Sizing hot water tank for in floor workshop

bhilton

Hi Guys, I was planning on buying a 39 gallon Rheem 240v water heater for my 4 loop 1/2” pex lines. Total 985 ft. (240 ft or so per loop). Closed system with glycol and distilled water mix.

Is 39gallon too much? Im wondering how wasteful it will be to keep that 39 gallon up to temp 24/7. Maybe i shouldnt worry?

Also, what do you guys recommend for setting the temp of the tank? Ive heard people set it from 90F to 110F, but I dont really know the pros n cons.

I plan to keep the slab 60-65F and I have a wood stove that I plan to supplement with because I love using my wood stove!


Thank you!

Bill

kcopp

Did you do a heatloss on the building?
Did you insulate the slab underneath? Did you insulate the perimeter?
With a 4500 watt element you may be pushing the limit on it keeping up. Approx 15k btu...

GroundUp

Buy an electric boiler and do it right. What you save buying a water heater you'll spend on 37 extra gallons of glycol. The fluid capacity has zero bearing on the output. That unit is only 6kw, or 20,000 BTU which may or may not be enough depending on your heat loss calculation. You can get an electric boiler with an H stamp that's designed for radiant heating for less money than the extra glycol all the while saving the space. A 6kw electric boiler is smaller than a box of beer and hangs on the wall

bhilton

Kcopp,

Slab has r12 insulation underneath.

The perimeter has r12 too.

4.5-5” thick slab.

I don't need 37gallons of glycol, since I am mixing it with water AND I have a left over jug of glycol from when I built my house.

hot_rod

the capacity of the WH has little to do with the operation. I've used 6 gallon tanks, upgraded to 5500W elements, or two 5500W if you have two 30A circuits.

That said, an electric boiler will be built for the application and have listings. Most have staging controls and our. Look at Thermo 2000 or Electro MN, they are are not all that $$ consider the features and piece of mind.

bhilton

Hot rod, yeah but my question is more about the fact that keeping 39 gallons of liquid up to temp 24/7 vs. keeping 10 gallons up to temp 24/7. It will likely be more efficient to keep 10 gallons up to temp than 39 gallons. That said, having 39 gallons of 110F liquid sitting ready could be a good thing to pump through the 10 gallon volume of floor, that's 4 cycles of liquid ready in the tank, that should result in the tank not having to work much to maintain temp. A n00b like me would think having a 10 gallon tank or 20 gallon tank would end up working a bit harder during heat calls to get back up to temp, but overall would use less energy to heat the smaller amount of liquid during the non-heat call hours.


I will look into the small electric boilers "H" stamp boilers, see if I can get something reasonably priced. The 6Kw hot water tank I'm looking at is only $350 ($260 USD). I don't think I need as many components to plumb it in as I would with the boiler either.

pecmsg

Why electric at all, a LP boiler will do the job for a lot less $'s Oil or NG will be even cheaper!

It really doesn't matter how many gals of storage you have, in floor radiant is very slow to react to temperature changes, once you get the slab to temp you maintain it there with little if any changes!

nibs

Am watching my new boiler heat my house, just a little bigger than your slab, have come to realize that the floor slab is the reservoir of heat. Your proposed tank holds no reserve in comparison to the massive heat reserve in the 5" slab.

bhilton

nibs, thanks, that's great advice, that makes sense!

bhilton

pecmsg said:

Why electric at all, a LP boiler will do the job for a lot less $'s Oil or NG will be even cheaper!

It really doesn't matter how many gals of storage you have, in floor radiant is very slow to react to temperature changes, once you get the slab to temp you maintain it there with little if any changes!

So you're saying the additional gallons of storage will just be a waste to keep up to temp 24/7.

And I believe the same thing hot rod was saying, it's more about the efficiency of the elements and no storage is needed.

Seeing as how my walls aren't vapor barrier'd yet, I can easily run any electrical.

