Heat Pump Anarchy

[Rain Man]

Any heat pump gurus on here?  Here's the problem:  700 sq. ft. cabin, new build, 6' crawlspace underneath.  The layout is single story, two bedrooms, one bath, small laundry/utility room and a great room with vaulted ceiling.  

The cabin will have a woodstove for backup, due to frequent power failures.  

However, for good energy efficiency, we want to use a heat pump.   The current design concept is a tri-zone mini-split with one 9K unit in the great room and a 7K unit each in the bedrooms.  The bathroom will have radiant floor heat on a timer/thermostat, and the laundry/utility room a small baseboard heater for the rare times it will be used.

Outside air temperature averages out to no colder than -5 C at night in winter, but there might be a few nights per year of -20 C where a heat pump will struggle.  

The heat pump will be used for aircon in summer, but not often.  

The problem is what to do with those few -20 C nights.  Are we better to stay with the mini-split (cheap, easy to install) and use baseboards for backup, or go with a ducted system and electric furnace which uses the heat pump coil and resistive backup so we can have heat even when it is -20 C?  We will be there for some of those nights and could use the woodstove for backup, but the cabin will be empty on some very cold nights as well.  For those, we just don't want the pipes to freeze.

I have also read that there are now mini-splits available that will still work well in -20?  Anyone know if those systems really work?

TIA

 

[Foreverslow]

how much you want to spend and how often do you have such extreme weather.

The new modern units are getting really good, but -20c is getting damn cold.  Most quality units have an emergency backup heater (eg baseboard heat) built into them, but when that kicks on, your bill will go up by a factor of 4.

You can use water based heat exchangers that pull heat out of the ground and never have to worry about cold nights.  Much more expensive initially (neighbor needed 16 big deep holes dug), but they are for heating in cold climates and/or big homes.  Likely overkill for your needs.

Or you can put a small stove in the corner for when it gets real cold.  All of the  houses in my neighborhood use heat pumps, but are backed up with fireplaces or stoves "just in case".

 

[Rain Man]

how much you want to spend and how often do you have such extreme weather.

The new modern units are getting really good, but -20c is getting damn cold.  Most quality units have an emergency backup heater (eg baseboard heat) built into them, but when that kicks on, your bill will go up by a factor of 4.

You can use water based heat exchangers that pull heat out of the ground and never have to worry about cold nights.  Much more expensive initially (neighbor needed 16 big deep holes dug), but they are for heating in cold climates and/or big homes.  Likely overkill for your needs.

Or you can put a small stove in the corner for when it gets real cold.  All of the  houses in my neighborhood use heat pumps, but are backed up with fireplaces or stoves "just in case".

What actually happens to these systems when it gets that cold?  Do they simply not produce much heat, or none at all? As long as the heat pump can still keep the house above freezing, we are ok.  If we are there we will put the airtight stove on to help the heat pump keep things comfortable.  If we are not there we just don't want the pipes to freeze.

There really are very few nights this would happen - 3-4 in a year at most.  We aren't worried too much about the cost of the electricity on those nights.  The cabin we are tearing down and replacing had almost no insulation, and in a cold month the electricity was about $150 to keep the house just above freezing using baseboards.  This cabin will be much better insulated.

I don't think we can justify the cost of, or afford an in-ground system.  We could afford a ducted system.

 

[floating dutchman]

Heat pumps lose efficiency in colder temps.  I don't know about -20, you will have to look at the manual.  The book of words for mine says down to -15 but some are rated to work in lower temps.

Our night time temps are not as low as yours and I installed a (bit undersized) heat pump, more for convenience.  We use the fire place as main heating and have too when it gets really cold.

If I were you, I'd plan on doing the same.

The two great advantages we got with the heat pump was, when it's not very cold and the fire is just overkill.  And the timer in the morning, it's great waking up to a heat source already on.

 

[tommays]

I have all the heat sources over the years and the heat pump is definitely my favorite BUT in January when it only got down to Zero at night with moderate days the electric bill DOUBLED 

BUT with oil and gas so high now heat pump may still work out 

 

[cyclone]

Include a quick drain-down for the water pipes in the build design. That way you can drain your pipes, splash the sinks, toilet, shower, washer with RV antifreeze and leave the cabin  unoccupied without worry of power failure.

 

[Bump-n-Grind]

doesn't your mini-split system have a resistance emergency heat capability?

I'm with Cyclone on the drain the pipe theory 

also maybe add some heat tape to overexposed pipes 

is the power fairly reliable there? 

heat tape on lower runs of pipe can also avail itself of convection to circulate warmer water upward within the pipes. 

for a place that size, you will gobble up a lot of real estate inside with ducted system. 

 

[mikewof]

View attachment 500062 Any heat pump gurus on here?  Here's the problem:  700 sq. ft. cabin, new build, 6' crawlspace underneath.  The layout is single story, two bedrooms, one bath, small laundry/utility room and a great room with vaulted ceiling.  

The cabin will have a woodstove for backup, due to frequent power failures.  

However, for good energy efficiency, we want to use a heat pump.   The current design concept is a tri-zone mini-split with one 9K unit in the great room and a 7K unit each in the bedrooms.  The bathroom will have radiant floor heat on a timer/thermostat, and the laundry/utility room a small baseboard heater for the rare times it will be used.

Outside air temperature averages out to no colder than -5 C at night in winter, but there might be a few nights per year of -20 C where a heat pump will struggle.  

The heat pump will be used for aircon in summer, but not often.  

