Help with heat loss and boiler sizing

[Tracs]

Hi, This is a long first post. I have been reading threads for days on this site about these topics but thought I would reach out.

I built a 24'x30' detached garage with the following specs:

6" slab with 12" thickened edge. R-10 rigid pink foam around the perimeter and R-8 ISORAD foam under the pad. This is the foam with groves the Pex sits into. Expensive but very easy to use. There is a thick poly barrier under all the foam and pipes.

720 sq ft.
2x6 Wood framed walls with R-20 batts. R-50 in the ceiling.
2- 9'x8' T16 overhead door -R-16. -144 sq ft
1- 36"x80" insulated steel entry door. - 24 sq ft
3- 2'x5' PVC double pane windows. - 30 sq ft
Per local building codes there is 6mil poly vapor barrier on walls and ceiling. Everything is sealed with Acoustical sealant.

3 Loop PEX system with equal length for all 3 loops. 240' per loop

The garage is built to and exceeds some local standards. The building inspector actually told me he wished the large home builders in the area put as much effort into the houses they build as I did for the garage.

In Manitoba it can get down to -35C (-31F) during the winter. On the coldest of cold days I would like the garage to maintain a temperature of about 15C (60F). So about a 90F rise above the coldest outside temp. If I don't plan on being in the garage working on something the temp could be turned down to 7-10C (45-50), 75F rise.

Now to the problems and second part of the thread title.

Has to be an electric boiler. Electricity is about $0.07/kWh. The electrical in the garage is already done. It has a sub panel that is connected to my home 100amp service from a 60amp breaker fed by new #6 copper wire.

Some local supply companies say that a 6kw (20,473btu) boiler will be adequate for my goals and some say I would need a 8kw (27,297btu).

Here is a link to the boiler company I will be going with.
http://www.conceptmfginc.com/leb.html

Both cost the same, but my electrician said I wouldn't be able to run a 8kw boiler with the electrical service I have in the house/garage. He said with a 6kw boiler I would be able to run other larger devices in the garage at the same time, but anything larger than a 6kw and I wouldn't be able to run much more than just the lighting.

Now back to the first part of the thread title.

I have used some online heat loss calculators and have got wildly varying results. Anywhere from 13,500btu - 22,000btu. I really just don't know enough about them to use and understand them properly.

What has me worried is that a friend built a 832 sq ft garage that is almost the same as mine in size, materials used, and construction methods. He had a heat loss calculation done and his came in at 29,303btu. That included 14,319btu for infiltration. He put a 6kw boiler in his garage and can't get the temp past 10C (50F). He is upgrading to a 9kw boiler (30,000btu) hoping that will get the temps inside up.

Is there anyone in similar cold climates that have a similar sized garage and floor setup that have real world results with boiler sizing and heat loss?

TL;DR Will a 6kw boiler be able to produce enough heat to keep the garage near room temp in the middle of winter if necessary?

 

[danski0224]

As I understand it, the biggest problem with slab heat is loss around the perimeter. Next is a thermal break from the interior heated slab to the footing or thickened edge.

A monolithic pour makes it complicated. You basically have to put 2" foam under everything and up the exterior.

Having a perimeter footing/foundation and then a separate slab inside is much easier.

Just based on your description, I think you have heat loss issues from the slab into the thickened edge, and you only have 1" foam around the outside perimeter.

The heat load calculations fail when there are construction defects.

If your friend has heat migration from the interior slab to the thickened edge, the bigger boiler probably won't help.

Then there is 1/2" vs 5/8" pex.

A glycol solution reduces capacity and increases head pressure.

If you have slab construction/insulation problems, then you may need to get a fan coil and use the boiler for forced air. It won't warm the slab a whole lot, but you will have heat.

 

[Tracs]

As I understand it, the biggest problem with slab heat is loss around the perimeter. Next is a thermal break from the interior heated slab to the footing or thickened edge.

A monolithic pour makes it complicated. You basically have to put 2" foam under everything and up the exterior.

