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CH and bath heating

Postby scass » Thu Jun 07, 2007 2:29 pm

Hi there

My first post on this forum. It is a bit off-the-wall and rather long I'm afraid. First, some background:

- I'm living in a renovated stone house in central Portugal (which is colder in winter than you might think!). The house is piped (pex?) for central heating but as yet we haven't fitted radiators and a boiler. (We have a separate on-demand gas water heater.)

- I'm building a Japanese-style bath, which is basically a deep bath maintained at temperature (up to 45°C) by circulation (like a tiny swimming pool, but hotter). Generally, the same bathfull of water might be used for a couple of days in winter then discarded. There's no chlorine or other treatment. Bath volume will be around 650 litres.

- Gas is propane from bottles. Heating oil is also available. I'd like to be able to use some kind of biofuel in future (don't want to use fossil fuels more than necessary).

- I'm pretty practical but know little about UK/European central heating systems. I may well import a boiler and other parts from the UK (where I'm from originally).

What I'd like to do is put in a central heating system that will also heat my bath. Ideally I'd like to have a nice wall-mounted control for the bath water (on/off; timer; temperature) in addition to whatever is necessary for the central heating.

It seems to me that the bath circuit is similar to a hot water circuit (though with a lower temperature and greater volume). One difference would be that the heat exchanger would part of a secondary pumped circuit (rather than simply a coil in a tank). I have found what appears to be a suitable swimming pool heat exchanger (available from Jacksons Camping website; 15kW at 0.75 m3/h).

I've very roughly calculated out that this heat exchanger should give a bath heat time of 3h from about 4°C. (In fact, a bath like this is often filled with hot water -- I could use the on-demand heater for this -- and then kept to temperature and reheated using the recirculation system.) I've also found that koi carp keepers use a similar (but lower temperature) system to heat their ponds. (See www dot koicymru dot co dot uk /heating.htm)

This will be quite a challenging project for me, but I think it is within my capabilities. However, my lack of knowledge of central heating systems means I'm rather unsure about the control system. I'm also not sure what I should be looking for in a boiler and control system. Finally, I'm not sure what best to do as regards minimizing fossil fuel consumption. We get quite a lot of sun here, but the winter sun (best bathing time!) is weak.

My specific questions are the following:

Type of boiler: A system boiler seems like a relatively easy and tidy way to install central heating. Also, the pipework already installed in this house assumes a sealed system (no provision for a header tank). But would I be able to configure a system boiler to heat my bath in this way and/or would it impose other limitations?

Control systems: I don't really know how a central heating boiler is controlled. For my bath circuit, I suppose I'd need a thermostat + timer, valve and pump. When the thermostat/timer says it's time to heat the water, it would open the valve and turn on the pump. I assume it also has to provide a signal to the boiler. How does that work? What kind of signal does a boiler need? Indeed, am I correct in assuming that getting a hot water flow requires three conditions be met: valve open, pump on and correct signal to the boiler?

Sorry for the length of this. I hope someone finds it interesting (different) enough to help out...


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