Using mini-split to complement in-floor heat

[75gmck25]

My brother-in-law has a 30'x54' building going up in North Dakota and plans to split it into a 30'x30' area for living/office area, and a 30'x24' area for shop/garage. He will be installing in-floor radiant heat with separate zones for the two areas, and the living area will have much higher R value for its insulation and windows.

Since he will only use the building on weekends, it may take a while to warm it up the living area with radiant heat in the winter, and he may also want A/C in there during the summer. I suggested to him that it might be good to install a mini-split heat pump in the living area, so he could use it for A/C in the summer, and have the option of using it to warm up the apartment area if the radiant heat takes too long.

Does anyone else think the mini-split idea makes sense (to supplement radiant), or am I just dreaming up a solution?

Thanks,

Bruce

 

[Fueler]

I do. My radiant is overhead. By fiddling with both thermostats I have managed to cut my winter bill down a good amount.

I may be wrong but turning the floor off may be false savings.

Once he sees how a mini split air works he will want it in both sections.

 

[ForceFed70]

Sounds like a good idea to me.

 

[Radix2]

If you want the A/C, the heat part on a split is not much of a cost add...so yes.

 

[theoldwizard1]

The only issue I see is how cold does it get in ND ? Even the best mini-split heat pumps start loosing efficiency below 0F. By -15F they are only 75% efficient, so on the coldest days, they are not going to help a boiler/radiant heat setup.

 

[Coach James]

My gym is ~13,000 sq ft and we heat/cool it with ten 2 ton mini split units. It doesn't get super cold here, but the summers are very hot and humidity is terrible. Those units do a good keeping it comfortable.

Coach

 

[walrus]

You can't turn radiant up and down and expect it to recover, the mass of the floor keeps recovery times long and slow. I suppose you could keep it at a low value and use the mini split to get the temps up in winter. It would depend on floor plan, open and it might work, all split up into rooms, not so much without multiple indoor units

 

[dsimatt]

My house has hot water and the main reason I got the hyper heat mini split was to keep the whole house cooler and run it in the living area where we are most of the time.

I think it will work out well for what you want and be able to quickly hand different temp needs and be a good compliment the the floor heat.

 

[jacks2000]

I would suggest using the in floor radiant to keep the basic winter temperature in the living area and shop around 55-60 deg. Then use the mini-split to bump the temp up to their comfort level on the weekends. Install the outdoor unit in the shop and you will avoid the efficiency issues of the sub zero outdoor temps. The air blowing out of the outdoor unit will have little affect on the temperature in the shop.

 

[yeldogt]

I use the radiant as a floor warmer and use the heat pumps to adjust the temp at my winter place that we don't use all the time.


I would recommend that slab insulation be equal in both spaces -- it will make a difference. pay for it once.

 

[Kaizen]

If he turns the slab off it will maintain that heat for days. Better to not have it up and down. I used a minisplit last year for primary heat in a 600 sq ft apartment. Below 40 it really had to work hard to keep up so I would recommend it. Above that temp it works well for heat and awesome for ac.


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[Dick in Wisconsin]

Since he'll have separate zones for the shop and for the living area, why doesn't he set the shop at 55 and living area at 72? I leave me shop (in floor radiant heat) at 55 and it's very comfortable.

Even though the temp will be lower in the shop, don't skimp on insulation. I have lots of insulation in the shop and it is noticed its benefits both in the winter and the summer.

Air to air heat pumps ( which I think is what you're talking about) become much less efficient at low temps. If electricity rates are low enough, the HP might have an advantage. You just have to do the math.

A2A Heat pumps work well south of say Chicago or Indianapolis. I just can't see them being cost effective in North Dakota. Too cold too long.

What kind of fuel will he use for the boiler.

 

[bruincounselor]

A2A Heat pumps work well south of say Chicago or Indianapolis. I just can't see them being cost effective in North Dakota. Too cold too long.

What kind of fuel will he use for the boiler.

Many folks in Minneapolis use them. ND is a bit cooler, but this is supplemental so good for a large chunk of the seasons. Contemplating doing the same for my shop in Fargo.

 

[Dick in Wisconsin]

Many folks in Minneapolis use them. ND is a bit cooler, but this is supplemental so good for a large chunk of the seasons. Contemplating doing the same for my shop in Fargo.

