Need opinion: Radiant & Mini Split

[340RedCoupe]

Long time reader first time post.
My plan is to keep the radiant heat on the low side maybe 50 degrees and when I'm in the shop and kick on the mini split on to add 10 or more degrees and also have the benefit of A/C in the summer.
Details: free standing building, 780 sq feet, 12 foot ceilings, new construction, R21 in the walls, R36 in the ceiling, ICF'S (insulated concrete forms) from the top of the footers (36" frost depth) to 2 foot above grade then stick built from there, 2" foam under the pad, 1" tubes in the concrete, 2 loops about 300' each, 3 bays total, 2 bays open area, one 4 post lift, 3rd bay shop area, bathroom w/toilet and shower and stairway to the unheated 2nd floor, 1" gas line in the ground to the building, 100 amp service in the building. Currently starting to insulate, most of the rough wiring done, no drywall yet.
Now I need to make some decisions before I go too much further. Does it make sense to add the mini split? Am I wasting money? A/C would be really nice. Now is the time to rough in the electric if I decide this is the way to go. Located in N E Ohio so we can go from a low of -5 to a high of 100. I do not have a heat source for the radiant yet that will be a later topic. I will need to set up the hot water for a sink and a shower. Any and all comments appreciated.
Thanks!

 

[fitter30]

There is nothing like floor radiate. Had on grade electric radiate in a family room last house. Came in from working outside all day running service hvac work - 40 below wind chill roof like a little bit of heaven. If you plan to work on cars without a lift heated floor gets my vote.

 

[jack stand]

Good plan overall.
You'll have a nice starting point for the minis to "bump" up from. (And the a/c!)
Depending on your electricity rates, the performance at lower temperatures of the M/S AND the fuel cost of your radiant heat source (gas, oil, electric) will come into play as the temperature drops as to any savings or comfort.
A quick warm up to 60*-65* during a cold snap may not happen as quickly (or cheaply) as you want.
The "shoulder" seasons (before you fire up the radiant) will be where the M/S will shine, without a doubt that's the way to quickly to take the chill off.
AND YOU'LL HAVE A/C

 

[340RedCoupe]

Thanks for the responses. I’m going to go ahead with the rough wire for the M/S since I have left over 10 GA from wiring the lift. I’m looking at a Mitsubishi 18,000 BTU, sound about right?

 

[jack stand]

I know that's good equipment, otherwise sizing is up to you or your installer to advise. The other question might be asking about a low temperature HP.
Maybe their all this way, I only experience is with my ms system and deep winter heating was not necessary. From December on we heat the radiant with a wood fired boiler. Our decision was primarily based on the a/c aspect. Prior to experiencing a ms, "heat pump" was a 4 letter word but the ms do a great job in the shoulder seasons. Nothing like the ducted heat pumps of the 80's, 90's and early 2000's.

 

[isb cornbinder]

Long time reader first time post.
My plan is to keep the radiant heat on the low side maybe 50 degrees and when I'm in the shop and kick on the mini split on to add 10 or more degrees and also have the benefit of A/C in the summer.
Details: free standing building, 780 sq feet, 12 foot ceilings, new construction, R21 in the walls, R36 in the ceiling, ICF'S (insulated concrete forms) from the top of the footers (36" frost depth) to 2 foot above grade then stick built from there, 2" foam under the pad, 1" tubes in the concrete, 2 loops about 300' each, 3 bays total, 2 bays open area, one 4 post lift, 3rd bay shop area, bathroom w/toilet and shower and stairway to the unheated 2nd floor, 1" gas line in the ground to the building, 100 amp service in the building. Currently starting to insulate, most of the rough wiring done, no drywall yet.
Now I need to make some decisions before I go too much further. Does it make sense to add the mini split? Am I wasting money? A/C would be really nice. Now is the time to rough in the electric if I decide this is the way to go. Located in N E Ohio so we can go from a low of -5 to a high of 100. I do not have a heat source for the radiant yet that will be a later topic. I will need to set up the hot water for a sink and a shower. Any and all comments appreciated.
Thanks!

I think you have this figured out, Good job!

