Heater Size Up or Down?

[Showkey]

^^^^^^^^^^
Heating any building to a given temperature takes a certain amount of BTUs.
Holding the heat in.......is a function of the insulation and air exchange rate.

So as far as run time to heat and bankruptcy:

120,000 btu for one hour
60,000 btu for two hours
30,000 btu for four hours

Cost about the same in fuel. Electricity varies slightly because for fan on time but slightly because a larger unit will likely have a larger fan motors.

Bankruptcy is a BIG heater and ( no) poor insulation or leaving the door open in the winter.

Run a reasonable sized heater and insulate and seal the building.
Your other conclusions are spot on with seal burners, in floor, constant heat, higher cost of the units with more features.

 

[TwoToque]

I am certainly in agreement of needing the insulation and no leakage along with controlled air changes per hour. By controlled I mean an HRV or as some call them ERV unless of course you are needing large exhaust fans and MUA. I am by no means recommending a huge heater as that is unnecessary but I cannot personally get behind the notion that a heater should be sized small enough that it runs continuously to heat a space. I guess I am old school.

 

[Bert_]

Interesting read for sure. I will not offer up any heavy wisdom without doing more of my own research outside of this site. I will mention a few things though. For those who want the heater to run continuously you must be rich. That would bankrupt a working man. A few members have clued in to modulating vs non modulating. A newer house uses the technology of a modulating gas valve and variable speed blower motor to try and create a uniform feel of heat. This is rarely installed in garages in my opinion. The parts of these modulating furnaces can be expensive to replace when breakdowns occur. A unit heater is common to have a two stage gas valve which is not the same as a modulating valve. Some more food for thought: radiant heat is more efficient than forced air as it heats objects first then the air. It is also much slower at raising the temperature as they are more for maintaining a constant temperature. This is true of in floor hydronic radiant heat as well as a radiant tube heater. Another thing to consider is what you are planning to do in the space. Woodworking produces fine dust that is highly flammable. Metal dust is also highly flammable if it is a fine enough dust (too hard to get it fine enough from normal grinding). Painting produces highly flammable vapours. Unit heaters usually have an open burner but are availble as a sealed unit for twice as much.

You have got to be joking. You realize a 50,000 btu heater running constantly uses the same amount of fuel as a 100,000 btu running 50% of the time...

The smaller heater running longer will probably save some fuel since you lose efficiency with more starts, stops and short run times.

 

[yeldogt]

You have got to be joking. You realize a 50,000 btu heater running constantly uses the same amount of fuel as a 100,000 btu running 50% of the time...

The smaller heater running longer will probably save some fuel since you lose efficiency with more starts, stops and short run times.

When a forced air heater is running -- the space will be more comfortable. Try this is any shop at any temp ... heater running = more comfort.

What happens as you oversize the air heats and the heater shuts off ... you go from hot to cold. With a modulating heater it's able to match the load and run all the time.

It's really hard to figure out the cost difference doing one over the other in a well insulated shop ... my shop is a few hundred dollars over the season. Again it's hard to pull it out of the overall cost. For me the extra $150 is worth working in a nice temp when I want.

 

[Bert_]

When a forced air heater is running -- the space will be more comfortable. Try this is any shop at any temp ... heater running = more comfort.

What happens as you oversize the air heats and the heater shuts off ... you go from hot to cold. With a modulating heater it's able to match the load and run all the time.

It's really hard to figure out the cost difference doing one over the other in a well insulated shop ... my shop is a few hundred dollars over the season. Again it's hard to pull it out of the overall cost. For me the extra $150 is worth working in a nice temp when I want.

Yes. Small heater running often is more comfortable.

The only positive thing about an oversized heater is quick recovery from a large setback. This is one reason why I really like overhead radiant. You feel the heat as soon as it kicks on. Doesn't matter as much that the rest of the building is still 40-50 degrees.

 

[TwoToque]

You have got to be joking. You realize a 50,000 btu heater running constantly uses the same amount of fuel as a 100,000 btu running 50% of the time...

The smaller heater running longer will probably save some fuel since you lose efficiency with more starts, stops and short run times.

For some reason my first response did not post because I unknowingly got logged out. Here is a shorter replacement. No Bert I am not joking but your numbers are correct. The other posts were comparing 45,000 BTU running for a 7.5 hours a day (the boss is out of town lol) vs 75,000 BTU running 5 minutes every 45 minutes. This means the 75,000 BTU is running 50 minutes in the same 7.5 hours. I think that is cheaper which is why I cried bankrupt with 7.5 hours runtime. I did some research and yes modulating unit heaters do exist so it is possibly for the one I found (50,000 BTU 3:1 turn down) to run at 16,667 BTU to maintain the temp for 7.5 hours which would be affordable. I personally prefer the radiant tube heater which I bought for my shop. It is 45,000 BTU and better not run for 7.5 hours straight. I fully support insulation and no air leakage which is why I think a heater should not run continuously unless it is a modulating unit.

 

[yeldogt]

Yes. Small heater running often is more comfortable.

The only positive thing about an oversized heater is quick recovery from a large setback. This is one reason why I really like overhead radiant. You feel the heat as soon as it kicks on. Doesn't matter as much that the rest of the building is still 40-50 degrees.

The same can be said with floor -- more tubing/ closer spacing allows faster response. This is very true with thin slab wet bed tile. It will heat faster than people think ...

I have an electric overhead radiant on a patio -- great for that application

 

[Bert_]

For some reason my first response did not post because I unknowingly got logged out. Here is a shorter replacement. No Bert I am not joking but your numbers are correct. The other posts were comparing 45,000 BTU running for a 7.5 hours a day (the boss is out of town lol) vs 75,000 BTU running 5 minutes every 45 minutes. This means the 75,000 BTU is running 50 minutes in the same 7.5 hours. I think that is cheaper which is why I cried bankrupt with 7.5 hours runtime. I did some research and yes modulating unit heaters do exist so it is possibly for the one I found (50,000 BTU 3:1 turn down) to run at 16,667 BTU to maintain the temp for 7.5 hours which would be affordable. I personally prefer the radiant tube heater which I bought for my shop. It is 45,000 BTU and better not run for 7.5 hours straight. I fully support insulation and no air leakage which is why I think a heater should not run continuously unless it is a modulating unit.

It doesn't work that way. It takes the same amount of heat (BTU) to heat a building no matter the size of the heater. Aka a small heater running a lot of a large heater running a little.

In your example, if the 45000 needed to run constantly to maintain temp then the 75,000 would run 27 minutes out of every 45 minutes to maintain the same temperature. The two heaters would consume a similar amount of fuel. There is no free lunch.

Alternatively if the 75000 only ran 5 minutes out of every 45 then the 45000 would run a little over 8 minutes out of every 45 minutes. Both heaters are oversized if this example takes place on the coldest day.

 

[TwoToque]

Sounds like rule of thumb design. X sq ft = X size heater regardless of building material, insulation, outdoor temperature, height of sun maximum solar load time of the day (used more for heat load for cooling applications). The numbers in the above example were presented by other posters earlier in this thread not mine. A BTU is the imperial measurement of the amount of energy to raise the temperature of 1 pound of water 1 degree fahrenheit. I would design and build a building such that it should almost never need heat to keep warm. Designing a building that requires a heater to burn continuously ( in this case the thread is saying 45,000 BTU) would bankrupt me to pay that bill. If it is a modulating heater maybe a break can be caught when it turns down to only 16,000 BTU. To get the building to the desired temperature will require heat. The amount of BTU's will determine how long it will take. All of the design factors I considered are perfect lab conditions. Poor insulation and or air leakage will ruin all else and too small of a heater will never get to the desired set point in that case. These are my final thoughts.

 

[Bert_]

Come on man do you even read these posts...

You do a heat load calculation. As an example it tells you the building needs 20000 BTU on the coldest day of the year to maintain 65*.

I don't care what size heater you put in it is going to take a certain amount of fuel to maintain the temperature no matter if it's a 20000 BTU heater or 120000 BTU. the more oversized the heater is the less efficient it's going to be and the less comfortable the space is going to be.

An oversized heater wastes fuel it makes the space uncomfortable. A big heater absolutely will not save you money just because it runs less.

Designing a building that requires a heater to burn continuously ( in this case the thread is saying 45,000 BTU) would bankrupt me to pay that bill.

