[hunter63]

I was asked a question about plugging off two of four burners on a four burner furnace. (propane) to cut down the btu's (over-sized).

I guess I couldn't really find a problem with that, BUT:
Would the other burners receive too much gas? Bad burn? (check pressure at orifice?)
Uneven temps on heat exchanger cause cracking?

Been involved in HVAC for a while, but never really thought about it.
Any manufacturer will tell you nothing or they will hang up on you.

Any ideas???

 

[Windy in Kansas]

I would be more concerned about cubic foot per minute of air flow for the remaining burners than too much gas. The gas pressure is regulated so won't change, meaning plugged off orifices will not force more gas to the others.

If you plugged off outer burners and left the heating in the middle I don't think, but don't know, that the heat exchanger would give problems.

With burners plugged off and the fan pushing normal flow will the efficiency of the furnace drop?

Good questions and my answers are only speculation.

 

[Nevada]

Would the other burners receive too much gas? Bad burn? (check pressure at orifice?)

The orifice back pressure is controlled by an upstream reducing regulator. Blocking off other burners won't increase the back pressure, because it's maintained by the regulator. The flame height should not be effected.

Uneven temps on heat exchanger cause cracking?

While that's a possibility, and the temperatures will certainly not be as uniform as they were, I think it's unlikely that the heat exchanger would fail as a result.

 

[hunter63]

I would be more concerned about cubic foot per minute of air flow for the remaining burners than too much gas. The gas pressure is regulated so won't change, meaning plugged off orifices will not force more gas to the others.

If you plugged off outer burners and left the heating in the middle I don't think, but don't know, that the heat exchanger would give problems.

With burners plugged off and the fan pushing normal flow will the efficiency of the furnace drop?

Good questions and my answers are only speculation.

Yeah, I hear you, as furnaces are designed to operate between a set of temperatures (heat rise). Example 90-185 degrees w/limit switch

The furnace in question is an older (free) 80%-er no vent/inducer motors to deal with (no pressure switches), just a high limit/fan control.

I haven't looked that close at it, but heat generally runs on "low speed" or "med low" and being older I don't think there are many choices as far as slowing down the blower (variable speed motor).

Neighbor (also a sheet metal guy) putting it in his garage, and has asked around to the various service guys he works with and still no consensus.

He is going to try it, so I guess we'll find out pretty soon.

 

[farminghandyman]

I would suggest to turn the fan on to continuous run if it is a only use while you are working in the shop, when I run mine in the shop and it is cold the fan switch kicks on and runs for a few Min's and it cools off the heat exchanger so fast, that it shuts down the fan, thus the fan short cycles and all it is doing is heating the exchanger up and cooling it off, by turning it on continuously it takes the heat out and into the room until it can maintain the heat and the furnace cycles,

the make two stage furnaces now that use the valve to (guessing change pressure) to change the BTU input,

I do not think it would hurt any thing, (besides possibly stress the heat exchanger), but in the long run I do not know if it would help efficiency, my guess is if the fan is not continuous run that it would take longer to heat up the chamber to kick on the fan,
thus lower efficiency, (yes I know the furnace that is sized to heat loss that runs continuously is in theory the most efficient), but until is extremely over sized, I do not think you will improve it any by plugging off burners or orifices, if it extremely over sized, may be look in to the wall mount vent-less heaters, I have two thermostatic controlled units in the back rooms of the barn and (I like the infrared over the blue flame), there up to about 30,000 BTU,

I do not think you would do any damage to the furnace by plugging off the burners, but I do not think it would help you any either, (say it is a 100,000 unit and you plug off halve of the burners orifices, to make it a say a 50,000 input), but since ever other one of the heat exchangers vents are now venting cold air up it, I jsut do not not think the efficiency would be there, I think it would drop considerable, and if it was only 70% to start with it may drop to less than 50%, I think I would rather have it cycle, or find another furnace that is more closely matched. than try to alter, it. (altering it would lead to insurance problems as well if any thing ever happened),

 

[hunter63]

FHM, that was my suggestion to the guy, but he got it for free, so you know.

I don't think it will make much difference in gas use if he was to just leave it as is.
A BTU is a BTU. Load is a load, doesn't matter how you get the load satisfied, still going to require the same amount of fuel.

As I said, there isn't a manufacture in the world that will give you an answer to a question like this, for all the right reasons, so I thought I would just put it out there.

I heat my garage/shop with wood, gives even heat (for the most part), and just heat it when out there.
These guys want to keep it heated all the time.

Another friend uses a propane furnace, but you can really tell when the thing will cycle on, (gets chilly). has T-stat by the door, bad location as far as I'm concerned.

Still another uses an electric "milk house heater" (should say used to use), as his electric was about $300 buck per month last winter. His wife about shot him.

His line of thinking was that it was just a "small heater", and he had spent a lot on insulation.
Also had moisture problems as the temp didn't get very high, too much of a load for the size of the unit.

So thanks, guys it seems that we all concur that it probably ( key word being "probably") won't "hurt anything, but won't make much difference either.

 

[Nevada]

has T-stat by the door, bad location as far as I'm concerned.

That's nothing. I knew some people who bought a grandfather clock and wanted to put it where the thermostat was. They hired a handyman to move the thermostat around to the backside of the wall. Unfortunately the backside of the wall was in a closet.

:stars: