Radiant Heat - Heat Pumps? Air-to-water

[tsm1mt]

I have a new shop underway here in Montana, and we're about to start digging for the mono-slab, then next month the building goes up.

40x44x16 building, post-frame but I'll have to do some interior framing to sheetrock over the 5.5" of Corbond foam and such..

But today's topic is Radiant Heat.

We're going mono-slab - with a single 14x14 door, it was going to take a pump truck to pour the concrete after the fact, so the slab is going in first.

Looking at 16-18" mono-slab perimeter depth.
2" of R-10 XPS foam around the perimeter (at least to the 16" mark since the 4x8 boards snap apart at 16)

I then went with R-10 Pex boards - I wasn't super enamoured with the mushroom nubs for the PEX, but it compared favorably to buying more 2" XPS blue-board and then wiring the PEX in place.

Home Depot had the best price, and delivered for free (~$300 delivery cost) - 7 pallets of the stuff. Each stack of 20 is light enough you can move it around, but.. 7 pallets of the stuff..

So, R-10 foam under the pad, R-10 foam around the edges.

I bought 2000' of 1/2" Oxygen Barrier PEX to put in the floor (1760 needed) and have a tube layout written down that has loops from 220' to 270' or so.

I bought a 7 loop manifold. Then realized I needed to buy the 1/2" PEX adapters (When it said "no compression nuts" I thought none were needed.. rather than included)

I'm going to slip the PEX into some 3/4" PVC conduit elbows, and add another chunk of PVC for the temp probe, where it goes into the concrete just to help make the bend uniform.

So in the end, 7 loops of PEX in the bottom of the slab over the insulation into a manifold.

Now... how to warm up this water.

I ran 200A electric, and at least 100k BTU natural gas to the new shop.

I'm inclined to run the electric (24kw of solar planned on the roof to offset), but might go gas.

But what I really want to know is...

Does anyone have a recommendation on an Air to Water heat-pump?

And/or using two heat sources in series?

I can find a few ~30k BTU ATW units for under $3k. I probably need 2-4x that size, plus I need an auxiliary heat source for when it's -20 outside and the heat pump comes up way short.
(ArcticHeat has a big enough unit - $6k +s/h for 60k BTU, but I think there's a 25% tariff on heat pump imports from Canada so that becomes $7500 - and maybe a small unit for mild days and a big backup heater is a smarter choice?)

When I plumb this, can I run through a small-ish ATW heat pump, then into ... a regular gas or electric boiler? Or to an on-demand water heater (gas or electric)? Then use a two-stage (or outside temp sensing) thermostat, to use the heatpump when I can, but turn on the auxiliary heat when it isn't enough?

The heat pump has a chance to save some electricity (If I can produce a surplus after heating the shop, I can start adding electric resistance elements into the house to use up the extra power), but it ALSO means I could chill the floor in the summer. :D

We have low humidity here, and I know I couldn't freeze out the shop or I'll have water condense on the floor, but the current shop stays reasonably cool as it stands (R-21 walls, R-50 ceiling, vs the new shop planning R-35 and R-50) but if I can help cool it a bit, why not?

So again, anyone have a good recommendation on an Air to Water heat pump?
Any obvious problems using one plus an on-demand heater? or conventional boiler?

I'm also looking at some cheap suspended forced air electric heaters to augment the radiant - if I keep the floor at 40degF, it may make more sense to heat the air up a bit as needed rather than try to keep the whole place at 60.



One other thing I'm doing "weird" - I'm planning a 1" run of PEX down one side of the floor, to eventually go to another manifold for ice-melt of the approach to the shop. I struggled with how to get the hot water to the far side - run a pipe through the wall? Outside the wall? And settled on just putting it in the floor, where I'm already heating the floor. That should keep it safe, and any heat loss would just heat the floor.

 

[[486]]

do not put XPS below grade, it is open cell and takes on water, makes the R value tank HARD
EPS (the cheap shit) is closed cell and does not take on water

you don't mention where you're at, where I'm at the only heat pump that'll work in the winter is a ground source
I've put thought into some 'strange' refrigerants like r-170 and r-1150 to deal with the low air temps but haven't tried anything yet, also, it would be inoperable as an A/C in the summer.

 

[Betarocker]

XPS is the closed cell. EPS is the white open cell. Closed cell under the slab is desired. When I install heated slabs on grade, I don't use less than 8" thick of insulation. Ground is a huge heat sink and without a large thermal barrier, will require higher water temperatures.
My HVAC guy is installing Air to Water heat pumps in my current build. Info is on my work computer and not going there until next week.

 

[YotaAtieToo]

Side note

One day I'd like a house and shop with radiant heat. I'd like to have a large wood fire furnace deal inside the shop to supplement heat the water system, why not right? My fear would be over heating the water and making steam/pressure. Anyone have any info on this?

 

[tsm1mt]

;n95864]do not put XPS below grade, it is open cell and takes on water, makes the R value tank HARD
EPS (the cheap shit) is closed cell and does not take on water

you don't mention where you're at, where I'm at the only heat pump that'll work in the winter is a ground source
I've put thought into some 'strange' refrigerants like r-170 and r-1150 to deal with the low air temps but haven't tried anything yet, also, it would be inoperable as an A/C in the summer.

I'm in Helena, MT. Low humidity, but plenty of cold.

The Arctic EVI ATW heat pump is supposedly good to -13F, but I know the efficiency drops at lower temperatures.

We usually have a week or so of "high of -10" to deal with, but it's often in the 20s at least.

With a big thermal mass, I could choose to run the heat pump more during the day when it can be more efficient, and bank the heat.

But I still need an adequate backup heat source for when the heatpump just won't cut it.

When I install heated slabs on grade, I don't use less than 8" thick of insulation. Ground is a huge heat sink and without a large thermal barrier, will require higher water temperatures.

I'll be greater than 2" but no where near 8. The Ampex Panels that I bought are only R-10, they are 3 3/8" high but more like 2.75" of foam.

Running various calculators I see now that throwing more 2" XPS under for R-20 total may have been a smart move, but the gravel is going in today. (I thought I was doing good compared to all of the uninformed "you don't need any insulation at all, use the ground as a big (expensive and unlimited!) heatsink" )

I can change the perimeter insulation - pondering adding another 1" of XPS for 3" total / R-15 on the perimeter of the mono-slab.

Still noodling on what combination of heat sources to use. I ran gas, and 200A of electric (with 24kw of solar on the roof)

I see some 30k BTU ATW heat pumps for <$3k and then the Arctic Heat 60k for about $8k (25% tariff)
$2300-ish would get me a big enough electric boiler and pumps.
I could try the smaller heat pump, with a gas on-demand water heater to augment. Or the small heat pump with a medium sized straight electric boiler. Or just go gas and be done with it.

A small surge tank is necessary, of course, but with this much concrete to heat in a single zone, I don't think I need a modulating system.

 

[[486]]

XPS is the closed cell. EPS is the white open cell. Closed cell under the slab is desired.

https://www.insulfoam.com/the-dirt-on-below-grade-insulation/
https://mcsmag.com/grade-slab-building-insulation/
Always use EPS below grade

Bonus is that most all flat roofs up here got it under the membrane, and lots of places that re-roof will just toss out the old stuff that had gotten wet due to mold concerns. It being under a slab means mold don't matter one bit.