I am redoing the bathroom and I want to heat a steel bathtub so that the water does not grow cold in the bath. I also obviously want to heat the room!
I was thinking about putting a radiator next to the bath and then also concealing it and venting it into the room. Or perhaps getting a nice looking radiator and just sort of framing it so that most of the radiator is visible in the room. There are certainly a lot of options for radiators these days.
Or, I can get a small long 7cm high radiator and put it directly under the bathtub, but then I have two radiators. The room is not huge and one radiator should be enough.
Forget about heating the bath water once it is in the bath; no radiator, or any other device, is capable of doing that. Even if you placed radiators under the bath the amount of heat gained by the steel bath will be much less than the amount of heat lost from the bath. If the bath water becomes too cool then the only solution is to add more hot water, or let some cool water out and replace with hot.
I recon that it cannot be done. It would take some working out and a lot of guesstimates to make a equation but I believe the heat losses would be too dig for a radiator to work this way.
It can be modelled but it is far easier to put in a known quantity of water at the desired temperature into the bath itself and time how long it takes to drop in temperature by a couple of degrees. That allows the heat loss to be calculated very easily at near enough the actual conditions of use. It allows for the effect of evaporation as well as convection. The underside of the bath obviously needs to be well insulated or have a surrounding air temperature maintained at the the water temperature, during measurement.
Then seal and insulate the airspace under the bath after having put in a radiator with at least 150% that rating (to allow for other losses).
The temperature differential between heated air and the steel underside of the bath will be much higher than that between water surface and ambient air. The thermal transfer function between hot air and steel will be much higher than between hot water and ambient air. So, provided the radiator at least provides the heat loss from the water, found by measurement, the water temperature will not fall.
A bit naughty from the electrical safety viewpoint, but I've used a 1kW fan heater just blowing away at the underside of a steel bath in an otherwise unheated bathroom. I assure you that the water temperature didn't fall.. quite the opposite.
I did think that an electrical heater would be best, it could be turned on and off on demand. There must be a safe alternative for this as well as a way to turn it off and on safely, perhaps with a timer. I will research that. In the meantime, Any ideas?
I was more thinking of a a plumbed-in radiator or two under the bath, with a hand-valve outside to turn on and off.
Water and electricity is a very dangerous combination. There are heaters (typically tubular ones) that could perhaps be utilised, hard-wired into a dedicated circuit and with a controller outside the danger area. But I would suggest sticking to a non-electrical solution, unless you have a very competent tame electrician.
You may also want to consider actually buying a hot tub - which will heat the water as it circulates.. That can give you the long soaks that you seem to want - with a standard shower unit in addition to actually use to get clean..
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