Unit Heater Sizing

[2fst4u_7]

Just dried in my 40x80x16’ shop and was hoping to get some feedback on heater sizing. I purchased a 125k Mr Heater and every calculator I have looked shows it will be big enough but I am skeptical. Any feedback would be appreciated! Thanks

 

[WildBill]

Insulation type and amount? Concrete insulated?

 

[u3b3rg33k]

Insulation type and amount? Concrete insulated?

if it's insulated, the heater is probably oversized.

 

[Bert_]

More than enough. I would be thinking 80,000 off the cuff if you are leaving the heat on. Might do it with 60 if it's well sealed and insulated.

Have a 100,000 tube heater in a 40x100x16 and it's a little oversized in my opinion. Even when we have highs below zero it only runs ~70% of the day.

 

[2fst4u_7]

Insulation type and amount? Concrete insulated?

R21 walls, ceiling will be between r38-r49. Concrete is not insulated.

 

[Firebrick43]

The type of doors installed, size and type of sealing have. I have a neighbor with a building that size with a huge door and if its below 20 degrees and the wind is blowing from the west his 100k furnace isn't enough because of the wind going past the seals of the door when its pushed back

 

[2fst4u_7]

The type of doors installed, size and type of sealing have. I have a neighbor with a building that size with a huge door and if its below 20 degrees and the wind is blowing from the west his 100k furnace isn't enough because of the wind going past the seals of the door when its pushed back

That is good to know. Our cold is in 0-20 range and only a short time usually.

There is 2 12x14 r10 garage doors that are sealed up pretty well but I will keep an eye on it.

I forgot to mention there will a wood stove in the corner for weekends and extended projects.

 

[2fst4u_7]

And the 125,000 is only input, output is 100,000. In assume most btu calculators are assuming output btu.

 

[Bert_]

That is good to know. Our cold is in 0-20 range and only a short time usually.

There is 2 12x14 r10 garage doors that are sealed up pretty well but I will keep an eye on it.

I forgot to mention there will a wood stove in the corner for weekends and extended projects.

Honestly that's very similar to the building I mentioned earlier. I have (2) 12x14 insulated doors, insulation in the rest of the building is pretty much the same. I don't have any insulation under or on the edges of the concrete. I generally keep the heat at 50* and will turn it up to 55* if someone is spending much time in the shed.

I looked up the design temperature for your area and it looks like 9 degrees. In NW Iowa where I am I we design for -11.

Yes you are correct that most of the time people are speaking of input btu. If you do a load calculation it will tell you how much heat you need to put into the space, output btu.

 

[PoorUB]

A bit oversized considering the insulation and the part of the country you live in. I would put in a 100,000 BTU.

 

[u3b3rg33k]

Honestly that's very similar to the building I mentioned earlier. I have (2) 12x14 insulated doors, insulation in the rest of the building is pretty much the same. I don't have any insulation under or on the edges of the concrete. I generally keep the heat at 50* and will turn it up to 55* if someone is spending much time in the shed.

I looked up the design temperature for your area and it looks like 9 degrees. In NW Iowa where I am I we design for -11.

Yes you are correct that most of the time people are speaking of input btu. If you do a load calculation it will tell you how much heat you need to put into the space, output btu.

that's design temp for residential, which is 68F IIRC(?). so if you're designing to 55F, the BTU load is much lower. add back in opening an overhead door and doing calcs for a home brings you right back to right/oversized.

 

[2fst4u_7]

What’s your thought on mounting height? My first thought was somewhere around 14’?

 

[Bert_]

that's design temp for residential, which is 68F IIRC(?). so if you're designing to 55F, the BTU load is much lower. add back in opening an overhead door and doing calcs for a home brings you right back to right/oversized.

I was quoting outside temperature. The temperature outside my house is the same as the shop two blocks away.

You can use whatever indoor temperature you want if you're doing a calculation

 

[u3b3rg33k]

I was quoting outside temperature. The temperature outside my house is the same as the shop two blocks away.

You can use whatever indoor temperature you want if you're doing a calculation

I was referring to the outdoor design temps. the standard calcs are for a minimum indoor temp on said "Design day" e.g. 68F indoor at 100% duty cycle of the heating appliance.

I have yet to see a system that wasn't significantly oversized for the design day - which means everything is oversized for the other 99.99% of days - unless you have modulating equipment.

 

[Bert_]

I was referring to the outdoor design temps. the standard calcs are for a minimum indoor temp on said "Design day" e.g. 68F indoor at 100% duty cycle of the heating appliance.

I have yet to see a system that wasn't significantly oversized for the design day - which means everything is oversized for the other 99.99% of days - unless you have modulating equipment.

I'm not really sure what we are discussing. In NW Iowa I'm designing for an outdoor temperature of -11. If I want an indoor temperature of 55 then that's what I'll input. That's a difference of 66 degrees.

If I want to figure for a house I'm going to be using 68 or whatever. That's a difference of 79 degrees, will need more heat than raising 66.

 

[finn]

I have in floor radiant in the 32x75’ work space in my shop in the UP, but only use that to maintain a base temperature roughly 40 degrees.

To actually heat the shop to working temperature, I have a 75 k btu hanging heater. That size is more than adequate to get from, say, 42 degrees to 65 degrees in a half hour, or certainly less than an hour.

Something on the order of 100k btu should be more than adequate in the op’s case. If your unit heater is oversized, it’s going to short cycle, which is annoying.

FYI, in addition to that work area I also have radiant in the office and bathroom, another 250-300 sf. The 125k boiler we installed will heat the entire shop to working temperature, including the office and bathroom, easily in sub zero ambients. Fortunately, the turn down ratio of the boiler is pretty good, otherwise it would be oversized and subject to short cycling.

 

[u3b3rg33k]

I have in floor radiant in the 32x75’ work space in my shop in the UP, but only use that to maintain a base temperature roughly 40 degrees.

To actually heat the shop to working temperature, I have a 75 k btu hanging heater. That size is more than adequate to get from, say, 42 degrees to 65 degrees in a half hour, or certainly less than an hour.

Something on the order of 100k btu should be more than adequate in the op’s case. If your unit heater is oversized, it’s going to short cycle, which is annoying.

FYI, in addition to that work area I also have radiant in the office and bathroom, another 250-300 sf. The 125k boiler we installed will heat the entire shop to working temperature, including the office and bathroom, easily in sub zero ambients. Fortunately, the turn down ratio of the boiler is pretty good, otherwise it would be oversized and subject to short cycling.

I love my modcon. rinnai i120CN. min-fire is the same across the lineup so i went big for DHW reasons.

 

[finn]

I love my modcon. rinnai i120CN. min-fire is the same across the lineup so i went big for DHW reasons.

I think that’s why we oversized the boiler maybe seven or eight years ago, but I never got around to hooking up the domestic hot water loop. Maybe this summer, but that’s what I say every year.

 

[Bert_]

Here's why you don't want an oversized heater. Replaced this reznor last fall. Think they said it was 15 years old. Multiple rust holes in the heat exchanger.

 

[u3b3rg33k]

I think that’s why we oversized the boiler maybe seven or eight years ago, but I never got around to hooking up the domestic hot water loop. Maybe this summer, but that’s what I say every year.

the max reported BTU output from my FCU that i've seen has been been about 54kBTU/hr output, at 140F water temp in stage 2 (high airflow). add on a 2GPM shower for 60-70kBTU (with 40F incoming water or so), and there's still overhead for the dishwasher or washing machine.