using mini-split or heat pump for radiant slab

[bikesandcars]

using mini-split or heat pump for radiant slab?

I have a situation that I don't think is very uncommon, but a solution seems hard to find. I'd like to heat my radiant slab with a heat pump.

60x40 insulated garage (post frame) with radiant slab. Insulation is very good (2" spray foam walls, R38 ceiling, insulated doors).

Right now I'm running an electric on demand water heater to heat my slab. It works nice and is quiet but it does chew up the electric. It was cheap and a way to keep the slab from freezing.... turns out we use the garage so much I want to keep it warmer than planned.

next year it's in the budget to replace my window unit with a mini-split or other A/C - heat pump system, I'm thinking 36kbtu

Has anyone heard of or used a mini-split or traditional heat pump system that has a heat exchanger to heat water directly so I could use it for my radiant slab? My total calculated worst heat demand is 44Kbtu, I get by on much less than that based on how my current system cycles.

I don't want to cool the slab, that would lead to issues with condensing water on the floor which I don't want.

 

[Lonnies Performance]

They make air to water heat pumps, but they are not common in the US.

I looked at the same thing a while ago, but I need a huge one that is not easy to come by.

http://www.spacepak.com/hydronic-heating-and-cooling

These work well in moderate temperatures for about 1/3 cost of electric.

 

[walrus]

This company is local to me. They made an add on unit for residential water heater. You hooked it up to existing water tank so you didn't have to buy complete water heater if you didn't need a new tank. They couldn't get energy star approved because of that. I have one that goes with my solar system perfectly. Have it set run at night depending on tank temps. No energy star means no state rebates which meant no sales, killed the unit. Too bad quality piece of equipment.

They make bigger air to water units, not sure on cost
https://www.nyle.com/wp-content/uploads/2019/05/C-Series-Water-Source-Sales-Sheet-2019-1.pdf

They have been in business a long time. Their main business was lumber drying using heat pumps. They guy who founded the business was a refrigeration guru.

 

[Jackfre]

Daikin pulled their air to water from the US market a couple years ago. I spoke with the folks from Gree a couple years ago and they said theirs was entering the market. Perhaps JustinRoyal here can give you more insight. He installs Grees and likes them a great deal. Doing straight resistance heat I can understand your desire to find an electrical alternative, but I would not want to be the crash test dummy for a small manuf entree until they prove the product out.

 

[86turbodsl]

Radiant and heat pumps are a good match. Radiant usually wants low emitter temps, and that suits heat pumps. Your COP on low temp quality heat pump can approach 5. I have GSHP in my home and heat predominantly radiant. It works pretty good. I would recommend a pro to size and install it though... these things get complicated fast.

 

[theoldwizard1]

Mitsubishi sells then in AU and NZ. Same compressor unit.

 

[yeldogt]

For whatever reason none of the big players have entered the USA market for air to water -- have looked into it more than a few times. The last I looked at was a system that worked like a package unit -- everything was outside and it pumped in the hot water

I have seen them come and go in SA .... I have solar DHW and a storage tank for partial radiant. The explanation I got was cost ... They can't make them cheap enough and when they do .. they don't hold up.

Geothermal -- is a small player and eveything is already in place except the heat exchanger

 

[fitter30]

https://www.hotspotenergy.com/air-cooled-chiller/
or heat pump water heater
https://www.acwholesalers.com/A.O.-...df9-U5mxm4rrrZ5KdnYfw9mbaY2VLJJoaAqx4EALw_wcB

 

[theoldwizard1]

... or heat pump water heater ...

Those "steal" the heat from INSIDE your house !

 

[dcg9381]

> traditional heat pump system that has a heat exchanger to heat water directly so I could use it for my radiant slab?

They were offering heat pump water heaters - apparently still are. I realize this is probably not the same as what you use in your slab. I point it out because they're much more expensive than conventional water heaters - and I assume they are more complex - so it may be a while before you get your money out of them. Your use case might be right though if you're heating constantly.

I've also done solar hot water - but MI - the sun is pretty low and you guys have lots of trees... I'd do it for residential, but not sure for heating a slab.
There is always geothermal - I think that's the right solution for the MI climate.


Mini splits are good - probably not as good as a "warm floor" but there are hyper heat units that can produce heat in very cold MI weather.

Could switch to propane too...

 

[bikesandcars]

Thanks all!

I saw some of the new LG and MItsubishi units produce heat down to -13 and full heat down to 5 degree's. Certain models have HSPF over 10, approaching 12. In my environment that's more than adequate. What's funny is that 2 individual 24K high efficiency LG mini split outdoor and indoor combo's (HSPF 11) cost less than 1 outdoor unit and 2 indoor units and are also more efficient.

These guys make the heat exchanger (the key part) and they are very affordable. http://aquasystemsinc.com/ They sell to residential / retail customers, approx $250

The issue with getting a water/air heat exchanger to work is the indoor units signal digitally back to the outdoor unit based on the thermostat and demand. To make this work I would probably need to buy a multi-zone outdoor unit, install 2 traditional indoor units (For AC). Then buy an additional indoor unit just for the guts and combine it with an aquasystems heat exchanger. Then add some additional relay logic so my circulator turns on when I click the remote for the radiant zone. Sounds messy!

Much easier to just install 2 high efficiency single units (20 amp circuits each) and walk away... but still fun to think about.

 

[bikesandcars]

1000kwh per month is my fall/spring average usage of mostly resistance heaters(water heater, stove, dryer, etc all electric, minimal A/C).

My peak combined (house and garage) usage during the coldest month is 6300 kwh or approximately $750. Still not bad for an all-in combined bill with no other bills, but I could probably trim $150/200 off of that with mini splits in the garage. My home has an older 14 SEER Trane heat pump, probably a 8-ish HSPF. The house is 90's construction, not very tight, leaky windows, etc.

My garage is better insulated and more efficient(even with the garage doors) than my house.

Calculated costs per month for 3000 kwh (or 10.2MMbtu's) which is my yearly average usage taking standard efficiencies into account (on the higher side):

-Electric Resistance Heater: $381 per month
-Elec Heat Pump 11 hspf: $118 !!!! (322% efficient)
-Propane (2019 current): $238 (95% efficient)
-Oil 2019 (current): $253 (85% efficient)
-wood pellet: $219 (85% efficient)