I'm surprised they are used that far north. Learn something every day.

But won't the user want the supplemental heat when its really cold out? When the efficiency of the A2A heat pump is at its lowest? If the cost of electricity is high, that makes the equation worse.

Did the OP poster tell us what kind of fuel source is available to this build?

 

[ForceFed70]

I'm surprised they are used that far north. Learn something every day.

They are common in Canada.

Not all mini splits are simple heat pump design. They come in 3 types:
1) Traditional heat pump design that you are thinking of.
2) Traditional heat pump design with electric resistance heat backup for when it get's too cold.
3) AC only design, with electric resistance as only heat source.

In Canada, we usually only use type 2 and 3. #2 is more efficient, #3 has a lower purchase cost.

 

[75gmck25]

Thanks for all the good information. I think he would leave the radiant heat set for a 50-55 degree minimum in the winter, and then bring the temp up higher in the living area when he goes there on weekends. He has a choice of propane or electric heat for the radiant, but hasn't made a choice yet.

It sounds like the mini-split could work fine to bring the air temp up in the living area in months where it only gets down to about 15 degrees. During really cold weekends he would be running on electric resistance if he used the mini-split, but could then transition to radiant heat as the slab heats up.

Any recommendations for good mini-split brands for this application (probably type 2 from the post above)? Even a brand that is more common in Canada may work for him, since he's close to the Canadian border.

Bruce

 

[justinjoyal]

They are common in Canada.

Not all mini splits are simple heat pump design. They come in 3 types:
1) Traditional heat pump design that you are thinking of.
2) Traditional heat pump design with electric resistance heat backup for when it get's too cold.
3) AC only design, with electric resistance as only heat source.

In Canada, we usually only use type 2 and 3. #2 is more efficient, #3 has a lower purchase cost.

We dont install HPs with resistance heat, unless what you're referring to is the heating element used to melt the ice away from the condenser during winter...

Mini-splits dont have backup heat, unlike, say, PTACs and such..

 

[finn]

Use a relay to tie the thermostats together so that the supplemental (mini split) thermostat controls both heat sources.

The radiant thermostat can then be left at the lower "maintenance" or base temp, but when the supplemental, stat is bumped, it will signal both heat sources to turn on.

 

[Kaizen]

I'd be interested in the time to increase a slab that size by ten degrees. Thinking even a top of the line Mitsubishi will be blowing just warm air at thirty and below. Not enough to bring a space up to temp. I would suggest a loop off his radiant to wall radiators with a blower. Put it on a seperate loop with thermostat. They have in wall ones. Couple of those would bring the place up to temp could even have a programmable thermostat


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[bzinsky]

Was stationed in ND for a few years

We had a big digital temperature sign on base that I saw everyday, would usually hover around 0-10F. Certainly a capable range of modern heat pumps.

I'd be interested in the time to increase a slab that size by ten degrees. Thinking even a top of the line Mitsubishi will be blowing just warm air at thirty and below. Not enough to bring a space up to temp. I would suggest a loop off his radiant to wall radiators with a blower. Put it on a seperate loop with thermostat. They have in wall ones. Couple of those would bring the place up to temp could even have a programmable thermostat


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below 30 degrees?

It's 2017, even the cheaper inverter heat pumps will perform beautifully at 20 degrees.

Top of the line mitsu is just starting to flex its muscles at 0

 

[Kaizen]

Was stationed in ND for a few years

We had a big digital temperature sign on base that I saw everyday, would usually hover around 0-10F. Certainly a capable range of modern heat pumps.



below 30 degrees?

It's 2017, even the cheaper inverter heat pumps will perform beautifully at 20 degrees.

Top of the line mitsu is just starting to flex its muscles at 0

Not what I experienced last winter. Mine worked awesome down to freezing but then was running at max speed to try and keep up. Supplemental heat was needed. As air temps drop the btu's per hour drop. No matter which one it is. In my region its normal to have supplemental heat if the mini is the primary. Just saying they are not going to be getting 18k btu's of heat out of it at 10 degrees. I haven't seen a range for Mitsubishi. I know they work i'm just saying since he's got the main heat source running the slab i'd put up a couple more loops into the walls and run some radiator type heaters with blowers. Worst case run them in case needed for future.

