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Old 12-12-2014, 10:46 AM   #15
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Originally Posted by mefly2 View Post
They suggest not using the heat pump below 45 degrees F. No heat (as said above) flows underneath the floor in the plumbing area unless you fire up your furnace.
I find that the heat pump works fine down to about 30 degrees. Mine is supposed to change over to furnace in the mid-30s, but it fails to make the change and it isn't worth it to me to troubleshoot the problem. 30 degrees covers just about everything we ever camp in in central Texas. I wouldn't worry about the tanks unless it gets down to the 25-degree range.

Main thing is to protect the sink and shower traps as well as the commode plumbing. I blow out the commode plumbing and put pink antifreeze in the sink and shower traps if a real cold spell is predicted. The shower trap is the most critical since it is a bear to replace if it freezes and splits.
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Old 12-12-2014, 11:15 AM   #16
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That begs the question, as an AS design issue, why not force some of that heat-pumped air through the underfloor ducts to the benefit of the tanks? Maybe have a valve to switch from furnace to ambient air supply?

That would make it possible to use the heat pump safely as the predominantly resistive heat source in very low temps.
As a mechanical engineer who has designed a couple of HVAC systems, I'm still not an expert, but I'll take a stab at answering the question:

Basically, a heat pump is an air conditioner running in reverse. Without going into a thermodynamics lesson, an air conditioner extracts heat from the interior air through a heat exchanger, and transfers that heat to the outside air. In order for this to work, the freon in the hot part of the duty cycle has to be hotter than the outside air. The closer the temperature of the freon to the temperature of the outside air, the less effective the AC becomes.

In heat pump mode, it runs backwards, extracting heat from the outside air, and transfering it to the inside air. In order for this to work, the temperature of the Freon in the cold part of the duty cycle has to be colder than the outside air. The closer the temperature of the Freon is to the outside air temperature, the less effective the heat pump becomes.

Given the typical efficiency of an RV AC/heat pump, about the time the temperature drops below freezing and you need to warm the tanks, the heat pump doesn't produce enough heat to do the job. So ducting air from the heat pump to the tanks wouldn't help.

But getting back to Riverdrifter's problem…

It's always going to be more efficient to run the Airstream's propane furnace than to try to heat the entire pole barn. First, it's heating a smaller volume, so you don't need as many BTUs to raise the temperature above freezing inside the trailer than you would to raise the temperature inside the whole barn.

Second, the temperature of the Airstream will never drop below the temperature of the barn. The interior of the barn has to fall below freezing, and stay that way long enough for the interior of the trailer to also fall below freezing.

Between these two facts, the trailer's thermostat will start the Airstream's furnace later than the barn's thermostat would have to start the barn's heater, and the trailer's furnace would have to run for less time and burn less propane than the barn's heater.

So rather than running the Airstream's heat pump, run the Airstream's furnace, in place of the pole barn's heater. As long as you also have electricity to make sure the trailer's house batteries stay charged, that is.

But ventilate the barn; the trailer's furnace exhausts into the barn, and you don't need to get gassed with carbon monoxide in between entering the barn and entering the trailer.
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Old 12-12-2014, 12:01 PM   #17
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Seems like a solid analysis Protagonist, at least the first half treating my post. Without knowing the real-world quantities, I supposed that a heat pump is most efficient heating given that that its heat transfer efficiency is strictly determined by the temperature drop, either way, but that while heating it adds waste (resistive) heat from the compressor to the pumped for the total. And some heat pumps supplement that with a resistive heating strip.

The advantage of using that arrangement is that it is entirely electrical, giving off no fumes.
It might be sufficient to maintain a temp just above freezing, as in storage.

The enclosed space angle is particularly interesting to me since I wondered if in my northern clime, it would be advantageous, if I wintered in my coach, to house it in some structure like a garage bay while the furnace was running, but I thought it would be necessary to -- somehow -- exhaust the effluent through a duct to the outside, with another to bring outside air to the furnace.

PS. Engineering is also my background (nuclear).
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Old 12-12-2014, 12:42 PM   #18
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We had a span of low single digit temps earlier this year here in the Idaho Rockies. I set the temperature to the lowest setting and ran the furnace in my 20ft FC....nothing froze. And it was reassuring to hear the furnace run when I passed Daisy in the driveway. I have since stored her after winterizing her...but found that the low setting/furnace running will get me safely through short periods of very cold weather
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Old 12-12-2014, 01:10 PM   #19
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I wonder how difficult it would be to install a simple relay that would start the furnace fan only, when the heat pump is running. It would draw the warm air from the trailer and circulate it through the ducts by the tanks. I haven't looked closely, but is there a way to do this through the thermostat that controls both the furnace and heat pump?

