Tank Heater vs.Recirculating Hot Water

JiminDenver wrote:
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,

You must be clairvoyant. In my shop is a partially built hydronic heat register (along with a nice pump, backflow valve, and tubing) for my trailer bathroom (that doesn't have forced air heat). I should have just bought a Aqua Hot register, but I (foolishly) tend to spend more time and money building things. :h

Steve

Steve, when I went to school for HVAC in the late 70's, passive was all the rage. Lets just say I've designed a few systems over the years. :) Of course that was a long time ago so my tech speak is bit weak.

There is a possible advantage to doing that.

Tap the hot water line AT THE SHOWER, adn recerculate just enough to keep the water HOT at the shower.

Usually this is done by recirculating to the water heater inlet by the way.

That said. I do not recommend it.. WHY:

There are bacteria that really LOVE hot water.. And multiply in the water heater (This can lead to stinky hot water) and I'd rather keep 'em out of the fresh tank.

wa8yxm wrote:
There are bacteria that really LOVE hot water.. And multiply in the water heater (This can lead to stinky hot water) and I'd rather keep 'em out of the fresh tank.

Wayne,
Thank you for bringing this up.
I did some research on this and learned that.
- The bacteria do not survive at (or over) temps of 140 degrees F. (I was thinking of increasing the temp of the water heater anyway)
- Do not multiply very rapidly in cold environments (Fresh water tank)
- Are easily killed with a diluted bleach solution (typical RV tank/system sanitation???)
So, based on this, I continue to be comfortable with this approach.
But, thank you again for bringing this to my attention.

Steve

SteveAE wrote:
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

Your math looks solid but establishing Delta T at 80f isn't necessary (unless you want the water in the fresh water tank the same as the water heater). To establish a continual temp of over 32f (perhaps 40f is a better target), you'll need to know the amount of heat loss at ambient adjacent to the tank and supply lines. Your experimental method of temp measuring in tank will probably bring best results. Off hand, if camping at 0f with cabin temps above 65f, with assumed fresh water temp of 40f, assumed tank placement and lack of total insulation, circulating hot water into the system a couple of times during the night should get it done.

westend wrote:
Your math looks solid but establishing Delta T at 80f isn't necessary (unless you want the water in the fresh water tank the same as the water heater). To establish a continual temp of over 32f (perhaps 40f is a better target), you'll need to know the amount of heat loss at ambient adjacent to the tank and supply lines. Your experimental method of temp measuring in tank will probably bring best results. Off hand, if camping at 0f with cabin temps above 65f, with assumed fresh water temp of 40f, assumed tank placement and lack of total insulation, circulating hot water into the system a couple of times during the night should get it done.

West,
Thanks for the confirmation on the calculations.
I had no intent or reason to raise the temperature of the fresh tank above 40 degrees. The reason for Delta T was to determine how many BTUs would be transferred if the tank got to that temperature so I could roughly compare it to a typical RV tank heater. I don't know how many KW a tank heater uses, but I see that they heat the tank to a median of 54 degrees (cycle between 44 and 64 degrees).
My tank is insulated on five sides but the bottom only has a thin layer of corplast over it. So, of course, the water always has contact with all of the surface area on the bottom of the tank, which is also where the most heat transfer will occur.
I like your WAG (unless it is more than that???) of my only needing to run the system just a couple times per night to get the job done.
I guess that I'll just have to implement it and then wait until it gets cold enough to test it out.

Steve

Aw-haw. I see that many tank heaters are under 200 watts. So, 1 gallon per hour (or even less) should be comparable. My water heater can easily keep up with that usage and still have plenty of heat to spare for other heating projects yet to come.

Steve

SteveAE wrote:

westend wrote:
Your math looks solid but establishing Delta T at 80f isn't necessary (unless you want the water in the fresh water tank the same as the water heater). To establish a continual temp of over 32f (perhaps 40f is a better target), you'll need to know the amount of heat loss at ambient adjacent to the tank and supply lines. Your experimental method of temp measuring in tank will probably bring best results. Off hand, if camping at 0f with cabin temps above 65f, with assumed fresh water temp of 40f, assumed tank placement and lack of total insulation, circulating hot water into the system a couple of times during the night should get it done.

West,
Thanks for the confirmation on the calculations.
I had no intent or reason to raise the temperature of the fresh tank above 40 degrees. The reason for Delta T was to determine how many BTUs would be transferred if the tank got to that temperature so I could roughly compare it to a typical RV tank heater. I don't know how many KW a tank heater uses, but I see that they heat the tank to a median of 54 degrees (cycle between 44 and 64 degrees).
My tank is insulated on five sides but the bottom only has a thin layer of corplast over it. So, of course, the water always has contact with all of the surface area on the bottom of the tank, which is also where the most heat transfer will occur.
I like your WAG (unless it is more than that???) of my only needing to run the system just a couple times per night to get the job done.
I guess that I'll just have to implement it and then wait until it gets cold enough to test it out.

Steve

Gotcha, I misunderstood the reasoning for calculating the Delta rise. Yeah, one gallon per hour or two six gallon transfers per night into the volume of the tank should keep everything above freezing.
If there is any way to put even 1/2" of foam insulation under the tank, you will use a lot less energy to heat the water. Anything you can do to break the thermal conductive areas is going to be a winner.

Your last post may have alluded to other uses for hot water. Have you looked at hydronic heating, heat exchangers, and such? With a 30 gallon steel tank stuffed under a dinette seat, it is beckoning me to try out some sort of hydronic system. Lately, I've been looking at solar heating systems.

Are you off grid? Direct tank heater works fine for me as at these temps I will be plugged in. Would seem using propane heat would be better primarily if no utility power.

Smk,
Yes. Our cold weather camping is off grid.
Steve