 

[walrus]

I'd be interested in the time to increase a slab that size by ten degrees. Thinking even a top of the line Mitsubishi will be blowing just warm air at thirty and below. Not enough to bring a space up to temp. I would suggest a loop off his radiant to wall radiators with a blower. Put it on a seperate loop with thermostat. They have in wall ones. Couple of those would bring the place up to temp could even have a programmable thermostat


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My Fujitsu will blow hot air at 10 degrees and will heat my upstairs with no issues . The new ones will put out rated btus at 20 degrees.

 

[brewchief]

Some of the minis are still putting out 100% of capacity at 0 degrees and others will be far less, some more like 50-60%, you have to look at the specs for the exact unit you are looking at to see what it will put out.

Even if the unit is putting out 100% it may not be enough, needed heating capacity can be much higher then cooling capacity in some areas.

If the area is well insulated I can see it working pretty good, we have a 25x35 shop area in my friends shop that is kept at 65 degrees with the infloor, just having all the lights on for several hours the temp will climb. A mini split heat pump in that space could easily bring the temp up but it's a very well insulated space within a larger building.

If you do this use a stat with a slab sensor for the infloor, if you don't the aux heat can bring the temp up and the infloor will shut off and the slab will start cooling off even though you still need the heat.

 

[yeldogt]

Not what I experienced last winter. Mine worked awesome down to freezing but then was running at max speed to try and keep up. Supplemental heat was needed. As air temps drop the btu's per hour drop. No matter which one it is. In my region its normal to have supplemental heat if the mini is the primary. Just saying they are not going to be getting 18k btu's of heat out of it at 10 degrees. I haven't seen a range for Mitsubishi. I know they work i'm just saying since he's got the main heat source running the slab i'd put up a couple more loops into the walls and run some radiator type heaters with blowers. Worst case run them in case needed for future.

It depends on the type and the size. If you need 20k of heat and it's a 12k unit -- it's going to fall behind and feel cold. Also -- if it's not designed to operate at full output at 0 degrees --- and you need full output .. it's going to fall behind and feel cold.

Mine are blowing hot air and having no problems at 0 degrees.

 

[Kaizen]

It depends on the type and the size. If you need 20k of heat and it's a 12k unit -- it's going to fall behind and feel cold. Also -- of it's not designed to operate at full output at 0 degrees --- and you need full output .. it's going to fall behind and feel cold.



Mine are blowing hot air and having no problems at 0 degrees.

Makes sense. But isn't that oversizing going to cause the ac to cycle more and shorten the life of the unit? Always have heard oversizing causes a lot of issues with central hvac. Not with minisplits?


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[yeldogt]

Makes sense. But isn't that oversizing going to cause the ac to cycle more and shorten the life of the unit? Always have heard oversizing causes a lot of issues with central hvac. Not with minisplits?


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HVAC is sized for both heating and cooling -- obvious if you live in a very hot or cold part of the country the loads will be somewhat off. It's not unusual for a house to need 3T of cooling and 4T of heat in a colder climate .. and yes this can be a issue with a conventional system. Years ago HP's could not work in cold weather so they sized for AC and added a furnace ...or if going cheap upfront electric strips for really cold weather. If you size for heat -- it will be oversized and can cause humidity comfort issues.

Oversized AC units will short cycle and not run long enough to remove humidity if operating in a humid area. Actually, the same is true for heat -- oversized creates an uncomfortable hot cold cycle. Ideally, you want output to match load on the building ........so the unit runs over a longer period.

In your case the unit may not be under or over sized -- just not a model able to run in colder temps. It's all in the specifications.

Models are available today with variable speed compressors and fans -- they are able to ramp up and down to fit the need of the space at a given time. My split system can run down to 25% -- so a three tone AC is now running at under one ton. Mini splits do the same thing. That's why the new split systems can be zoned so effectively -- they send what's needed to each part of the house. The compressor speeds up and down to match this need -- along with the fan.

My three head Mitsubishi hyper heat compressor has three 12k heads attached to a 30k unit. While the unit can actually run at 110% .. the reality ... all three heads are never calling for full output. SO I can have the output needed in each area when needed w/o going to a larger unit.