No combustion gases to worry about, and you would benefit from the higher efficiency of the heat pump compared to a resistive heater.
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Old 12-12-2014, 01:49 PM   #20
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A unit of flexible hose could be run from outside to the inlet of the furnace to supply fresh air for combustion and a second section of piping to exhaust the results from burning a fossil fuel (propane). This would allow you to used the furnace safely without the fear of carbon monoxide poisoning.
This is for storage inside a building. not needed if outside.
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Old 12-12-2014, 02:21 PM   #21
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I would drain the holding tanks, especially the water tank. The water tank drain valve is at the bottom of the tank and exposed to cold air, no matter how you heat the trailer. Some models (our 25 for example) have the drain valves for the hot and cold system under the tailer, exposed to cold air, so I would open those as well, let them drain and leave the valves open.

It is not unusual for these exposed valves to break from freezing and they are very difficult to replace.
So is there risk of the freshwater drain valve breaking if you camp in freezing outdoor temperatures? I have a 25' International that I'd like to take to a high elevation ski town in CA (Mammoth Lakes) this winter to stay in for a few days of skiing. I would plan to leave thermostat set at 55 while I'm out skiing and warmer when I'm in the trailer overnight. What risk of plastic breakage or something freezing exists if overnight temps are very low, such as single digits or teens?
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Old 12-12-2014, 03:17 PM   #22
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So is there risk of the freshwater drain valve breaking if you camp in freezing outdoor temperatures? I have a 25' International that I'd like to take to a high elevation ski town in CA (Mammoth Lakes) this winter to stay in for a few days of skiing. I would plan to leave thermostat set at 55 while I'm out skiing and warmer when I'm in the trailer overnight. What risk of plastic breakage or something freezing exists if overnight temps are very low, such as single digits or teens?
You could insulate the drain by getting one of those Styrofoam domes that you put over an outside faucet at home, and attach it to the drain. That would provide at least a small chance of the interior heat keeping the drain warm enough not to freeze. I plan to get a couple of those domes for winter camping, so I can put one over my exterior shower connection and one over my municipal water hookup.
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Old 12-12-2014, 03:45 PM   #23
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Where can the styrofoam domes be gotten? I'm not familiar with them.
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Old 12-12-2014, 03:48 PM   #24
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Where can the styrofoam domes be gotten? I'm not familiar with them.
Thermwell Products Co. FC1 Faucet Cover-OVAL FOAM FAUCET COVER - Walmart.com
FROST KING FC2 Faucet Cover,Plastic and Foam,1 In - Walmart.com
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Old 12-12-2014, 04:22 PM   #25
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So is there risk of the freshwater drain valve breaking if you camp in freezing outdoor temperatures? I have a 25' International that I'd like to take to a high elevation ski town in CA (Mammoth Lakes) this winter to stay in for a few days of skiing. I would plan to leave thermostat set at 55 while I'm out skiing and warmer when I'm in the trailer overnight. What risk of plastic breakage or something freezing exists if overnight temps are very low, such as single digits or teens?
We were in Breckenridge for a week last year just after Christmas. The high for the week was around 17 degrees and it was typically 0 to -4 degrees in the morning. I ran the furnace the whole time and nothing froze- freshwater drain valve and outdoor shower included. 30 lbs tank of propane lasted about three days. We've also camped in Taos several times when it was below freezing for 3-4 days at a time. No problems there either. No improvised skirting, light bulbs, hay bales, etc. Just parked and turned on the furnace. Go and enjoy yourself.
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Old 12-12-2014, 06:51 PM   #26
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For what its worth… We live south of you 150 miles, store ours like you (since 2008). We don't winterize but do open all cupboards, closets, lift the bed and put a small space heater on the bedroom floor set to run when the inside dips into the 40s. Area temps have (rarely) been as cold as 12F. Freezing spells are short… less than a week. We do check on it but so far have had no problems. I had not considered the drain valves and may be just lucky so far. If I get nervous about them, two options come to mind: put a muff over them like hose bibs or squirt some RV antifreeze up each briefly opened valve.
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