View Full Version : Thermosiphon

12-14-2008, 06:04 PM
A couple years ago I put in a master bathroom. I embedded some pex in the mortar bed on the floor for future heat. It is cold here, darn cold and my wife is tired of the cold bathroom floor. The basement is insulated fairly well but until I finish it it is not heated so it is cool.

I am tight on cash so when I was offered a 12 gallon electric water heater for nothin I got it. I am thinking about putting it in the basement and connecting it to the PEX loop in the bathroom floor above.

After bleeding all the air out of everything would warm water from the tank thermosiphon up to and through the loop in the floor?

12-14-2008, 07:03 PM
12 gallon electric huh? Sounds like a waste of time and effort.

12-15-2008, 07:19 AM
You will probably not have too much luck getting circulation through your floor loop without a pump. Look up references on gravity recirculation loops to see what is necessary for flow without a pump (all lines need to be pitched, etc.)

Since you are just trying to warm up the floor of the bathroom, it is quite likely that a small electric tank can provide enough heat. Compare to the electric floor heating systems which use relatively little power.

12-15-2008, 12:03 PM
It would be convection, not thermosiphoning, but it would only work if there were absolutely no air pockets in the system, which is not likely. A 12 gallon electric heater would be completely inadequate for your use.

12-15-2008, 12:23 PM
A 12 gallon electric heater would be completely inadequate for your use.

A bit of an understatement huh HJ?

12-15-2008, 12:30 PM
It does not appear that the intention is to use this floor loop as a primary heating source for the room - he only wants to warm up the cold floor.

Electric radiant floor heat uses only 10-15 watts / square foot.
So, for a 100 square foot bathroom floor, this would be 1000-1500 W.

A 12 gallon electric water heater may likely have a 1500W element, so it may well be enough heat to keep the bathroom floor warm.

12-15-2008, 12:33 PM
Maybe in the Sahara!

Anywhere else it has a snowballs chance in hell!

12-16-2008, 05:33 AM
You obviously do not know how long it takes a 120 volt heater to just heat the 10 gallon tank.

12-16-2008, 07:26 AM
It doesn't really matter how long it takes to warm 10 gallons of cold water. Once the water in the tank is hot, the stand-by loses are quite small, and nearly all of the 1500W can be used to heat the floor.

I did not say that it would bring the floor up to a warm temperature quickly. The electric in-floor heat has a very slow response time as well (can be hours to warm up a cold floor). However, once it is warm, it does not require much energy input to maintain temperature. The floor in question as above an insulated basement, and the house has its own heating system.

In steady-state, there is no reason that a 12 gallon electric water heater with a 1500W element cannot provide plenty of heat to maintain a comfortable floor temperature for a single room.

The electric in-floor heat that we installed in our bathroom (over a very cold unheated crawlspace) keeps the bathroom floor at a very comfortable temperature with a couple hundred watts of power (it is only about 25 square feet of floor space).

12-16-2008, 08:14 AM
Electric floor heat vs. water heated by a tiny electric water heater are two very different things.

WV Hillbilly
12-16-2008, 10:52 AM
I can't answer your question as to wheather the water would circulate without a pump but a friend of mine installed pipes in a first floor cement floor ( not a slab ) & connected the pipes to a 30 gallon lp gas water heater using a pump to circulate the water . He heated his whole house very comfortably this way . Had a thermostat on the first floor to turn the pump on & off . He had a small gas stove in the basement & a gas range & a tankless water heater for domestic use . Said he paid about $600.00 a year for the lp gas . I know there are more efficent ways to heat water for a radiant floor , just telling you what he did . His house was new ( built it himself ) well insulated & was 2 stories above a full basement . Probably about 2000 sq feet not counting the basement . It has been about 5 or 6 years since he told me what he was paying per year for the lp gas & I don't have any idea how much more per gallon it is now than then . All that being said I would think a 12 gallon heater would heat a small loop under a bathroom sized room . Might need a pump to circulate the water though . I could also be totally wrong too .

12-16-2008, 01:44 PM
The floor is 62F and I would be happy if it was 72F. I don't care if it were to raise the temp of the room 1 degree or not. My wife just wants the floor to be a bit warmer.

Pex is cheap so I will give it a shot and see what happens. The worst thing will be wasting $20 on some pex. If it don't work I will have a water heater when I build a bathroom in my shop someday.

When I built this bathroom I planned on an electric tankless and a circulator to heat this floor, 30sqft. In the future I want to do the same thing in the main bathroom when I remodel it.

Unfortunately when I tore into the floor and ceiling of the room where I built the bathroom I found what the anus previous owner did. This room was an addition. I spent my heated floor budget fixing this tards work. Now the foundation is level, and the roof is properly supported but I have a cold floor.

Thanks for all your replies.

12-16-2008, 03:14 PM
Things will likely last longer if you get pex with an O2 barrier, rather than the stuff for potable water. Depending on the ambient temp of the room and its heat loss, the floor may not get warm. To initially raise the temp of the water, you have to be supplying more heat than you are losing, and with only 1200W (and the pex isn't the most efficient at transferring the heat it does carry), you may not ever raise the tank's temp much, thus, it might only maintain, rather than heat.