Radiant and mini split questions

[nautique1228]

So after 9 months or so, we’ve closed on our property, got our easements and signed a builder on for our new shop that will be done in August! We’re building a 45x70x14 in Kansas City. R-19 walls, R-30 ceiling and the slab will be edge insulated and probably just 8ft perimeter of 2” foam. I would like to do radiant for heat. I’m in a debate between 7/8 and 1/2” pex. I would need 11 300’ loops of 1/2” which just seems excessive. I was kind of set on 6 circuits 400’ of 7/8”.
But other than radiant tec or radiant company, 7/8 seems rare. It’s also way more expensive than 1/2” oxygen barrier pex from big box stores of some online suppliers. Planning on a propane tankless hot water heater for heat source.

My next question is about A/C. Would 2 18k minisplits work? Not trying to cool it to 65* but just be workable in the summer. We will have a 14x20 “woodshop” in one corner. Put one head in there and one by the lift/work area. I could do like. 9k in the wood shop and a 24k by the lift.

My plan is to keep the shop at 50-55* in the winter with radiant and then use heat pumps in the mini splits to bring it up to 65-70. Or would I be better off just using the radiant to bring it up to temp. How long do you think radiant would take to raise the temp 10-15* in a shop this size? Would 1/2” vs 7/8” affect that. I’m under the impression 1/2” pex has a faster recovery rate.

Thanks for your help
Ryan

 

[Jking24]

I was advised to still keep loops under 300' but my loops are at 16"oc vs 12 with 1/2". Trying to raise the temp that much could take a few hours and be very costly if done on a regular basis. The key to radiant is maintaining one consistent temp. I went with radiantec. I spent a little more money but after taking with Don i felt like it was a good choice. If you purchase from them they will design your system layout for free. They also claim their 7/8" is thin wall and has a higher heat conductivity than standard 1/2". the way i looked at it is you generally here one of two things and not much in between. Either is the greatest thing ever or it's the worst thing they ever did lol so i swung for the fences. My hope is that my extra money spent will be recovered in efficiency and fuel savings. My 40x60 has 6 loops

 

[nautique1228]

Yes 7/8” would be 16” on center and 1/2” would be 12” on center. I had radiant in my previous shop. It was only 1/2” pex but 24” on center. It was built by the previous owner. It worked ok but it struggled when it got cold. It was Learning experience and I would like to get it right this time. But by the time I price all this out I feel like I should just go with a forced air furnace and ac unit.

 

[yeldogt]

Yes 7/8” would be 16” on center and 1/2” would be 12” on center. I had radiant in my previous shop. It was only 1/2” pex but 24” on center. It was built by the previous owner. It worked ok but it struggled when it got cold. It was Learning experience and I would like to get it right this time. But by the time I price all this out I feel like I should just go with a forced air furnace and ac unit.

24" on center is way too far apart. Even with super insulated buildings and slabs.

There is no benefit to 7/8 PEX ... the transfer to the slab is not much different and the long lengths work against you -- in both circulation and delta.

What you want to do and what you need to do ... ??

1/2 PEX is the sweet spot -- so it's used most often .... so it's the cheapest. Adding a loop or two is the way to go and cheap -- especially today with very affordable manifolds.

With constant circulation and proper controls. You can get away with a poorly built slab -- PEX too far apart and a big boiler. Trying to raise temps quickly is not going to work.

Mini splits need to be sized to the loads .... sized too small for AC .. they will never have the capacity in the winter .. especially in a cold climate where a hyper is needed. You need to do load calculations.

You should fully insulate the slab ... even if cost force thinner foam in the center. The reason 2" is typical is this is also the sweet spot in keeping the radiant heat going up to the building and not heating the ground.

Ideally -- radiant works best with a modulating boiler and constant circulation with a small pump ... close spaced tubing. If you have NG available -- nothing will be cheaper ..... proper insulation pays for a lifetime.

many on here use various heat sources .... w/o modulation on the boiler ... you need to slightly overheat the water to raise the slab temp and then the system stops ... the cycle repeats. Without enough tubing these systems don't work well. It tales a lot of heat to raise a slabs temp

Also -- different tubing sizes don't change the way radiant works .... bigger only increases possible loop length ... it's not more efficient or faster/ better.

