Will the heat pump keep the tanks from freezing

[Riverdrifter]

I have a 2011 30' international. If I keep the heat pump (rear A/C unit) on will it keep the holding tanks and water system from freezing? Thanks!

[murreywalker]

The only heat going to the holding tanks is vented from your furnace.

The A/C's heat pump doesn't.

[timhortons]

Well that's a great design.

[Tincampers]

Given that most heat pumps are ineffective at freezing temps...well.

[Riverdrifter]

Thanks for the quick response. I have my trailer in heated pole building but thought when it gets below freezing I thought it would be cheaper to heat with the heat pump rather than heating the whole 30' x 50' building with propane. I'm in western WA so we only get below freezing a couple times each winter. BTW I have a heat pump (newer mini split unit) at my cabin in Winthrop WA were it gets cold in the winter and it is good to about 20 degrees.

[AnnArborBob]

Originally Posted by Riverdrifter

Thanks for the quick response. I have my trailer in heated pole building but thought when it gets below freezing I thought it would be cheaper to heat with the heat pump rather than heating the whole 30' x 50' building with propane. I'm in western WA so we only get below freezing a couple times each winter. BTW I have a heat pump (newer mini split unit) at my cabin in Winthrop WA were it gets cold in the winter and it is good to about 20 degrees.

If you have infrequent, predictable and short duration periods of freezing weather, you could consider running the Airstream's furnace. The furnace has some of it's heated air diverted into the belly pan and that is designed to allow you to camp during freezing temperatures. We've successfully camped in temps as low as 19 degrees and the furnace did its job. Of course, you'd have to make sure that your pole barn was vented sufficiently to provide fresh air to the furnace and also to vent out the products of combustion.

[polarlyse]

I think you could get the same heating effect from a small space heater and likely use a lot less electric. Of course you would also not get the benefit of heat to the tanks but the tanks are not the freeze problem. It's the trailer plumbing that you need to be concerned with. Maybe a space heater ( or heat pump if that's your choice) and a small fan to circulate the warm air ????

[idroba]

I doubt very much that your rig would ever get cold enough to freeze tanks or pipes inside an enclosed pole barn building in western Washington.

The heat pump would do fine with the small pipes in the body of the trailer, and the mass of the liquids in the water and holding tanks (if not drained) would be so large that freezing would not occur.

However, as I said to begin with, I doubt that you would ever have and problem inside a building of any type in Western Washington. You would have to have sustained temps below 32 F for anything to happen.

[dkottum]

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.

[TinShack]

Where I live we the winter lows are generally in the 40's and daytime highs in the 50's and 60's. We get occasional night time winter temps that dip into the 20's and on sometimes into the teens although for the most part. This is generally not a problem if it happens for just a night or two. The problem is when you have extended below freezing temps like we get sometimes.

I got careless one year and did not run my furnace as I thought a portable heater would suffice during an extended freeze. Needless to say I had some issues with some frozen pipes.

Last year we had about a two week cold spell so I ran my furnace...just set it to 50 degrees... and had no issues at all. In two weeks of subfreezing I almost emptied my propane tanks, but in my opinion, it was worth the cost for peace of mind and no frozen pipes.

[Bbelk]

Originally Posted by mrprez

Given that most heat pumps are ineffective at freezing temps...well.

This is totally true. On my 2010, the unit will switch itself over from the heat pump to the furnace somewhere in the mid 30s.

For the short duration freezes we have in Texas, I have a small electric heater that keeps the living space above freezing which protects most of the plumbing.

[mefly2]

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.

[ROBERTSUNRUS]

Hi, maybe a make-shift skirt of some kind and a few lights under your trailer; This could work and draw way less power than most anything else.

[BearScream]

Originally Posted by mefly2

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.

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.

[Pahaska]

Originally Posted by mefly2

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.

[Protagonist]

Originally Posted by BearScream

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.

[BearScream]

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).

[B00merang]

Riverdrifter
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

[StokerDoc]

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.

[macofpei]

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.