This has got to be the absolute stupidest thing I've ever read. Nobody even remotely suggested this. the whole point of this conversation is that you should pick the smallest heater that will maintain temp in the building. that means a heater that will run nearly continuous on the coldest date of the year. Sometimes in small buildings and it's not always possible to get a heater that is sized correctly. But not many garages need a 75000 + BTU heater.

 

[TwoToque]

This site works different from another one I have used and I keep getting logged out. This is my second attempt at a reply. There has never been a mention of coldest day of the year in this thread. When I did the math for my shop it says I needed 105,000 BTU for a 720 sq ft shop 10 foot walls. I know this is not needed because my house only has a 68,000 BTU. My coldest day is -40C and I want to maintain 20-25C on some days for the type of work I am doing. Painting and staining stuff. I bought a 2 stage 45,000 BTU tube heater which will likely run constant on the coldest day of the year but during average seasonal temps that can be from -15C to -23C it better turn off for a while for me or I will go broke. -15C and running constant is a bad deal for me. A 20,000 BTU heater won't get it done for me during the cold months. Gas is expensive in Canada but cheap in the Mid West USA from what I hear. Yes I have chatted with someone from SE Iowa. It has been a fun chat hope to see you at the next thread.

 

[Bert_]

My coldest day is -40C and I want to maintain 20-25C on some days for the type of work I am doing. Painting and staining stuff. I bought a 2 stage 45,000 BTU tube heater which will likely run constant on the coldest day of the year.

Then we agree. You picked a heater that is sized right., as small as possible for the building. The earlier posts really sounded like you were against this.

All heating calculations are based on the coldest day of the year.

 

[TwoToque]

Yes we agree and I am sure we will get along fine. Happy to of met you here. I hope the original poster understands the coldest day of the year part as it was never mentioned. My heater is still on the larger size but so is my insulation. R24 in 7 inch thick walls and I will have a minimum of R40 in the ceiling. I will likely go more there too. Once it gets to temp the first time I hope it stays off for a couple of hours on average cold days. Only time and more money spent will tell.

 

[walrus]

. Once it gets to temp the first time I hope it stays off for a couple of hours on average cold days. Only time and more money spent will tell.

It won't stay off if there are no other sources of heat.

 

[yeldogt]

Yes we agree and I am sure we will get along fine. Happy to of met you here. I hope the original poster understands the coldest day of the year part as it was never mentioned. My heater is still on the larger size but so is my insulation. R24 in 7 inch thick walls and I will have a minimum of R40 in the ceiling. I will likely go more there too. Once it gets to temp the first time I hope it stays off for a couple of hours on average cold days. Only time and more money spent will tell.

The greater the delta -- the more insulation comes into play. With your very cold temps -- the buildings thermal envelope becomes the most important item in the formula. You have a different situation ... in my case it can get into the single temps ... many years we don't see any. But, you have to have a plan.

 

[toyotadriver]

I am certainly in agreement of needing the insulation and no leakage along with controlled air changes per hour. By controlled I mean an HRV or as some call them ERV unless of course you are needing large exhaust fans and MUA. I am by no means recommending a huge heater as that is unnecessary but I cannot personally get behind the notion that a heater should be sized small enough that it runs continuously to heat a space. I guess I am old school.

You don't need a HRV/ERV for a shop. If your shop has a garage door then there's plenty of fresh air coming in even if properly sealed because it's essentially impossible to seal a garage door. As soon as you open a garage door, your heat starts rapidly escaping and air exchanges quickly.

 

[TwoToque]

You don't need a HRV/ERV for a shop. If your shop has a garage door then there's plenty of fresh air coming in even if properly sealed because it's essentially impossible to seal a garage door. As soon as you open a garage door, your heat starts rapidly escaping and air exchanges quickly.

I was merely trying to cover all angles. Excessive welding smoke or rattle can paint jobs on cars requires fresh air. Opening the garage door drops the temp too much in the winter and will ruin the paint job. I highly doubt most residential garages would need mechanical air changes for typical hobby usage.

 

[58Yeoman]

I've got a 75k Big Maxx in my 24x40 pole building. It has 6" batts in the ceiling and 1 1/2" styrofoam in the walls covered with osb, one 8' door and one 16' door, both insulated steel, and one man door with two small slider windows. I don't heat it all the time, just when I'm working out there. I usually heat it up and turn it off. But, yesterday, I was working out there quite a while and noticed the short cycling, never noticed that before. I guess I should've gone with the 45k.