Having a perimeter footing/foundation and then a separate slab inside is much easier.

Just based on your description, I think you have heat loss issues from the slab into the thickened edge, and you only have 1" foam around the outside perimeter.

The heat load calculations fail when there are construction defects.

If your friend has heat migration from the interior slab to the thickened edge, the bigger boiler probably won't help.

Then there is 1/2" vs 5/8" pex.

A glycol solution reduces capacity and increases head pressure.

If you have slab construction/insulation problems, then you may need to get a fan coil and use the boiler for forced air. It won't warm the slab a whole lot, but you will have heat.

The PEX is 1/2" and is what everyone around here uses in thier garages.

I had another look at the foam around the perimeter and I was wrong. It is 2" R-10. Under the pad is also the same. But when I had the pad contructed and poured there is no foam under the bottom of the tickended edge. I asked the builder about that and he said no one in the area does is like that. I asked the building inspector and he also confirmed he never sees foam under the tickended ege, only on the sides of the perimeter and under the main pad.

The PEX piping runs throughout the pad but stays away from the thickended edge.

Thats why i was wondering if someone with similar construction in a cold climate had any real world advice.

There has to be 100 garages in my city and 1000s in the prairies/northern US built the same way that are slab heated. Most of them have to be working OK or it would be known.

 

[danski0224]

The PEX is 1/2" and is what everyone around here uses in thier garages.

I had another look at the foam around the perimeter and I was wrong. It is 2" R-10. Under the pad is also the same. But when I had the pad contructed and poured there is no foam under the bottom of the tickended edge. I asked the builder about that and he said no one in the area does is like that. I asked the building inspector and he also confirmed he never sees foam under the tickended ege, only on the sides of the perimeter and under the main pad.

The PEX piping runs throughout the pad but stays away from the thickended edge.

Thats why i was wondering if someone with similar construction in a cold climate had any real world advice.

There has to be 100 garages in my city and 1000s in the prairies/northern US built the same way that are slab heated. Most of them have to be working OK or it would be known.

Then it is done wrong.

Is the ground warmer 6" further down under the thickened edge? If it is, then you will not have heat loss there.

No one does it because it is a pain/takes longer/costs more.

It takes longer to do a perimeter foundation/footing and then come back to do the floor slab. That means it costs more. But it makes the thermal break issue much easier to solve.

The systems that are working OK either have enough reserve boiler capacity, more/bigger loops or are not located in an extreme climate like yours. Doing it wrong over and over again does not make it right. I bet you can find some section views on how to properly prepare a slab for radiant heat. Odds are, your construction details will not match.

Your boiler will never overcome the heat loss through the bottom of the uninsulated thickened slab.

Good luck.

 

[dfiler2]

I built a 24x24 about 10 years ago with the same specs you describe except all my foam is 1.5". I heated mine with an electric water heater for a while, I think it would have been around 30'000 BTU's, kept up easily. I switched to an outdoor pellet boiler after a couple of years which heats my home and the garage. Any chance you can back up with propane or pellets or wood, to take a little stress off of the electric if needed? My heat demands are pretty close to yours, I am located about 20 miles south of the border in MN, we are generally considered Southern Canadians.

 

[totaledmyGN]

A buddy built a pole shed just in time to experience the polar vortex last year. It's a 60x40ft with I think 18 foot walls or around 43,200 cubic feet. I talked him into upgrading his insulation a little bit from what the builder usually does. 2 inches under slab and on perimeter, 6 inch bat insulation in the walls but no tyvek, and blown fiberglass to r50 in the ceiling. 1 22ftx16ft r16 overhead door with 2 small windows, 1 insulated walk-in door and 4 ceiling fans. The boiler he installed is a 15kw lion boiler and it keeps the temperature at 10-12 degrees celsius no problem. I do not know what the maximum temperature rise he could achieve with this set-up. It's hard to tell exactly how much it cost to run last winter because there is an old farmhouse and several cattle waterers on the same meter. He guesses around $200.00 month but he also no longer has 2 tractors plugged in either so that cost could be added to the $200.00 month. This in in southwest Manitoba.

