Zinger ZT33BH Furnace High Limit Cycling Fix (with pics)

Gdetrailer wrote:
Dead wrong.

Get one of those "smoke sticks" used for testing smoke alarms.

My money is on ALL (or nearly all) the air coming from the new hot vent will be completely sucked into the return vent.

This is a good idea. Now to find somewhere to buy them...

Gdetrailer wrote:
At the same time by reducing the intake size you increase the velocity of air coming into the return vent. Restricting the inlet size will increase the over all temp withing the furnace and cabinet.

The old cold air return has an area of roughly 30x24, but is 50% obstructed with the horizontal slats. That gives a return air area of 360. I suppose I have cut the cold air return in about half, but I am still well above (three times) the minimum requirements of the furnace, and still greater (about double) than the "optimum" size listed in the manual. I could add additional cold air return area easily enough though.

Gdetrailer wrote:
I would have looked for another place to add a vent and done it correctly.

See my previous post where I asked where folks here would suggest. Here's the floor plan: clicky. The furnace is located under the pantry. A few other ideas that surfaced were to duct the third one in to the floor as well, and cut additional floor registers in the main living area. I dismissed that idea as being too large a task for myself. So, barring that and barring the idea of ducting it to the bunkhouse, what are your thoughts?

Gdetrailer wrote:
You are more than welcome to ignore any and all good advice :E

I haven't ignored your advice...yet... ;-)

This tidbit in the manual was also interesting:

Use of 2? ducts does not count toward achieving minimum discharge
requirements. Ducting in dead air space with no return air, such as
holding tank areas, does not count toward achieving minimum discharge
requirements.

I am almost certain my ductwork in the floor is 2". In that case, I'm still not meeting the minimum requirements.

Another tidbit is this:

DO NOT install floor registers within 2 feet of return air openings

In my case, I have no floor registers anywhere near the furnace, and I cannot find anywhere else in the manual where it says not to supply hot air near the cold air return (Except when using in-floor hard ducting).

My personal opinion on the matter is that because the amount of air flowing out of a register is not very forceful, the furnace can pull that heated air directly back in to the furnace. In the case of my installation, the air coming out of the duct above the cold air return moves up and away at a rate faster than the furnace draws fresh air in.

Either way, it may be a good idea to rebuild my access panel as I mentioned above, with the cold air return vertically on the left and the supply over on the right to at least satisfy the 2' "in floor ducting" rule.

Other than ducting into the bunkhouse, which I've already stated is not an idea I'm willing to explore, where *else* would you suggest I add supply?

bmupton wrote:

I am still wayne_tw wrote:
You may still have an air flow issue as you have severely restricted the intake air volume from its original capacity.

The manual for the furnace states that the minimum requirement for cold air return is 65 square inches (80 is optimal) and that ducts should not be located within 2' of cold air return.

The only one of those things I am not following is the 2' to the cold air return from a duct. My cold air return is 192 square inches. And in this orientation it is blowing the air up and away while drawing air from the floor. It's not technically 2' away, but it appears to be working.

The solution from Crossroads was to cut another vent through the wall in to the bunkhouse and duct in there. Let me tell you, the bunkhouse was already a "hot zone" when the furnace was running. Ducting another vent in there would add hot air there and remove even more from the front of the trailer. When I say "hot zone" I mean the trailer would be a chilly 20 degrees, and the bunkhouse would be 30, much too uncomfortable for the kids to stay in there.

The fact of the matter is, I ran the furnace for close to half an hour, raising the temperature from around 20 degrees up to 28 and the furnace did not short cycle at all. The hot air being supplied through my new duct was being blown up and away from the cold air return, and it was drawing air off the floor. Prior to this, the furnace would run *maybe* three minutes before it was short cycling. And once it started short cycling, it would only run about a minute between cycles.

I suppose an alternative design would be to place the cold air return vertically on the left side of that panel, and the supply duct over on the right, which would work out to better than 2' away. I'm not sure how much of a difference that would make overall however.

Also, it may not seem like it, but I do appreciate the criticism.

Dead wrong.

Get one of those "smoke sticks" used for testing smoke alarms.

My money is on ALL (or nearly all) the air coming from the new hot vent will be completely sucked into the return vent.

At the same time by reducing the intake size you increase the velocity of air coming into the return vent. Restricting the inlet size will increase the over all temp withing the furnace and cabinet.

I would have looked for another place to add a vent and done it correctly.

You are more than welcome to ignore any and all good advice :E

I am still wayne_tw wrote:
You may still have an air flow issue as you have severely restricted the intake air volume from its original capacity.

The manual for the furnace states that the minimum requirement for cold air return is 65 square inches (80 is optimal) and that ducts should not be located within 2' of cold air return.

The only one of those things I am not following is the 2' to the cold air return from a duct. My cold air return is 192 square inches. And in this orientation it is blowing the air up and away while drawing air from the floor. It's not technically 2' away, but it appears to be working.

The solution from Crossroads was to cut another vent through the wall in to the bunkhouse and duct in there. Let me tell you, the bunkhouse was already a "hot zone" when the furnace was running. Ducting another vent in there would add hot air there and remove even more from the front of the trailer. When I say "hot zone" I mean the trailer would be a chilly 20 degrees, and the bunkhouse would be 30, much too uncomfortable for the kids to stay in there.

The fact of the matter is, I ran the furnace for close to half an hour, raising the temperature from around 20 degrees up to 28 and the furnace did not short cycle at all. The hot air being supplied through my new duct was being blown up and away from the cold air return, and it was drawing air off the floor. Prior to this, the furnace would run *maybe* three minutes before it was short cycling. And once it started short cycling, it would only run about a minute between cycles.

I suppose an alternative design would be to place the cold air return vertically on the left side of that panel, and the supply duct over on the right, which would work out to better than 2' away. I'm not sure how much of a difference that would make overall however.

Also, it may not seem like it, but I do appreciate the criticism.

ScottG wrote:

I am still wayne_tw wrote:
You may still have an air flow issue as you have severely restricted the intake air volume from its original capacity.

Thats what I'm thinking plus now it's ingesting its own heated air.

X2

1 Return intake vent NEEDS to be the same size as the "original" opening.

2 The furnace will now pull the heated air from the "new" output vent right back into the furnace.

While you may have "corrected" the over temp for now you most likely will have it again and at the same time will use more fuel and battery to do the job.

To correct both problems you need to do the following..

1 Put original return ven back into place.

2 Locate a NEW place for the output air vent SEVERAL FEET away from the return air vent.

3 READ THE MANUAL! The furnace manufacturers actually put the correct ducting info into the manual for how to install. It is specific for the BTU SIZE of the furnace.

Make sure that your furnace TRULY "needs" three (or more)duct connections. Otherwise you could be hiding a serious issue with your furnace.

This is not the place to play around and hope you fixed it, a overheating furnace is a fire hazard and needs to be addressed properly.

I am still wayne_tw wrote:
You may still have an air flow issue as you have severely restricted the intake air volume from its original capacity.

Thats what I'm thinking plus now it's ingesting its own heated air.

You may still have an air flow issue as you have severely restricted the intake air volume from its original capacity.