Using an indirect cylinder backwards?

[Keef]

I've got a static caravan that I'd like to install central heating and a bath on the cheap but It's only got bottled Propane gas and electricity supplies and Propane boilers aint cheap.

If I put a heavily insulated indirect fortic cylinder high in an outside shed and heat the water with the electric element, is there any reason why I couldn't use the hot water to heat the indirect coil within the cylinder for central heating?

Obviously I'd need a pump at a low point and a vent and fill to the top of the fortic but other than that can anyone think of any other problems?

[Julian]

No reason that I can see, you're just using it as a heat store.

However,  fortic cylinders don't have a good reputation.  Why not use a pressurised system?  What you describe will use a lot of power why not go the whole hog, make bio and install an oil fired boiler all same James and Jules with electric backup, you seem to usually have a good supply of oil.

[julianf]

The surface area of the indirect coil will probably be about 0.34m sq.

The heat transfer will therefore be pretty minimal, and hence the input into your radiators be poor.

Am i missing somthing - if youre heating the water with electricity, why not just get a fan heater?

[Keef]

No reason that I can see, you're just using it as a heat store.

However,  fortic cylinders don't have a good reputation.  Why not use a pressurised system?  What you describe will use a lot of power why not go the whole hog, make bio and install an oil fired boiler all same James and Jules with electric backup, you seem to usually have a good supply of oil.

Pressurised system? Which bit would be pressurised?

Unfortunately, the site owner is a bit paranoid about fire risk so a bio boiler is out of the running.

The surface area of the indirect coil will probably be about 0.34m sq.

The heat transfer will therefore be pretty minimal, and hence the input into your radiators be poor.

Am i missing somthing - if youre heating the water with electricity, why not just get a fan heater?

This is why I'm throwing the idea out there. In my head, the coil is the same as when used the other way round (ie. used to heat the DHW in the cylinder) and that seems to work ok so why would the transfer be so much more inefficient if you used the hot water to heat the coil?

I'm just thinking that to fill a bath using the current propane water heater would take about a week.
My next thought was to have a cylinder with immersion heater, and if I've got a cylinder full of hot water, maybe I could use it as a heat store to provide central heating.

Once the DHW is up to temerature, surely I've then got a potential 3kw to provide heating throughout the caravan if needed which I could set up for very little money.

[greasemonkey]

I do heat a static with a oil boiler, but that doesn't mean I know what I'm talking about......

With the indirect tank, the coils heat the water in the tank, but the water going through the coils is much hotter than the water in the tank gets. So it's quite good at heating the tank water, providing your not trying to get it over 60c or so. Working it the other way, there isn't a great heat transfer from the hot water into the coils.
I think, that with the amount of water you will need to pump around the rads, through the coils to pick up the heat, that it will not have enough time to dwell in the coils, and get hot enough. If you slow the water down, the rads will take an age to get hot.

That may not matter, if it is left on semi permanently, they will get hot eventually, but you'll not be looking at quick heat, and it'll be costing a tump in electric.

Add to this the fact that even a well insulated static holds heat like a plastic bag in a fridge.

I might be wrong, but thats what I think.

You may get a better heat transfer using a FPHE between rads and tank, like the setup on a combi boiler, but both sides of the FPHE will have to be pumped. ( found this out from a project).

I have a Propane water heater. It would take an age to fill a bath, but it runs the shower just great. Has been for over three years now.

[1958steveflying]

You could do as you suggest and use the main store for when you want to fill a bath, you could add an inline heater to top up the central heating side if the coil alone is not enough.