 

[Tracs]

Then it is done wrong.

Is the ground warmer 6" further down under the thickened edge? If it is, then you will not have heat loss there.

No one does it because it is a pain/takes longer/costs more.

It takes longer to do a perimeter foundation/footing and then come back to do the floor slab. That means it costs more. But it makes the thermal break issue much easier to solve.

The systems that are working OK either have enough reserve boiler capacity, more/bigger loops or are not located in an extreme climate like yours. Doing it wrong over and over again does not make it right. I bet you can find some section views on how to properly prepare a slab for radiant heat. Odds are, your construction details will not match.

Your boiler will never overcome the heat loss through the bottom of the uninsulated thickened slab.

Good luck.

So trying to heat the slab would be a loosing battle because there is no insulation under the thickened edge portion? Should I be abandoning the in floor heat and come up with something else?

 

[Tracs]

I built a 24x24 about 10 years ago with the same specs you describe except all my foam is 1.5". I heated mine with an electric water heater for a while, I think it would have been around 30'000 BTU's, kept up easily. I switched to an outdoor pellet boiler after a couple of years which heats my home and the garage. Any chance you can back up with propane or pellets or wood, to take a little stress off of the electric if needed? My heat demands are pretty close to yours, I am located about 20 miles south of the border in MN, we are generally considered Southern Canadians.

Did you put insulation under 100% of the pad or did you do like me and have it under the center slab and vertically around the perimeter but not any under the thickened edge?

Did you tank cycle on and off as demand required or did it run constantly?

 

[Tracs]

A buddy built a pole shed just in time to experience the polar vortex last year. It's a 60x40ft with I think 18 foot walls or around 43,200 cubic feet. I talked him into upgrading his insulation a little bit from what the builder usually does. 2 inches under slab and on perimeter, 6 inch bat insulation in the walls but no tyvek, and blown fiberglass to r50 in the ceiling. 1 22ftx16ft r16 overhead door with 2 small windows, 1 insulated walk-in door and 4 ceiling fans. The boiler he installed is a 15kw lion boiler and it keeps the temperature at 10-12 degrees celsius no problem. I do not know what the maximum temperature rise he could achieve with this set-up. It's hard to tell exactly how much it cost to run last winter because there is an old farmhouse and several cattle waterers on the same meter. He guesses around $200.00 month but he also no longer has 2 tractors plugged in either so that cost could be added to the $200.00 month. This in in southwest Manitoba.

That sounds like pretty good heat for a large building. For what our electricity costs, $200/month means the boiler is running for 6hrs per day according to a electricity cost calculator.

My biggest fear if I go through with the install is that my boiler will run 24/7 and never be able to catch up.

 

[Platonic Solid]

Just an observation from a no real-life-experience with this scenario observer: Wouldn't it be easier to upgrade to 200A service and go with the 8kw boiler?

 

[totaledmyGN]

Tracs what is your friend doing with his 6kw boiler he is taking out? If it's priced right it may still be worth a try. I would get a quote for upgrading your electrical system for the house/garage as well.

 

[dfiler2]

Did you put insulation under 100% of the pad or did you do like me and have it under the center slab and vertically around the perimeter but not any under the thickened edge?

Did you tank cycle on and off as demand required or did it run constantly?

The insulation is not under the thicker part of the slab, there may be some heat transfer to the thicker area but the edges are insulated 16" down. The heat that does get transferred to that area is pretty insignificant IMO and will eventually move back up into the building. I operated the water heater at a constant temp then used a Taco 007 zoning pump, that can be operated with a 24V thermostat, to control the temp in the building. Any heat loss from the tank is lost into the building.

 

[BadgerBoilerMN]

The Manual 'J' heat load for my 40 x 60 modified pole barn is 30,000 btuh.