What do you think about this guy? I'm looking for something around this price point (CAD).

https://www.homedepot.ca/en/home/p.ie-11-electric-tankless-water-heater.1001110175.html

hot_rod

bhilton said:

Hot rod, yeah but my question is more about the fact that keeping 39 gallons of liquid up to temp 24/7 vs. keeping 10 gallons up to temp 24/7. It will likely be more efficient to keep 10 gallons up to temp than 39 gallons. That said, having 39 gallons of 110F liquid sitting ready could be a good thing to pump through the 10 gallon volume of floor, that's 4 cycles of liquid ready in the tank, that should result in the tank not having to work much to maintain temp. A n00b like me would think having a 10 gallon tank or 20 gallon tank would end up working a bit harder during heat calls to get back up to temp, but overall would use less energy to heat the smaller amount of liquid during the non-heat call hours.


I will look into the small electric boilers "H" stamp boilers, see if I can get something reasonably priced. The 6Kw hot water tank I'm looking at is only $350 ($260 USD). I don't think I need as many components to plumb it in as I would with the boiler either.

But if the tank is well insulated and within the heated envelop?? Any standby loss, which should be low on a tank or electric boiler a they can insulate the entire vessel, is inside the shop space.

If you are run-in g the tank or boiler at low temperature, maybe below 100F, losses will be low.

nibs

My 1200 sqft slab is only 2" thick, but we have massive thermal mass, concrete interior walls with some columns and very thick insulated exterior walls.
The 50K btu boiler responds to the night time drop in outside temp and runs for 4 to 6 hours. In the morning the boiler shuts off and the house stays warm all day. We are still working on insulation and are not nearly done with the construction.

KC_Jones

kcopp said:

Did you do a heatloss on the building?

This x1000 you can't choose anything until you do the heatloss on the building, otherwise you are throwing darts, blindfolded.

bhilton

hot rod, yes the tank will be standing on the slab itself. The walls are R22 but the ceiling is not very insulated right now. I actually don't plan to use the in floor heat for heating this winter, just a "warming" pad for my workshop and vehicles. I am putting new trusses & roof & attic insulation on the shop next year. For this winter the in floor will be used to keep it above freezing and then I'll supplement with my wood stove.

Solid_Fuel_Man

Once you get your first few electric bills you will change the water heater anyway.....trust me. You need to know your heatloss, but to be honest with what you are planning it doesn't really matter as whatever you heat the slab to will be sufficient even if it only maintains 50 when it's really cold out.

But....your electric bill will scare you! And you likely won't be warm enough....

bhilton

Solid_Fuel_Man said:

Once you get your first few electric bills you will change the water heater anyway.....trust me. You need to know your heatloss, but to be honest with what you are planning it doesn't really matter as whatever you heat the slab to will be sufficient even if it only maintains 50 when it's really cold out.



But....your electric bill will scare you! And you likely won't be warm enough....

I have a hot tub that I keep at 101F all winter, it's exposed to wind on three sides, I think the most it costs me during the -20C (-4F) months is $50-60 and we usually only have 1 or 2 of those months. The rest of the winter it only costs me $25-35 per month.

I am thinking this in floor heat, being protected from the elements and insulated better than the hot tub will use less electricity once up to temp. Hope I'm too far off.

GroundUp

Sounds like you've already made up your mind. If you can't afford to do it right, you can't afford to do it twice. Buy a boiler and do it right. All the components will be the same whether boiler or WH. 39 gallons of glycol solution versus 2 gallons of glycol solution, same price? Not. If this garage has overhead doors, forget about heating it properly with 6kW anyway. As others have mentioned, you need a heatloss calculation. At least look at an Electro EMB-S-9 for an idea to compare prices on equipment.

hot_rod

bhilton said:

Solid_Fuel_Man said:

Once you get your first few electric bills you will change the water heater anyway.....trust me. You need to know your heatloss, but to be honest with what you are planning it doesn't really matter as whatever you heat the slab to will be sufficient even if it only maintains 50 when it's really cold out.



But....your electric bill will scare you! And you likely won't be warm enough....

