Contractor Talk - Professional Construction and Remodeling Forum banner
1 - 11 of 11 Posts

·
Registered
Joined
·
400 Posts
Discussion Starter · #1 ·
I have a couple of hot-water unit heaters in my shop, and I am convinced that they would be more efficient if the fans ran slower. Much slower. I have been told that these motors are too big (1/2 HP) for a ceiling fan type controller. But there were these 2 baby VFD's left over after a job I was doing-also for 1/2 HP, and it set me to thinkin' and wonderin'.
 

·
Administrator
Maker of Fine Sawdust
Joined
·
55,706 Posts

·
diplomat
Joined
·
5,292 Posts
I hate it when people respond like this, but why do you think they will be more efficient?

I can assure you that virtually 100% of the electricity is turning into heat and 100% of the delta T of the water is also going into the room as heat.
 

·
Registered
Joined
·
400 Posts
Discussion Starter · #4 ·
Thanks to Leo G for that link-that was helpful reading.

And Golden View-maybe I phrased it wrong. I think a slower fan would more efficiently use the heat available. Reason being is 1) these unit heaters are designed for steam and 2) I only have a propane-fired water heater that on a good day only produces about 110-120 degree fluid. I believe that the high speed of the fan blows the air past the coils too fast to gather what little heat there is. I believe if the air was moving slower, it would heat up more. Maybe not, but when I choke it off with a piece of cardboard or something, it puts out more heat than just letting it rip full flow.

All this stuff has been salvaged from various jobs over the years, so I'm not really out anything either way, and between this system and my solar system, I can get my shop up to 60-65 when it's zero outside (as long as the sun is shining)
 

·
Capra Aegagrus
Remodeler
Joined
·
25,220 Posts
I believe that the high speed of the fan blows the air past the coils too fast to gather what little heat there is.
Imagine that you're about to take a bite of a really hot cheezy pizza. When you blow on it to cool it, do you use a gentle zephyr of open-mouthed exhalation, or do you purse your lips and give it a fair blast?

The higher speed is going to do a faster job of getting the heat out of the coils, but your perception is the opposite because you're feeling more draft. :thumbsup:
 

·
Administrator
Maker of Fine Sawdust
Joined
·
55,706 Posts
The faster you blow the cooler the air will be coming off the coils. Slower air flow will result in higher air temps off the coil
You will still be outputting the Same amount of BTUs
 

·
Contractor of the Month
Joined
·
26,075 Posts
Imagine that you're about to take a bite of a really hot cheezy pizza. When you blow on it to cool it, do you use a gentle zephyr of open-mouthed exhalation, or do you purse your lips and give it a fair blast?

The higher speed is going to do a faster job of getting the heat out of the coils, but your perception is the opposite because you're feeling more draft. :thumbsup:
This is an excellent analogy.
 

·
diplomat
Joined
·
5,292 Posts
You will always get more BTUs out with higher flow even though the air is cooler.

However it might be well past the point of diminishing returns with such cool water temps. Maybe there are easier options than VFDs. Modify the fan so the pitch is less. Clip the blades so they are shorter. Or replace with a lower RPM motor. You won't heat the room more, but there will be less draft, and with a slower motor at least, less noise.
 

·
Read Only
Joined
·
16,993 Posts
Imagine that you're about to take a bite of a really hot cheezy pizza. When you blow on it to cool it, do you use a gentle zephyr of open-mouthed exhalation, or do you purse your lips and give it a fair blast?

The higher speed is going to do a faster job of getting the heat out of the coils, but your perception is the opposite because you're feeling more draft. :thumbsup:

How I miss the zephyrs....
 

·
Registered
Joined
·
400 Posts
Discussion Starter · #11 ·
OK. You've convinced me. I need more BTU's, and that means hotter water. I purposely did not run natural gas when I built my shop, because I know myself and I would have a 80 degree shop all winter and a $200 a month gas bill just for weekend projects. And gas heat is way cheaper than electric heat, even if I had the amperage for it.

And since my original question has been answered, the electrical forum is not the place for me to continue to try to figure out how to get hotter fluid.

Thanks, all.
 
1 - 11 of 11 Posts
Top