In really large industrial buildings they use the larger tubing -- it's always constant circulation w/ bigger pumps. Again -- it's all based on load and needed boiler size

 

[nautique1228]

Thanks

I'm really just deciding on if 11 loops of 1/2" is going to be more of a PITA than 6 loops of 7/8". At this point I'm leaning towards 1/2"

From my calculations on loadcalc.net im at 68k btu heat loss and 32k btu cooling
From pex universe load calc I'm 88k.
That's if I'm doing it right.

How do you calculate how many btus actually go to the floor? Would a 100-120k btu boiler or instant hot water heater be enough? I initially thought I would be in the 180k btu area.

I will insulate the whole slab and Propane or Electric is my only option. Planning on propane at this time.

Constant circulation.. so when the slab comes up to temp, the boiler modulates to low heat and only puts out enough to keep the slab at desired temp. What kind of controller does this require and who sells it?

 

[yeldogt]

With propane you want to make sure you do it correct .... with NG so cheap currently the loss of 25% is still small. It's greater with propane because of the fuel cost. Electric only works in some parts of the country with very low rates. Electric also requires a lot of power.

What you are trying to do is transfer the heat the boiler is making to the slab ... that requires contact area and proper flow of water. What people don't get is pipe size. 3/8 tubing at 6" is going to outperform 1/2 at 12" ..and way better than anything at 16". The thing is 3/8 piping can only go about 225' and 1/2 is good to 300'. Also -- since 1/2 is more common ... it's cheaper. It's the spacing that matters.

You match the boiler to the load -- and remove the efficiency loss. If you need 60k and you have an 80% 80k boiler .... it has 64K to deliver in heat and 16k BTU goes up the chimney.

With constant circulation and outdoor reset (ODR) -- the boiler is set up with an outside temp sensor. The boiler knows the outdoor temp and runs based on a heat curve that you set -- you start with one based on your climate and then it gets tweaked. The boiler is then able to adjust it's temp to warm the slab to just what it needs to keep the space the temp you want. It can further be modified -- should you want to temp adjust lower.

Remember -- if you want to be able to lower slab temps .... unless you have tighter spacing it will take longer to bring everything back up. This also requires hotter water .. and that costs more .. so setbacks only work for longer periods or fewer degrees.

With wide spacing the water temp required is greatly increased ... Spending a bit for for the proper spacing/insulation and a real boiler will provide both comfort and savings.

Remember -- the boiler size is based on heating the space to the design temp (could be 72) on the coldest day your area sees. In my area that is often a day or two ... and maybe only seen every few years. But a few years ago -- we had it for much longer ..

With AC -- it's all about use. Turn a unit off and allow both heat and humidity to rise and the size of the unit needs to be very high. Removing water from the air requires lots of BTU's . That's why modulating systems (like a mini) should never be turned off ... you can raise the temp .... but the unit will keep removing humidity. Some systems will monitor the humidity and work based on that number. You can be comfortable at higher temps w/ low humidity

 

[Lonnies Performance]

I'll give you some ideas of what I did. I'm 70 x 96 so roughly twice your size. Dividing by 2 will give some good approximations...

I used 14 loops of 3/4 on 16" centers. My loops are all 300-450ft. I have no issue heating with a 200K BTU boiler, but the temperature drop of the loops is around 20 deg. For reference my heatcalc said I needed 100K with a 14 deg delta. This plays more of a part in the boiler sizing if you wish to maintain a certain water discharge temp. This limits the speed I can ramp up the temp in the space. I run my boiler on the lowest temp setting of 87 & it works, plus you are more efficient at a lower temp (less heat out the chimney).

I never used 7/8" PEX, but bending the 3/4 U-bends on 16" centers was not fun... I'm sure 7/8" would be worse.

For AC, I chose overkill in the way of more units instead of over-sizing less units. I used 4 30K units, but only run 2 for cooling unless it gets in the 90's out, then I may need a 3rd to hold 72 (2 units maintain about 76 when it's around 90 outside.). I did this for 2 reasons... individual smaller units are more efficient, than a multi-zone unit. Also, I may want to heat with them as the heating load is much more demanding, since output drops significantly as it gets colder outside. I typically only run the AC from 11am-9pm, running them more only wastes money.

Typically the heat pumps cost me less than propane in the fall months. With natural gas, your results may differ, but running an electric boiler will likely cost you 3X. I switch to radiant around December, holding 50-55 inside & use the heat pumps for supplemental heat, to around 60 when I'm working there short term.

 

[Lonnies Performance]

To answer your question, I would guess a pair of 18K heat pumps would pretty close to your needs for cooling & supplemental heat. When I try to ramp up the temp, I typically run both systems & can get an increase of approx 5 deg/hr. I typically bump up the radiant for an about hour to help (from 50-55), but let the heat pumps carry the temporary increase & let them modulate to maintain 60.

