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Old 12-13-2018, 07:37 AM   #1
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Atwood Furnace Efficency

Hi, there have been some posting about furnace venting ,I searched but can't locate them. My question is about vents and efficiency. My furnace is an Atwood 8940 (40K btu propane) The owners manual indicates that at least 4 vents should be used for best efficiency. The RV from the factory only uses 3 one to the bedroom , one to the bathroom and one to the floor ducts in the living area. I've blocked the bedroom register.
I'm now questioning if by blocking a register my efficiency has been reduced causing me to use more propane than necessary. Dose anyone know about how the heat exchanger works and how the efficiency changes.

CLIFFORD
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Old 12-13-2018, 09:09 AM   #2
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Blocking vents is a bad idea. It may end up causing the furnace to overheat.
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Old 12-13-2018, 09:28 AM   #3
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Blocking vents is a bad idea. It may end up causing the furnace to overheat.
This is what Iíve been told as well. In my case most of the warm air seems to be coming out in the bedroom and bathroom which gets nice and toasty (too much) while the rest of the mh stays a bit cool. Iíll use a small fan to move some of the heated air.
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Old 12-13-2018, 10:07 AM   #4
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I have found if you block a register the furnace will continually turn the high limit switch on causing the heat exchanger to turn off but the fan will keep running to cool the heat exchanger until the high limit switch turns off. If it is supposed to have 4 vents then you will need at least 4 for it to work properly. One of you vents is probably in the basement or to heat the holding tanks to make 4. I hope this makes sense
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Old 12-13-2018, 11:10 AM   #5
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If you have several floor ducts in the living area, they each count as a vent per your furnace manual. So it sounds like your RV meets the minimum guide line if you don't block any. There may be an additonal small vent or two that blows heated air into your basement some where to protect water lines and tanks from freezing.

IMHO you can block one vent and nothing bad will happen. No more than that.
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Old 12-13-2018, 12:40 PM   #6
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Hi Powercat, I have ample heat in my utility area (tanks,plumbing, etc. ) not produced be a vent , I think it's heat transfer from the plenum housing. I don't think I have heat coming from the furnace controled by the safety high limit switch cycling on and off.
I shut the bedroom vent off for two reasons, we like a cool bedroom and secondly to conserve fuel. If shutting down a vent makes my furnace less efficient I might as well open the vent and utilize the wasted energy.

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Old 12-14-2018, 09:56 AM   #7
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Sounds like to me you need to add another furnace and connect the bedroom ducting and basement to it. Use an old analog thermostat in the bedroom and make life easy.
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Old 12-14-2018, 01:08 PM   #8
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Efficiency is fixed......burner is preset to burn X amount of propane (btu/hr), fan provides X amount of airflow (combustion & room air...cfm) and heat exchanger transfer X amount of heat from hot exhaust to cold room air as it flow thru exchanger


Blocking off a vent has less to do with efficiency then ambient air temp..supply air temperature
Blocking off a vent can cause high temps in heat exchanger due to less air flow thru exchanger/less heat transfer------which YOU would realize due to furnace cycling ON/OFF, ON/OFF and fan NOT shutting down...just the burner


Wouldn't burner more or less propane (that is fixed based on btu/hr burner size and LP System pressure) it could cause heat stress to heat exchanger, high limit switch opening/closing...hot to cool back to hot etc. Cracks/CO leaks into room side




So if NOT cycling on high temp limit switch then close off vent as needed
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Old 12-14-2018, 03:38 PM   #9
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I closed the basement vent off, moved the hoses on the plenum to get better heat in living area.......this furnace doesn't cycle off and on till it hits the thermostat setting...there's enough leakage through the wall of the 3 other hoses to keep warm air in tank area, plus the heated tank pads.....nothing in the storage area that'll freeze
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Old 12-14-2018, 05:10 PM   #10
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This title is an oxymoron. Nothing efficient about a rv furnace.
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Old 12-14-2018, 10:10 PM   #11
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This title is an oxymoron. Nothing efficient about a rv furnace.
I don't know..... the furnace is probably fine. It's the duct tape and paper these things are put together with that really kills you. I've often joked an RV is the only place you can be too cold and too hot at the same time. The heat transfer through them it just so fast.
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Old 12-15-2018, 09:04 AM   #12
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That was my poor way of a joke. The rv furnace puts more heat in the yard than in the camper. It uses a lot of propane and a lot of watts.
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Old 12-16-2018, 06:23 AM   #13
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To calculate any furnace regardless of use...or any lp apliance for that matter. Read the tag. Find its lp btu/h input, and its output. Compare the 2 in representation of a % and that is the efficiency. Most are 70-85% efficient.
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Old 12-16-2018, 08:00 AM   #14
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Hi Wild Card, my furnace is 40K btu's in and 32.5 K out . I guess if the flow of hot air is reduced thru the heat exchanger the furnace run's longer to heat the rv and fuel gets wasted. If there's too many vents too much air passes thru the exchanger and air isn't heated enough and the furnace run's longer. So, I guess there's a need for balance.

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