Of course this is anecdotal, like the rest of this thread.

Every sound radiant floor systems starts with an ACCA Manual 'J' heat load analysis. It is required precisely because of the different climates, building locations, standards and requirements of the occupants.

Once insulated, in any way, the slab slab itself represents one of the lowest loads as a percentage of the whole. This is why the blanket "insulation" crowd get by selling garage bags.

The perimeter load can be significant if not detailed properly but heat load is calculated by surface area x temperature. As you can see the perimeter surface area of this slab would be a small percentage of the total.

First, I don't think you can get anything in a garage this size that would warrant a 6" slab.

Second, knob and tube insulation is a real time saver it that is what you are after. But I wouldn't pay for a knob at the expense of the R-10 minimum we generally use in the 'field' of the slab. The perimeter should be closer to 20.

Third, the garage door represents one of the most significant loads, more especially if is not well sealed and maintained. Mine is R-19, which is much closer than the imagined R-19 required by local building codes once the thermal bridging of R-4.5 studs and infiltration is accounted for.

Finally, you will not have the infiltration or fenestration (loss around window) since you don't have many. These facts confuse many, professionals and novices alike.

Get a real heat load. Get a good tight door. Get some more insulation under and more important, at the perimeter of the slab.

And don't forget outdoor reset, lowers the fuel bill no matter what the fuel source.

 

[DC73]

What has me worried is that a friend built a 832 sq ft garage that is almost the same as mine in size, materials used, and construction methods. He had a heat loss calculation done and his came in at 29,303btu.

TL;DR Will a 6kw boiler be able to produce enough heat to keep the garage near room temp in the middle of winter if necessary?

I think your friend's real world experience should play a major role in your decision. His heat loss sounds more reasonable.

I wouldn't worry about having no insulation under the thick slab edge. Most slab insulation details don't show insulation in that location. It's not a major component in your heat loss situation given everything else you've done and it'd be a lot of trouble to remedy at this point.

DC

 

[danski0224]

I found these articles with just a little bit of digging.

How deep is your tubing? More than 2" down?

http://inspectapedia.com/heat/Radiant-Slab-Heat-Mistakes.htm

What type of foam?

http://www.inspectapedia.com/Energy/Insulation_Polyisocyanurate_Slab.htm

What type of rocks under the slab? Quality moisture barrier or plain 6mil plastic?

http://www.finehomebuilding.com/ite...-rock-solid-well-insulated-garage-slab/page/2

I did find one or two illustrations showing foam under the thickened slab, but not many. The foam on the exterior certainly needs to extend further down than the thickened part, be tight to the concrete, not retain moisture... bla bla bla.

 

[Tracs]

Just an observation from a no real-life-experience with this scenario observer: Wouldn't it be easier to upgrade to 200A service and go with the 8kw boiler?

I talked to another electrician friend and he said that yes I could run a 8kw boiler off 100 amp main, 60 amp sub panel setup I have now. The only thing is that I wouldn't be able to run large items like a 240v air compressor or similar items at the same time without the risk of tripping the breaker.

So a 8kw boiler is defiantly in the picture now.

 

[Tracs]

Tracs what is your friend doing with his 6kw boiler he is taking out? If it's priced right it may still be worth a try. I would get a quote for upgrading your electrical system for the house/garage as well.

I asked him to check with the plumber what he will sell the used 6kw boiler for. An electrician threw out $2000 for upgrading to 200 amp service. That is a cost I don't feel like paying for a house I don't plan on staying in forever.

 

[BadgerBoilerMN]

inspectapedia. Seriously.

The articles are of limited use and do not address what the builder was trying to do. In fact, had they looked into the long proven and still used frost protected shallow foundation they would not have installed tube in the footing, who would, and they would know that the depth of the tubing has more to do with the depth of the slab and the specific application than output. Once you have enough your done.

Tube spacing, design water temperature, the heat load based on specifications.

