Tank Heater vs.Recirculating Hot Water

West,

You might be right that 5 gallons is overkill. I chose it for two reasons:
1 - it is the size of my hot water tank
2 - given my assumed delta T, it just happens to work out to ~1KW.

Here is the math I used, corrections or suggestions are appreciated:
Given/assume:
- Freshwater tank water temp of 40 degrees
- Water heater hot water temp of 120 degrees
Assuming no line losses or cooling due to replacement water into the water heater, this is a delta T of 80 degrees
Since: 1 BTU = 1 degrees (F) change of one pound of water
Then: 80 degrees x 8.34 lbs/gal (weight of water) = 667 BTU/gal
667 BTU/gal x 5 gal = 3336 BTU
Since: 3412 BTU = 1 KW
Then: 5 gallons of 120 degree water cooled down to 40 degrees will release ~0.98 KW worth of equivalent electrical energy.

I know that this is ideal and the losses will add up.

I am also assuming that all incoming water will be cooled down to 40 degrees. Depending on the outside temp and the amount of water in the tank, there will likely either be further cooling OR the tank water will slowly increase in temperature. But, this is perhaps out of my ability to determine given the data I have. So instead, I was just looking for the amount of heat transferred to the tank compared to a typical resistance (electrical) tank heater.

I suppose I could do heat loss calculations on the tank to determine how much energy is really needed, but again, I don't believe there is enough data to even get close (insulation of the tank to outside, heat transfer from the floor of the trailer to the tank, etc.). So I though it would be easier to use a little experimental data (measure the tank water temp) to determine what was happening instead.

The goal is to be able to eliminate furnace heating of the freshwater tank and still protect it from freezing in temps down to about zero (F). Much below that and I am out of there.

Steve

Now you are a step away from using the system for additional heat. Add a few Ts and a by pass valve to the hot side in the bath and you can run the hot water through a small baseboard heater, do it again after the kitchen sink and pump and you will get some benefit other than keeping your pipes from freezing.

Jim,

That is exactly the same loop I have (except the galley sink is located in the center of the trailer almost right on top of the tank and the head and water heater are in the back of the trailer), with the freshwater tank vent tube (which I was thinking of using as a return to the tank) run through the same cabinet as the galley sink. For my rig, it should take a valve, controller (switch, timer and/or thermostat) and about two feet of additional plumbing.

Steve

A few thoughts

I would cut the FW tank out of the loop unless you are worried about it freezing. A line from the last point of the hot water side to a winterizing valve between the tank and pump will do. Since I don't have the diagram for the plumbing in your rig, I'll explain it with mine.

As the FW is pumped to the back of the trailer, it hits the kitchen sink, then the tub, bath sink, toilet and finally the HWH. From the HWH the process is reversed and it ends at the kitchen sink. So I would run a line from the hot side in the kitchen to the valve between FW tank and pump creating a closed loop that will require less pumping and much less propane to keep the lines warmer.

I had thought about recirculating to the fresh water tank in a different context, operating as a passive heating reservoir. If it was possible to heat the water in the tank to an elevated level, the tank would give up that heat into the cabin interior during cold weather camping. Since I have a steel tank, my situation probably better fits this application than others. I've been investigating what is the most efficient way to heat the water and how much thermal transfer I would get. This is in the realm of hydronic heating, I would guess.

To just raise the temperature of the system so that it would not freeze and using the tank in the process, one wouldn't need to elevate the temperature much to accomplish this. Assuming that the bulk of the system is within the cabin and is also influenced by the cabin heating, just circulation would be enough. If applying the tank-included method to traveling with no cabin heating (may be where this is the most benefit), a guy would have to know the volume of the system, the ambient temperatures involved, and duration of the cycle.

Steve, if you can state the last three parameters mentioned, I think we can work out the physics necessary,i.e. amount and temperature of hot water circulated into the tank. Just ballpark, I think introduction of 5 gals. of usual water heater temperature every hour is overkill for most situations.

I figure if you just let it run to 50F before sleep the tank will not freeze overnight.
Run again in the morning.

I am not sure how to stop the pump after five minute run at some interval overnight while unattended. Maybe a timer every two hours?

Good comments, thank you.

Skip: Even though I have one, I don't use a generator, hence no 120 VAC...that I want to run a resistance heater with anyway.

SMK: Actually, it is during the night that I want the system running. Day time temps in the upper teens and low 20's really aren't an issue for me. It's those overnight lows that I fret about. My thought was to transfer the entire contents of the water heater into the tank as quickly as the plumbing will allow, once a hour (or so). So while the water pump will run for long enough to pump 5 gallons, it shouldn't take too long to do this. Otherwise the pump will be off. I have worked hard over the past couple years to quite the pump so there isn't much noise and the power consumption (2.3 Amps) is much less than that dang forced air furnace.

MLTS: From the transfer point into the tank will be all downhill, so the line (tank vent tube) will dry out sufficiently so no nasty green stuff can grow (it's very dry here most of the year).

Steve

On the Sportsmobile forums, there is a posting from someone who did exactly this, controlled by a toggle switch. The recirculating system does a good job at ensuring that the FW tank and other plumbing stays above freezing.

My only issue with a recirculating system is that when it is off, the water in the disused pipes may grow algae in the summer, similar to how water stored in the pipes behind sprinklers turns black over the years. The answer to that would be a check valve near the FW tank (so nothing can backflow into the pipe), and a valve near where hot water gets diverted so one could blow out that pipe with compressed air.

Water valve $7



Snap Disk switch $10



Probably open at 50F and close at 40F would work. Pump will be running for an extended period while heating.

Could add a manual switch so it remains silent overnight.
Aso may need a manual off if the valve passes too much volume and reduces pressure when needed.

Sounds like an expensive alternative (fuel costs) over electric tank heaters. My 120 volt heaters run with generator in eco mode and the 12 volt side are not noticed in fuel mileage.