 

[RichHawk]

Long time reader first time post.
My plan is to keep the radiant heat on the low side maybe 50 degrees and when I'm in the shop and kick on the mini split on to add 10 or more degrees and also have the benefit of A/C in the summer.
Details: free standing building, 780 sq feet, 12 foot ceilings, new construction, R21 in the walls, R36 in the ceiling, ICF'S (insulated concrete forms) from the top of the footers (36" frost depth) to 2 foot above grade then stick built from there, 2" foam under the pad, 1" tubes in the concrete, 2 loops about 300' each, 3 bays total, 2 bays open area, one 4 post lift, 3rd bay shop area, bathroom w/toilet and shower and stairway to the unheated 2nd floor, 1" gas line in the ground to the building, 100 amp service in the building. Currently starting to insulate, most of the rough wiring done, no drywall yet.
Now I need to make some decisions before I go too much further. Does it make sense to add the mini split? Am I wasting money? A/C would be really nice. Now is the time to rough in the electric if I decide this is the way to go. Located in N E Ohio so we can go from a low of -5 to a high of 100. I do not have a heat source for the radiant yet that will be a later topic. I will need to set up the hot water for a sink and a shower. Any and all comments appreciated.
Thanks!

I'm following right behind you. Almost identical setup, though 1100Sq Ft for me, insulated, Radiant floor heat, and 18,000 BTU minisplit.

What size radiant floor heat did you choose (BTUs or watts) ?
I did the heat transfer calculations, and for winter came up with 16,000BTUs total on a -15 deg Day and a 72 deg shop temperature.
I know the minisplit Heat pump says it can deliver that even on a -10 deg day, but I don't trust it will.

The calcs say I will lose 1300 BTHh through the slab to the dirt, and I'm considering hooking up a 10 gal / 2000W ( 6,800btuh) electric hot water heater as my radiant floor heat source. that leaves me 4,500 BTUs for heating, which is

I would love to hear from everyone on what size heat source they chose for their radiant floor heat. Who successfully heats via a hot water heater?

Thanks, Richard

 

[340RedCoupe]

I have not selected a heat source for the radiant as of yet. I’m still in the insulation stage and I have plumbing work to do for the bathroom. I do have a 1” gas line installed to the building but not connected yet. My plan is to finish this summer.

 

[HoosierBuddy]

If your gas line is NG there is no reason to consider anything else for the radiant source.

I mean, I hope you're not considering installing a heat pump and then turning around and running an electric resistance hot water boiler as a source for hot water to heat your floor.

That would increase your cost to heat the garage by around 400% vs just doing it with the mini split.

NG is going to cost on the order of 1/4 the price of power on a direct KW to BTU price check. That offsets all or part of the mini-splits"KW in per BTU of heat moved" advantage.

Your mini-split is going to have a very high COP (4? 5? 6?) which will make up for the higher cost of power....but an electrical resistance heater of any kind has a COP of 1.

 

[340RedCoupe]

I misspoke about using “heat source”. NG is the only way to go. I was thinking about sizing, with a boiler or tankless water heater, not energy source. The mini split is very high efficiency and will be only used for cooling in summer and heat early fall and late spring.

 

[HoosierBuddy]

I misspoke about using “heat source”. NG is the only way to go. I was thinking about sizing, with a boiler or tankless water heater, not energy source. The mini split is very high efficiency and will be only used for cooling in summer and heat early fall and late spring.

Should be the bees knees.

I'm using a fully condensing wall boiler for my garage radiant (since 2006) and it's awesome in winter. My AC solution is kludgy in comparison. If I had to do it over, I'd definitely be going the route you are with a mini-split for AC.

 

[theoldwizard1]

While mini-split systems that produce warm/cold air AND hot water have been around for years in other parts of the world, they are just NOW showing up in the US !

Nothing can beat the efficiency of a heat pump when the ambient air is 30F-70F. Depending on the make and model, those numbers will vary.

 

[Jackfre]

As to the MS you have the choice of the deep heating models down to -20 or the standard units which will heat down to 5*. In you climate you want the -20*. That will provide you back-up should the unforseen happen with the radiant. On the radiant I’d suggest a combi boiler. They settle your HW need and come with the controls built in for you outdoor reset temp control for the boiler. Most also come with the primary circ built in. I have had good luck with the Rinnai model I’s but it has been a couple years since installing them. There are many out there. They all need to be set up with someone who has and knows as to oxygen analyzers.
I would suggest that you use plywood set vertically rather than SR. An 8’&4” set vert. That way you can access the wall interior in the future. It cam in really handy in my shop. Seeing as how we are spending your money I would suggest that you look at adding solar, if the orientation is right. I self installed a 7.5kwh system on my shop and with a grid tied system zeroed my electric for the whole property. Free ac/heat