If the foundation were properly insulated, yes on the outside since most of the cold climates in N. America don't have termites, this would have been a different "study". As it is, it simply confusing bad construction with questionable radiant design.

The second article discusses, superficially, the use of polyisocyanurate underground. I say read the manufacturers' specs. Then some reference to a study of XPS as though it were not the current standard.

The last article shows a picture of a well built form with chairs suspending wire and presumably PEX after all. Though over-kill in most climates and residential, light commercial applications it does show professionalism. As for insulation under the slab, it depends on the inside design temperature.

Finally, neither the slab or the ground are "unlimited heat sinks" since all but the wettest ground will resist heat flux and will reach equilibrium at some point pushing heat back into the cooler space above. The main reason for sub-slab insulation is for to control response time.

Here in Minneapolis the year round ground temperature is 47°. Should you want to keep a storage building above freezing then the design water temperature for your radiant slab might be as low as 40°F, that is, below the ground temperature. It all comes down to insulation and doing the proper heat load up front instead of after-the-fact.

I would be surprised if your heat load exceeds 20 mbuth. This doing hundreds of loads here and all over Canada.

Keep in mind, the heat load represents less than 3% of the year, a few days. If you size the boiler properly and set and forget the thermostat, the boiler should keep up with design.

If you intend to let the building, slab and ground below drop 20° between weekends you should:

Raise the tube in the slab.

Insulate more under the slag--marginally.

Get an over-size boiler for the pickup energy you will require.

There is generally no penalty in over-sizing an electric boiler and if you need it for a few days you might consider taking them off.

Here is a link to a more relevant article.

http://www.finehomebuilding.com/des...ails/frost-protected-shallow-foundations.aspx

 

[Tracs]

The Manual 'J' heat load for my 40 x 60 modified pole barn is 30,000 btuh.

Of course this is anecdotal, like the rest of this thread.

Every sound radiant floor systems starts with an ACCA Manual 'J' heat load analysis. It is required precisely because of the different climates, building locations, standards and requirements of the occupants.

Once insulated, in any way, the slab slab itself represents one of the lowest loads as a percentage of the whole. This is why the blanket "insulation" crowd get by selling garage bags.

The perimeter load can be significant if not detailed properly but heat load is calculated by surface area x temperature. As you can see the perimeter surface area of this slab would be a small percentage of the total.

First, I don't think you can get anything in a garage this size that would warrant a 6" slab.

Second, knob and tube insulation is a real time saver it that is what you are after. But I wouldn't pay for a knob at the expense of the R-10 minimum we generally use in the 'field' of the slab. The perimeter should be closer to 20.

Third, the garage door represents one of the most significant loads, more especially if is not well sealed and maintained. Mine is R-19, which is much closer than the imagined R-19 required by local building codes once the thermal bridging of R-4.5 studs and infiltration is accounted for.

Finally, you will not have the infiltration or fenestration (loss around window) since you don't have many. These facts confuse many, professionals and novices alike.

Get a real heat load. Get a good tight door. Get some more insulation under and more important, at the perimeter of the slab.

And don't forget outdoor reset, lowers the fuel bill no matter what the fuel source.

I wouldn't even know where to get manual J heat load done.

The pad is built. There is no changing the details now. No way to add insulation under the pad. The perimeter rigid insulation was placed inside the forms before pouring the concrete and had plastic anchors installed. The R10 perimeter insulation is secured tightly. I might be able to double up the perimeter as it is not back filled yet.

The 2-9'x8' garage doors are R-16 value and will be installed tight with the best weatherstripping.

 

[Whiskydink]

Hey Tracs, my garage is nearly identical to yours except I have 2 - 18 x 72 triple pane windows, insulated the thickened edge and have 100amp in garage. Sized up for 7 or 8kw boiler. I went with the Leb8. For the most part I keep it at 10c but had it up to 20c for a week in Dec. while doing some painting and had no problem. The 7kw only pulls 4 amps more than the 6 and they both require the same size breaker and wire.