The problem is what to do with those few -20 C nights.  Are we better to stay with the mini-split (cheap, easy to install) and use baseboards for backup, or go with a ducted system and electric furnace which uses the heat pump coil and resistive backup so we can have heat even when it is -20 C?  We will be there for some of those nights and could use the woodstove for backup, but the cabin will be empty on some very cold nights as well.  For those, we just don't want the pipes to freeze.

I have also read that there are now mini-splits available that will still work well in -20?  Anyone know if those systems really work?

TIA

Three types of HVAC heat pumps; air-sourced, water-sourced, and ground-sourced. You're asking about air-sourced heat pumps, and their heating efficiency plummets when it gets cold. They tend to work around this by including a resistance element in the units. You will know when the resistance element kicks in, because your power bill will skyrocket. Yes, there have been improvements to cold-weather air-sourced heat pumps, the culmination of a ten year Department of Energy project, and they do work to a degree, but you'll have to balance the cost.

As for baseboard heat, all resistance heaters are exactly 100% efficiency. Cheap ones, expensive ones, they are all the same, and they are all the second most expensive way to heat a space after burning U.S. currency in the fireplace.

The cheapest way to do it is just use your woodstove to heat on the cold nights and install some electric pipe heaters or a fast drain system from the feed so your pipes don't freeze.

But before you spend any money, maybe get an estimate for a direct-refrigerant ground source heat pump. Total Green makes those, they took over from Earthlinked. https://www.totalgreenmfg.com/. When you pull and dump your heat from the ground rather than the air, it will work year round. But this might be better for a regular house, for a little cabin, maybe just a wood stove, a few fans, some electric tape for the pipes and leave it at that?

 

[Zonker]

Mini split with extra heating coils or baseboards. Baseboards are dirt cheap to buy/install. Have them on a separate thermostat to prevent freezing.

The Canadian Coast guard is refitting a lot of their icebreakers with mini splits because it is very easy to reconfigure spaces without running new ducts etc. And they work in low temperatures. 

 

[Grande Mastere Dreade]

View attachment 500062 Any heat pump gurus on here?  Here's the problem:  700 sq. ft. cabin, new build, 6' crawlspace underneath. 

6 foot crawlspace?     we call those basements around here..

 

[Rain Man]

Thanks for the feedback.  At this point I think the right way to go is heat pump for the primary source, and a few baseboards as backup for times when the heat pump isn't cutting it.  We definitely need the drain for emptying the water pipes in case the power fails.  It does fail in this location during the winter sometimes, hence the woodstove.  

We plan to use on-demand hot water rather than a hot water heater because it is more efficient and so we don't have to drain the tank every time we leave the cabin empty for a while.  

It might be worth getting a quote for a small in-ground system just to see the difference.   We are looking at about $4K for the mini-split plus installation.  

 

[Recidivist]

Include a quick drain-down for the water pipes in the build design. That way you can drain your pipes, splash the sinks, toilet, shower, washer with RV antifreeze and leave the cabin  unoccupied without worry of power failure.

No idea about your situation, but we are a bit remote and don't have town water or sewerage.  Our grey water is treated by a biocide system which I'm fairly sure would not appreciate a dose of antifreeze every now and then. We have to be quite vigilant as to what gets flushed down the pipes.   I'd suggest ensuring compatibility of whatever treatment system you install. 

 

[cyclone]

Fortunately most RV antifreeze is ethanol based and considered safe for septic systems in limited amounts. It takes less than two gallons to winterize our garage/shop/apartment and it all gets diluted when the water is turned back on and the traps get flushed.

 

[tommays]

I just fired up mini split heat pump #2 for a 14 x 20 room addition as I wanted it to be completely independent 

Its and R410 unit with 24.4 SEER so we will see how much it cost to ran as I have 3 years of bills on the house cost 

 

[billy backstay]

Our 1,400 SF Ranch house has a 3 ton Mitsibushi split system for AC, but for a few hundred dollars extra it has a rarely used heating option.  We keep the thermostat and bedrooms cooler in winter and warmer in summer, and use the gas log fireplace, or nearby split system heat to take the chill off main living areas on cold winter mornings, or cool the main living area in summer heat.  If it's really hot and humid, I place a fan in the hallway to send cool air down the hall to the bedrooms.  Main living area is an open L-shaped living - dining room just off the kitchen. 

 

[Foreverslow]

Youtube's ubbr geek (Eddie Muster love child look with a tweed jacket and T-shirt)  just released an episode on heat pumps.

He has some interesting ways to look at their use.

 

[mikewof]

12 minutes ago, Foreverslow said:

Youtube's ubbr geek (Eddie Muster love child look with a tweed jacket and T-shirt)  just released an episode on heat pumps.

He has some interesting ways to look at their use.

But when the rubber meets the road, in those handful of cold days where air-sourced heat pumps don't work, the users tend to get incredibly upset with the technology when their power bill quadruples (or more) when the resistance elements kicks in. Without cold-weather performance, air-sourced heat pumps can't see wide adoption.

A better approach is to find cheaper ways to install ground-sourced heat pumps rather than shove air-sourced heat pumps into people's homes when they can't trust the technology. The payback with waterless ground sourced heat pumps is so short now, it's silly to exclude them from the conversation.

This guy can't promote residential air-sourced heat pumps and ignore the reality that they have to use resistance heat. He then calls the realists like us "curmudgeons." Cold-climate heat pumps could be great, but they don't seem to be there yet. Given that, this video is good.