I have a hot tub that I keep at 101F all winter, it's exposed to wind on three sides, I think the most it costs me during the -20C (-4F) months is $50-60 and we usually only have 1 or 2 of those months. The rest of the winter it only costs me $25-35 per month.

I am thinking this in floor heat, being protected from the elements and insulated better than the hot tub will use less electricity once up to temp. Hope I'm too far off.

I'll bet your tub is much better insulated and smaller BTU load than the shop My sister had one of those electric outdoor tub and it had something like 10" of foam all around and a 8" thick insulated cover.

I doubt that you will experience much heat from a slab in a building without any ceiling insulation? Or that you will even warm a small portion with 6 kw

As my mother always said, "close the door, I'm not paying to heat the great outdoors!"

heat load calculations and energy required to warm a building are pretty predictable and knowable numbers these days with all the load calc and simulation software available.

bhilton

GroundUp said:

Sounds like you've already made up your mind. If you can't afford to do it right, you can't afford to do it twice. Buy a boiler and do it right. All the components will be the same whether boiler or WH. 39 gallons of glycol solution versus 2 gallons of glycol solution, same price? Not. If this garage has overhead doors, forget about heating it properly with 6kW anyway. As others have mentioned, you need a heatloss calculation. At least look at an Electro EMB-S-9 for an idea to compare prices on equipment.

I don't mean to sound decided, I'm definitely open, that's why I went to this wall! For the record, I still am not sure what direction I want to go yet.

bhilton

hot rod said:

I'll bet your tub is much better insulated and smaller BTU load than the shop My sister had one of those electric outdoor tub and it had something like 10" of foam all around and a 8" thick insulated cover.

I doubt that you will experience much heat from a slab in a building without any ceiling insulation? Or that you will even warm a small portion with 6 kw

As my mother always said, "close the door, I'm not paying to heat the great outdoors!"

heat load calculations and energy required to warm a building are pretty predictable and knowable numbers these days with all the load calc and simulation software available.

hot rod, as far as the hot tub goes, I installed mine, had the covers off it, and it only has this plastic bagged insulation that's not even 2" thick. I highly doubt it's insulated any better than my garage - other than the lack of attic insulation.

I see people all over using radiant heat in their concrete driveway all winter long. My workshop is going to be a million times more energy efficient than that right? For this winter, I want to achieve a slab that stays above freezing a few degrees. You don't think I would notice any warmth when I enter the garage when it's -4F outside and the slab is operating at 36F? It would be a complete waste?

With all your opinions, I might just not do anything this winter and stick with the wood stove as needed.

Thanks!

KC_Jones

bhilton said:

I don't mean to sound decided, I'm definitely open, that's why I went to this wall! For the record, I still am not sure what direction I want to go yet.

We know, without a heat loss of the space that's impossible, again you could keep guessing, but that's not the proper way.

If you have the numbers post them and you will get much better input, if you don't have the heat loss you need to calculate and post back.

Zman

I think the discussion between a water heater and an electric boiler is missing the mark a bit.
The biggest energy waste in your system is the energy lost as your slab overshoots (and subsequently undershoots) the target temp.
If this were mine, I would seek a way to control the supply water to the slab using outdoor reset. An electric boiler with staging or SRC controls could easily do this as could an electric tank with a outdoor reset control in place of the aquastat.

pecmsg

Much larger Thermal Mass in the garage floor then the Hot Tub!

bhilton

KC_Jones, it's hard to do a heat loss when I have no proper attic insulation. In fact, next summer I am ripping the trusses off and putting a completely new roof on with a loft built in to "attic trusses". I don't have the insulation planned out yet, which is why at this point I'm leaning toward doing leaving the in floor rough-in alone this winter. Use the woodstove, and worry about in floor heat next fall after the new roof and proper insulation is done.

hot_rod

bhilton said:

hot rod said:

I'll bet your tub is much better insulated and smaller BTU load than the shop My sister had one of those electric outdoor tub and it had something like 10" of foam all around and a 8" thick insulated cover.

