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Old 12-30-2009, 07:25 PM   #1
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High efficiency furnace(s)&multi speed blowers

Just spent a long weekend in 32 degree weather, which is cold for Eugene, Oregon. Used the furnace a lot. It is a 30k btu unit. and it went through a fair amount of LP Gas. So, I took a quick look around for a high eff rv furnace like the 90% unit in my house-there was a LOT of heat going out the vent of the furnace, which bummed me out. Did not find anything so I am questioning the forum-is there a 90-95% eff furnace out there for RV's? Also, my old unit cycled quite a bit-again, I think a constant low blower speed with the burner going on and off would provide a better heating operation inside the trailer. Any body mod their unit for constant low speed operation?
ol' bill
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Old 12-30-2009, 07:44 PM   #2
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Old 12-30-2009, 08:02 PM   #3
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.................................................. ..

I think a constant low blower speed with the burner going on and off would provide a better heating operation inside the trailer. Any body mod their unit for constant low speed operation?
ol' bill
I am now on my third TT and the furnaces in all have behaved like that.
They blow a hurricane force wind and drive the temperature up before the thermostat has figured out what is going on. Then they shut off and do the whole thing over again a few minutes later. I have decided that I was going to look into slowing the fan down, but hadn't thought of letting it run constantly at a slow speed. There are obviously things to be considered, such as keeping the heat exchanger from overheating and burning up, and would running the fan continuously run the battery down too quickly when do on shore power? I have often wondered if RV manufacturers are putting too large furnaces in the RVs. In any case I am definitely with you. I would like to find some improvement.
Looking forward to reading some good ideas,
Ken
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Old 12-30-2009, 08:45 PM   #4
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Another forum member just suggested looking into this : http://www.precisiontemp.com/pt_rvmd_junior.html .I just took a peak and it looks interesting.
Regards,
Ken
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Old 12-30-2009, 09:54 PM   #5
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This link may work better: PrecisionTemp.com: For Recreational Vehicles and Boats

It sounds very interesting, but regardless of what the unit actually costs, I sense that installing a complete heating system for a TT might wind up being quite a significant expense, not only in the hardware, but also the labour to install.

If anyone does investigate further, I would love to hear cost estimates.
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Old 12-30-2009, 10:07 PM   #6
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If anyone does investigate further, I would love to hear cost estimates.
How about this http://www.precisiontemp.com/images/2009_Pricing.pdf
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Old 12-30-2009, 10:16 PM   #7
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If you want something over 90% eff you need to consider two issues. First you need to have a secondary heat exchanger which most likely means an increase in the furnace size. Once you get that in you then have to deal with a much lower temp exhaust which means some type of PVC venting system and condensate drainage.

Typical non eff furnaces send exhaust out at about 400 degrees. Eff furnaces vent in the 80 to low 90's. That means the moisture in the exhaust condenses and condensate which is acidic must be drained out. That works okay in a house where it drains into your basement sewer through a plastic hose. Try draining liquid out of a trailer in the freezing cold. The exhaust itself is routed through PVC due to the fact that the acidic moisture in the exhaust will destroy typical galvanized components.

I don't think in the near run you will find anything like a 90+ furnace in a trailer until you can address these issues.

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Old 12-30-2009, 10:31 PM   #8
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I would think it would be very difficult to put hydronics under the floor, but what about a thin hydronic "pad" running the length of the trailer, on top of the floor? I bet it could be made less than one inch thick. Then you could really take full advantage of a unit like the one in the link above.

I'm partial to wood heat myself, in fact I'm sitting by my woodstove in my livingroom while I type this. A friend of mine who used to be a fisherman told me to check out the small marine woodstoves before I buy a new furnace. He says they would burn for four to six hours on a small chunk of hardwood, and heat a small boat cabin well. I saw one in an Airstream on the forum here when I first started to lurk. I can't remember who it was now.

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Old 12-30-2009, 10:33 PM   #9
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Rodney,

Thanks for the splash of reality. I'll pass on that system.

Jack,

Very good points. And considering that even if one lives in the TT full time in colder climes, I can't imagine a payback happening any time soon.

All in all, I would be happy with just a two-speed fan on a new Suburban furnace. I think that a system like that, one that would keep air circulating all the time that I want it to, and only kick the speed up when the thermostat calls for heat would do a great job of taking the rough edges off the simple system an AS comes with.

In my opinion, this would make a more comfortable (read: even) temperature in the TT, and shouldn't cost much more than a standard system. Judicious adjustments of the airflow through the rather crude system that AS installs would help too, I feel.

I just don't know if a compatible two-speed motor and controller exist to achieve this.

Anyone?
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Old 12-30-2009, 10:40 PM   #10
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Quote:
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I think that a system like that, one that would keep air circulating all the time that I want it to.................

Anyone?
A cheap(er) approach is to run a 12v fan or the fan on the AC to keep the air moving.
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Old 12-31-2009, 07:06 AM   #11
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There are installations of this; Uwe, I think on one of his retrofits. Granted, that would be one deee-luxe setup, but the trailer would gain by eliminating the space/housing for the water heater. This is the sort of appliance one expects, I suppose, the $150k level and above. One would have to be a dedicated cold weather camper.
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Old 12-31-2009, 07:40 AM   #12
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You will have a problem with a 2 speed fan there is a safety device in RV heaters that shuts off the gas valve when the fan slows down called a sail switch.
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Old 12-31-2009, 08:18 AM   #13
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Jack is absolutely right. To get up above 85% you have to have a condensing furnace which adds complexity, weight, and size.

A smaller furnace might help. Units with fewer BTUs often have smaller fans. As a rule of thumb the furnace should run at least half the time on a fairly cold day, or it's oversized.

At least some of the recent Atwood furnaces claim efficiencies in the high 70%s. Not bad, really, considering that 55-60% was common for household furnaces 25 years ago.
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Old 12-31-2009, 09:07 AM   #14
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One of the problems is the real lack of insulation. When it really gets cold, you know within minutes when that blower shuts down, that the cold is creeping back in. The other issue is thermostat placement. In my last two Airstreams, the thermostat was placed on the wall that houses the refrigerator. Any use of an exhaust fan, bathroom or stove causes air movement in that triggers the thermostat. In the summer the hot air in the compartment prematurely triggers the A/C. Like wise in the winter it triggers the cold air. I've put some silicone caulk into the hole that the wires pass through for the thermostat but it's not the end all to this problem.

In reality, I'd like the ability to have a two stage burner on the furnace with it's resultant two speed motor. My current home furnace has this feature and the way it was explained to me was that a typical single stage burner and fan have one goal in mind....to get the thermostat up to set point. In a two stage furnace it takes more time, but it allows the warm air to permeate the house better, which eliminates hot spots and makes the temperatures more even. If you think about it, most homes have their thermostat in a central core area. With a single stage furnace the areas outside the central core will hit set point sooner than the thermostat. If the heat is more gradual, both in temp and blower speed, you don't get the extremes in the outer areas prior to the thermostat seeing the set point.

In the trailer being able to select a slower blower and burner setting would keep things running longer, and would minimize the wild swings in temperature. As a bonus you wouldn't have those moments when the cold pours off the walls while you wait for the cold to hit that thermostat.

Jack
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