tech of how they work fascinates me.

 

[Highbeam]

Awesome points thanks guys.

0 F or below is quite the norm here (late dec- mid feb) at least through the nights, so I'm guessing that explains the lack of them. The tech of how they work fascinates me.

It's also a regional thing. These things are the only way to provide central cooling so you'll see them more in cooling climates where it is so hot and humid that people think that they NEED cooling.

The minisplits are new to the US compared to conventional split systems. The rest of the world is ahead of us on them.

 

[jdp1152]

Awesome points thanks guys.

0 F or below is quite the norm here (late dec- mid feb) at least through the nights, so I'm guessing that explains the lack of them. The tech of how they work fascinates me.

Ground source heat pumps will tackle that without issue. I barely used any wood last year to test out the system and my overall energy bill for the year was 4k less than the prior year. I can't attribute all of that to the conversion since I've replaced insulation in wall bays where it was damaged, air sealed the heck out of my house, and had my attics spray foamed.

 

[sesmith]

There's a few Geo-thermal systems around but they are pretty costly and really only found in new builds with a lot of space. I can't imagine you're all talking about those.
.

Ground source heat pumps (geothermal) make a lot of sense where you live. They also make sense as a replacement system in an older house, not just new construction. You have to look at the big picture, not just the installation cost. If you're going to be in your house a while, and your alternative is oil heat, it's almost sure to be cost effective to make the switch.

This is our third heating season with our geo system. No regrets. It will pay back the installation costs in less than 7 years total. It keeps the house within a degree of the thermostat set point,works well down below zero, does about 60% of our water heating, serves as a whole house hepa air filter, and gives us central air when we need it in the summer. It's a very efficient way to heat. I couldn't buy firewood for what the geo system costs us to run. Our system replaced a forced air oil furnace in an 1840's vintage house. I also still have a wood stove but don't use it any more.

 

[velvetfoot]

You'd also have to have ductwork rather than radiators/convectors, another cost if you don't have it.

 

[jdp1152]

Ground source heat pumps (geothermal) make a lot of sense where you live. They also make sense as a replacement system in an older house, not just new construction. You have to look at the big picture, not just the installation cost. If you're going to be in your house a while, and your alternative is oil heat, it's almost sure to be cost effective to make the switch.

This is our third heating season with our geo system. No regrets. It will pay back the installation costs in less than 7 years total. It keeps the house within a degree of the thermostat set point,works well down below zero, does about 60% of our water heating, serves as a whole house hepa air filter, and gives us central air when we need it in the summer. It's a very efficient way to heat. I couldn't buy firewood for what the geo system costs us to run for heat.

Like wise. I still like to cook myself out of a room with wood I cut mysel though

 

[jdp1152]

You'd also have to have ductowrk rather than radiators/convectors, another cost if you don't have it.

Ductwork is surprisingly cheap. For my install it accounted for 2k in materials. No clue on labor as it was only one line item on the invoice.

 

[velvetfoot]

Alas, I have a two story house.