I doubt that you will experience much heat from a slab in a building without any ceiling insulation? Or that you will even warm a small portion with 6 kw

As my mother always said, "close the door, I'm not paying to heat the great outdoors!"

heat load calculations and energy required to warm a building are pretty predictable and knowable numbers these days with all the load calc and simulation software available.

hot rod, as far as the hot tub goes, I installed mine, had the covers off it, and it only has this plastic bagged insulation that's not even 2" thick. I highly doubt it's insulated any better than my garage - other than the lack of attic insulation.

I see people all over using radiant heat in their concrete driveway all winter long. My workshop is going to be a million times more energy efficient than that right? For this winter, I want to achieve a slab that stays above freezing a few degrees. You don't think I would notice any warmth when I enter the garage when it's -4F outside and the slab is operating at 36F? It would be a complete waste?

With all your opinions, I might just not do anything this winter and stick with the wood stove as needed.

Thanks!

certainly 36F would feel warmer than -4F, I think we are concerned about how much electricity quantity and cost it will take to get you there!

I like your plan to use the wood stove until the shop is an a better "heatable" condition.

nibs

Yep, put the pipes in the slab, and burn wood until you know how best to heat the place.

bhilton

For those who were curious, I attached some pics. The pex is in the slab, the walls, windows, doors, are all closed in now. I'm in the process of doing electrical rough in, wood stove chimney and wall insulation.

nicholas bonham-carter

Once you figure out the heat loss, using the SlantFin app, you will see how many hot water heaters you need to handle the load.
A boiler is designed to provide a high volume of water with a 20-30 degree rise, whereas the hot water heater will provide a much smaller volume of hotter water-two different animals.—NBC

Voyager

bhilton said:

hot rod, as far as the hot tub goes, I installed mine, had the covers off it, and it only has this plastic bagged insulation that's not even 2" thick. I highly doubt it's insulated any better than my garage - other than the lack of attic insulation.

I see people all over using radiant heat in their concrete driveway all winter long. My workshop is going to be a million times more energy efficient than that right? For this winter, I want to achieve a slab that stays above freezing a few degrees. You don't think I would notice any warmth when I enter the garage when it's -4F outside and the slab is operating at 36F? It would be a complete waste?

With all your opinions, I might just not do anything this winter and stick with the wood stove as needed.

Thanks!

Insulation matters, but so does surface area. What is the surface area of your garage? I suspect it is 30X or more larger in area. Even if the garage has 4X more insulation, it still will likely lose 8-10X more BTUs/hr.

You really need to do a heat loss calculation. That will give you a fairly accurate indication of what you need for a heat source.

hot_rod

bhilton said:

For those who were curious, I attached some pics. The pex is in the slab, the walls, windows, doors, are all closed in now. I'm in the process of doing electrical rough in, wood stove chimney and wall insulation.

nice work, great to have a pumper instead of wheeling all that mud.

How thick is the slab? It's alway best, but hard to do, keeping the tube up into the slab. Strip type chairs help keep most of the tube up.

Output drops off a bit when tube is at the bottom of a 4" or any pour, probably cost you 15° or more additional SWT to get the same output.
Credit: Modern Hydronic Heating 3rd edition

bhilton

hot rod said:

bhilton said:

For those who were curious, I attached some pics. The pex is in the slab, the walls, windows, doors, are all closed in now. I'm in the process of doing electrical rough in, wood stove chimney and wall insulation.

nice work, great to have a pumper instead of wheeling all that mud.

How thick is the slab? It's alway best, but hard to do, keeping the tube up into the slab. Strip type chairs help keep most of the tube up.

Output drops off a bit when tube is at the bottom of a 4" or any pour, probably cost you 15° or more additional SWT to get the same output.
Credit: Modern Hydronic Heating 3rd edition

Thanks!