 

[mygarageone]

I wouldn't even know where to get manual J heat load done.

The pad is built. There is no changing the details now. No way to add insulation under the pad. The perimeter rigid insulation was placed inside the forms before pouring the concrete and had plastic anchors installed. The R10 perimeter insulation is secured tightly. I might be able to double up the perimeter as it is not back filled yet.

The 2-9'x8' garage doors are R-16 value and will be installed tight with the best weatherstripping.

There are on line calculators for heating and cooling.

 

[Tracs]

I thought I would post a update to this thread 1 year later.

Garage was finished, 6kw Concept Engineering Lion boiler installed last September. I set the air temperature thermostat at 63F and left it there. The system had no trouble keeping the room at that temperature even at the coldest -30C nights. I did some painting recently and had it up to 70F and it held no problem. Electrical bills have not been too extreme either. Also I waited since October to December for the blow in insulation guy to show up. So in those months I only had R20 in the ceiling and it still was toasty warm in there. On warmer days around 30F, daytime garage temps will even rise 2-4 degrees higher than thermostat setting. Nice little heat gain.

My friends garage as mentioned at the beginning of the thread, still can't keep the heat and will loose a few degrees on cold cold nights. His system must be serious junk.

 

[totaledmyGN]

Thanks for the update. I'm curious how much you think it costs you a month? I would think $125 a month wouldn't be too far off. Someday I hope to finish my garage and get some heat out there.

 

[Dick in Wisconsin]

Sounds like everything ended well.

Your $0.07/kWhr is pretty inexpensive.

Sad to hear about your friend. It would be interesting to hear about is wrong with his deal.

Isn't radiant heat nice? One of the things I've noticed is that my feet don't get cold working the shop anymore!

 

[BadgerBoilerMN]

You do know what anecdotal means?

I do proper Manual 'J' Heat loads and am licensed to install HVAC systems of all kinds. If you only trust what others have done, I can refer you to my own 40x60 pole barn. Radiant heat throughout with electric and propane boilers.

My heat loss is less than 30mbtuh with R-25 walls, R-50 ceiling and R-20 slab.

The heat load program models the building before construction--ideal--or as-built. Either way you are able to make an informed decision instead of trying to compare your situation with a raft of others.

What you describe is a common frost protected shallow foundation, which should be insulated from edge-to-edge including the area under the foundation. Builders want to put the concrete on the ground out of habit, not science.

In your case it will not likely change the heat load. Remember also that the boiler will only draw full amperage if you are at design conditions or set the thermostat back more than a couple degrees.

The rest of the year you will have extra amps to draw.

Finally, the LEB boiler appears to lack outdoor reset. You want this feature available from another Canadian manufacturer Thermolec.

 

[Tracs]

You do know what anecdotal means?

I do proper Manual 'J' Heat loads and am licensed to install HVAC systems of all kinds. If you only trust what others have done, I can refer you to my own 40x60 pole barn. Radiant heat throughout with electric and propane boilers.

My heat loss is less than 30mbtuh with R-25 walls, R-50 ceiling and R-20 slab.

The heat load program models the building before construction--ideal--or as-built. Either way you are able to make an informed decision instead of trying to compare your situation with a raft of others.

What you describe is a common frost protected shallow foundation, which should be insulated from edge-to-edge including the area under the foundation. Builders want to put the concrete on the ground out of habit, not science.

In your case it will not likely change the heat load. Remember also that the boiler will only draw full amperage if you are at design conditions or set the thermostat back more than a couple degrees.

The rest of the year you will have extra amps to draw.

Finally, the LEB boiler appears to lack outdoor reset. You want this feature available from another Canadian manufacturer Thermolec.

What I dont understand is why you bothered to post any of that....let me run out and buy a new boiler to replace the one that has been performing well for 5 months. You are spouting off like I can go back in time and change things. All I did was give an update of how things worked out.

 

[BadgerBoilerMN]

I do it for the benefit of others. Glad it all worked out.