 

[yeldogt]

I'll give you some ideas of what I did. I'm 70 x 96 so roughly twice your size. Dividing by 2 will give some good approximations...

I used 14 loops of 3/4 on 16" centers. My loops are all 300-450ft. I have no issue heating with a 200K BTU boiler, but the temperature drop of the loops is around 20 deg. For reference my heatcalc said I needed 100K with a 14 deg delta. This plays more of a part in the boiler sizing if you wish to maintain a certain water discharge temp. This limits the speed I can ramp up the temp in the space. I run my boiler on the lowest temp setting of 87 & it works, plus you are more efficient at a lower temp (less heat out the chimney).

I never used 7/8" PEX, but bending the 3/4 U-bends on 16" centers was not fun... I'm sure 7/8" would be worse.

For AC, I chose overkill in the way of more units instead of over-sizing less units. I used 4 30K units, but only run 2 for cooling unless it gets in the 90's out, then I may need a 3rd to hold 72 (2 units maintain about 76 when it's around 90 outside.). I did this for 2 reasons... individual smaller units are more efficient, than a multi-zone unit. Also, I may want to heat with them as the heating load is much more demanding, since output drops significantly as it gets colder outside. I typically only run the AC from 11am-9pm, running them more only wastes money.

Typically the heat pumps cost me less than propane in the fall months. With natural gas, your results may differ, but running an electric boiler will likely cost you 3X. I switch to radiant around December, holding 50-55 inside & use the heat pumps for supplemental heat, to around 60 when I'm working there short term.

Closer spaced tubing would have allowed faster response .... the delta is important .. but the overall water temp is as well. IE -- running 140 water and getting 120 back .... vs 110 and getting 90 back.

Radiant is forgiving ... the goal is efficiency at a practicable price

 

[Lonnies Performance]

I agree that the smaller tubing will allow more energy transfer, but in my situation the response is limited by the boiler BTU. Both are capable of heating the building without issue. I could still get faster response if the boiler put out more heat, allowing a higher delta & corresponding increased transfer to the floor, but on initial startup it is giving all it can.

 

[yeldogt]

I agree that the smaller tubing will allow more energy transfer, but in my situation the response is limited by the boiler BTU. Both are capable of heating the building without issue. I could still get faster response if the boiler put out more heat, allowing a higher delta & corresponding increased transfer to the floor, but on initial startup it is giving all it can.

What also occurs -- with long loop lengths the water temp drops. To keep the delta from falling you need greater flow ..

It's funny how everything in life works the same way ... it's a trade off.

insulate and as much tubing as you can afford.

My current project has an old level of about 1000 sf where I'm doing underfloor extruded plates -- I'm using 3/8 tubing. The ability to move and bend the 3/8 over the 1/2 is the reason. I just need to add a loop or two to get the same coverage

 

[nautique1228]

Thanks for the help everyone. Found a company local that helped steer us in the right direction and we bought all our supplies from. Went with NTI boilers, FTV 150 combi. It will do our DHW as well. Went 5/8 rehau pex a tubing. 9 circuits 12” on center 400’ total length. Happy to say the install went fairly smooth, no major problems. We tied the pex to rebar and we’re just to scared to saw cut it, if I had to do it again I would try and put it under the rebar, and do saw cuts. Tubing and concrete happened late august. Building went up and was mostly finished by Mid October. We’ve been doing the interior ourselves minus the ceiling. Fought with the electric company for quite awhile, it’s been a PITA. But everything is installed and operational. Didn’t have a single leak from any of my solder joints!! Insulation we did 2” foam board underneath and on side of concrete. 2” closed cell spray foam on walls, and r/38ish blow in in the attic. Been below freezing the last couple nights and was out there for 2hrs and the heat never kicked on once. Boiler is modulating, condensing and uses outdoor reset. I did not do constant circulation as the boiler controls the circulation pump and that is not an option. Here’s some pics

Don’t have one of it fully completed but the last on is close.
Built a wall about halfway across the shop. The end is the “boiler room”
Bathroom and bar on the far side. My lift and toolbox, work bench will go on the close side.

 

[yeldogt]

Boilers use constant circulation when doing outdoor reset .... otherwise they can't adjust.

the process starts depending on what you set the slope for ... if set low start temp and low slope ..it will stop.