Floor is 4.5" +/- thick and 1/2" pex should be sitting 1.25-1.75" up from the bottom since it's fixed to the top of the 1/4" wire mesh and the wire mesh is sitting on 1" foam pucks I made using off cuts (you can see them if you look closely).

Voyager

bhilton said:

Thanks!

Floor is 4.5" +/- thick and 1/2" pex should be sitting 1.25-1.75" up from the bottom since it's fixed to the top of the 1/4" wire mesh and the wire mesh is sitting on 1" foam pucks I made using off cuts (you can see them if you look closely).

2” chairs would have been better, but you should still be able to heat the space effectively. The real question is: Have you completed that heat loss calculation yet?

bhilton

Voyager said:

2” chairs would have been better, but you should still be able to heat the space effectively. The real question is: Have you completed that heat loss calculation yet?

I mentioned earlier that I don't have much attic insulation right now, I'm ripping the trusses off next summer and replacing them with higher peak and built in loft (see Attic Trusses).

I took a gander at SlantFin and filled out all the fields for a Room and it said 274,000 BTU/HR. But, it never asked me what insulation I was using... I'm not sure If this app is hand holding me enough.

Calculator.NET says You will need 35,212 BTU/hour or 10.3 Kw.

I went through some more online calculators which ranged from 6.8Kw to 13Kw.

Solid_Fuel_Man

I assume it's about 1,000 square feet as you said it is 985 total feet if tubing.

bhilton

Solid_Fuel_Man said:

I assume it's about 1,000 square feet as you said it is 985 total feet if tubing.

Yes, 28' x 36' = 1008'. I don't know how you guys measure it but with 7" thick walls, it's more like 27' x 35' inside area.

3 windows total 28' (Brand new, low-E)
2 car doors total 126'
2 man doors total 53.2' (one is a double door)

Wall insulation R22, everything air tight, cill gasket, spray foam at windows doors and all places holes are entering the building.

Car doors have R12 and weather stripping fits properly.

Attic will be complex because the center of garage ceiling will be a loft, with R32 (I'm guessing) up in the rafters but the outer edges of the garage ceiling along the shall portion of the trusses will be blown in R40~.

Voyager

I used Excel and the Zurn guide and it wasn’t all that hard. The main thing is deciding what assumptions to use for air infiltration and such. I came up with 40-48,000 for my 32’ x 48’ x 14’ workshop depending on the assumptions for air infiltration. About the smallest NG boiler available was the Triangle Tube CC50s so I used that. So far, so good, but haven’t had any really cold days yet. It was 22 F last night and that is the coldest thus far and it kept up with that easily.

Here is the Zurn guide. Lots of useful information all in one place.

https://www.zurn.com/media-library/web_documents/pdfs/zpm02101-pdf.aspx

GroundUp

Slant/Fin seems to pad their heat losses pretty considerably. I use a program called BuildItSolar that seems to be right on the nuts. I did your calculation with the numbers you gave, using R10 edge insulation and 10 foot sidewalls (not sure if you said or not, but I don't see it). Not sure on your location, so I used my own in central MN with -20F design temp. Came out to be 21,969 at design

Voyager

I ran the Slant/Fin calculator on my workshop and it gave an heat loss of 65,000, which is more than 50% higher than my Excel calculations which were in the 40s depending on what assumption I used for air infiltration. Air infiltration was one of the bigger levers in changing the estimated heat loss. Since the Slant/Fin program did not ask about that, they are probably using a very high value for air infiltration. The difference between 0.5 and 2 air changes per hour is substantial. I think I ended up using 1 for my building as it has a large sectional overhead door and they are hard to seal well.

bhilton

GroundUp said:

Slant/Fin seems to pad their heat losses pretty considerably. I use a program called BuildItSolar that seems to be right on the nuts. I did your calculation with the numbers you gave, using R10 edge insulation and 10 foot sidewalls (not sure if you said or not, but I don't see it). Not sure on your location, so I used my own in central MN with -20F design temp. Came out to be 21,969 at design

Thanks! That's right in the ball park of what I've been getting.