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Author Topic: Unit heater vs coil in furnace and sq ft calculation  (Read 2347 times)

shawbran

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Unit heater vs coil in furnace and sq ft calculation
« on: July 02, 2018, 09:58:06 PM »

Currently have a old furnance in the garage and it has a heat exchanger installed in it.  Looking at getting rid of it and installing a hydronic unit heater.   What’s your thoughts on both.

Also how do you size a unit heater?  How many BTU per square ft
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E Yoder

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Re: Unit heater vs coil in furnace and sq ft calculation
« Reply #1 on: July 03, 2018, 06:55:03 AM »

How many cfm's is the old furnace, and what size heat exchanger? How did it heat?  I think typically unit heaters aren't rated by CFM, so it can be hard to compare, but knowing the old equipment can can give an idea of what your heating load is.
Square footage is a place to start, but other factors weigh in. How is it insulated, how many garage doors (air infiltration), how high is the ceiling?
I've found unit heaters to be compact and low cost. But sonewhat noisy and don't push hot air down near the floor if they're mounted too high.
« Last Edit: July 03, 2018, 07:19:34 AM by E Yoder »
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shawbran

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Re: Unit heater vs coil in furnace and sq ft calculation
« Reply #2 on: July 03, 2018, 10:21:52 AM »

I actually have a 2 part garage, both have 8ft ceilings and are full insulated.  Both have 2 big garage doors, and 1 small door.  One garage's big door opens in to the other section only.  I'm going to look at installing two units.  One for where we park our vehicles and where the dog sleeps.  The other will be the workshop, it only needs to be heated in the evenings or during the weekends. 

I have no clue on the old furnace as I pushed it just to place the heat exchanger in.  Only reason I'm replacing is to more adequately heat the parking section of the garage.  Right now I have to heat both section just for the parking section to be heated and it, burns a lot of wood doing that.  I'm going to replace with individual unit heaters so I can zone it better.

One garage section is 30x48 the other is 30x36. 

I was looking a just buying two 100k BTU ones but that might be over kill.


Thanks for the help
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E Yoder

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Re: Unit heater vs coil in furnace and sq ft calculation
« Reply #3 on: July 03, 2018, 11:28:49 AM »

So that comes out to 40 btu's per square foot. Probably overkill but the flip side is it'll heat up quickly if you want to use on an occasional basis. There's only about $80 difference on eBay between a 50k heater and a 100k.
I think you're on the right track.  :thumbup:
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mlappin

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Re: Unit heater vs coil in furnace and sq ft calculation
« Reply #4 on: July 03, 2018, 11:36:06 AM »

Thats what I did in my shop, overkill that is, (2) 50,000 and (1) 100,000 BTU heaters, leave it at 40 at night or when we ain't working, doesn’t take long to get to 60-65 when we do work in the shop.
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shawbran

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Re: Unit heater vs coil in furnace and sq ft calculation
« Reply #5 on: July 03, 2018, 01:05:14 PM »

Going to install on line voltage thermostat  Especially the one garage,  where refrigerators don't like to work in the cold, we lost a lot of food last year because of that.  I plan on keeping one garage at 50 degrees.

So would you guys put 100K in both or 100K in one and 50K in the one,  the one with the 50K is the smaller garage on will be the one that keeps the parking area and dog warm!!


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E Yoder

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Re: Unit heater vs coil in furnace and sq ft calculation
« Reply #6 on: July 03, 2018, 02:01:39 PM »

Oops, I said 40 btu's per sq ft but it's 80 with two 100k heaters. I think the 50k will heat the smaller side fine, but you'll hear it running steady out there more. I'd probably spend a tad more to be able to bump up the temp quickly. But that's just personal preference.
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wreckit87

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Re: Unit heater vs coil in furnace and sq ft calculation
« Reply #7 on: July 03, 2018, 02:25:30 PM »

Nothing wrong with oversizing them aside from the added noise that often comes with a bigger unit. Your ceilings are low so that helps, but I think 100k in both sides would be just dandy. I run a 200k in my 40x40 and a 50k in my 20x40, the 50k runs a lot but is nice and quiet. The 200k runs for 10 minutes and will raise it from 50 to 80, both sides have a 14ft ceiling. I prefer the bigger units myself.
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greasemonkoid

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Re: Unit heater vs coil in furnace and sq ft calculation
« Reply #8 on: July 03, 2018, 06:05:06 PM »

You can always throttle a large unit down, but not the other way around. I'm a proponent of oversized units and then reduce the fan rpm to quiet the thing down, saves a bit on power consumption as well.

Maybe that's just my cockeyed philosophy though.

The motors on some of those things, sheesh, they're power hogs for their size.
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