# Sizing water-to-air hydronic forced air heater for pole building?



## MNBobcat (Nov 17, 2011)

Hi Guys,

I have a 40 x 76 pole building with 12 foot ceilings. I just had pole building metal installed on the ceiling and insulated with R44 cellulose and vapor barrier. The walls have +R19 foam. The building is very well insulated. There are three 16 foot wide garage doors which I'll be insulating with foam panels. One utility door. No windows.

My goal is to be able to keep the building at perhaps 40 degrees when not used. And then be able to turn on the heat and warm it to 70 degrees in about a half hour to an hour.

I have an outdoor wood boiler that I'll be using to heat it. It will pump 185F water to the heat exchanger(s). Originally I was thinking of putting in two water-to-air hanging heaters (the kind with a fan behind the HX radiator). I was thinking one on each end of the building.

The place where I buy from has 140K heaters in stock. They can get 220K heaters if they order them.

What I don't know is if I'll need two heaters or if one will be enough. The foam insulating company said it won't take anything to heat the building and one heater will be plenty. But I'd like to hear some other opinions.

I can go with two 140K heaters, two 220k heaters, one 220k heater or one 140k heater. Obviously I don't want to waste money and buy more than I need.

I'm in Minnesota so -10F to -20F temps are not uncommon.

What do you guys think?


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## PassionForFire&Water (Nov 17, 2011)

40 x 76 = 3,040 SF
For a residence (7 to 8' high ceilings) not very well insulated I most of the time take 35BTU/sf to get an idea on heat demand.
So 3,040SF x 35 BTU/SF = 106,400 BTU/hr
12' high ceilings is 50% more volume, so this would be 103,400 x 1.5 = 160,000 BTU/hr.
It looks to me that with your ceiling insulation R44 and wall insulation R19 you should fall within 140,000 BTU/hr, but it may be border line.
At the other hand, it's better to have a smaller boiler perfoming at peak then a bigger boiler at medium burn.


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## rkusek (Nov 17, 2011)

Passion's numbers seem to follow my experience.  My Morton building is a little smaller  30x60 with 14' ceilings,  18'x12' insulated door, and 2 insulated walk in doors (one has a window).  I put R19 fiberglass in walls and R38 fiberglass in ceiling.  I wanted to put pex in floor but had time constraints in late Oct 08 that didn't allow for that.  It is very easy to heat this shed, with foam insulation I would guess yours should be easier.  I'm using a homemade water to air heater using an old outside home AC (actually heatpump) condenser unit from the junkyard.  I changed the fan to 120V but I could have just placed a simple cheap box fan on top since the fan actually turns too fast.  It can take all my EKO 40 can put out when it is running at full tilt probably at least 100kBTU.  A single 140k heater with constant 185* water would probably suffice but I can't imagine you would need more than a single 220k heater even in Minnesota.  If you were using gasser/storage that would have lower water temps it would make sense to use the 220k heater.  Last winter from December to February when the boiler was running every day it kept the shed around 60* without the need to even run the fan just circulating the water through the coils plus some heat off the boiler too (indoor) I guess.  You are going to love a 70* heated shop when it is below zero outside, nothing like working in short sleeves.  Pricewise, it might make sense to start with a single 140k on the end of the building that you use the most.  If that is not enough you could add another on the other end.  I'm guessing the 140k are probably quite a bit cheaper than the 220k.  I would plan on keeping the building at 60* as a minimum which is still pretty comfortable and you can work in there while you are waiting for it to heat up to 70*.  This might allow for you to get by with the single heater.  We have been down to 20* the last 2 nights and my shed was still 48* this morning and I haven't even ran the boiler yet this year.  The concrete must be still pulling heat out of the ground I guess.  Insulation makes all the difference.


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## MNBobcat (Nov 17, 2011)

Thanks guys.  I talked to the place where I buy my stuff and I decided on the 140K unit.  I'm going to install it tomorrow or Saturday but with a TEE in the line so that if I need to it will be easy to install a second one.  I'm pretty much planning that I'll need two units.  Since they plumb in series, a single 220K might take so much heat out of the line that the second unit wouldn't get much for heat anyway.  These guys say two 140K units can coexist pretty well.

What I also did was I just ordered  a WiFi thermostat from Home Depot for about a $100.   If I get up on a morning and I decide I want to work in the shop, I can get on my computer or IPAD and set the temp in the pole building so that it warms up while I'm having breakfast.    That, or I can remotely program the thermostat the night before to have the building warmed up.


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## heaterman (Nov 18, 2011)

A single 140 will be fine provided you always have 180*+ water temp available. The only reason to go with multiple units would be quicker recovery from setback, redundancy and/or the ability to use lower water temp if you ever switch to a storage type system.


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## in hot water (Nov 18, 2011)

You can nail down the actual heat load number with a software program or this hand form from the PP building dept.

www.pprbd.org/plancheck/Heat Loss Table.pdf


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