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Mixed thermosyston and pumped central heating

Postby ericmark » Sat Jan 12, 2008 2:16 am

My father has a central heating system where a thermo siphon hot coil heats the domestic hot water and central heating is pumped. There is no thermostat on the cistern. There are no motorised valves. There is a programmer and a room thermostat. The boiler keeps cycling which since the pipes are under floor boards and the bed rooms get too hot. The hot water also becomes too hot. There seems to be two ways forward. Method one is just fit a cistern thermostat common to Boiler normally closed to pump and normally open to programmer hot water output. This will stop boiler cycling it would only run if central heating or hot water was required. But when central heating is running water will still heat in cistern but since pump should push most of the water around the central heating it may be very little? It will require one three and earth cable routing. Second is to fit cylinder stat which controls a motorised valve which has a change over switch incorporated that switch then controlling boiler Orange to boiler, Brown and grey to cylinder stat and White to pump. In both cases room stat also connects to pump. The latter needs 4 wires plus earth as it will require a neutral and extra cost of valve plus plumbing will be required to fit valve. I am sure I am not the first to be faced with this problem and wonder if the first method has ever been tried and how well it worked. Second method connects the valve in a way I have never seen in any books again has anyone tried it and how did it work.
I have built circuit in multisim and tested and shown to be electrically sound but no way that I have to simulate water flow.

Thanks Eric

PS I am sticking same post in plumbing and Electrics as it involves both trades.


Simply Build It

Postby rflight » Sat Jan 12, 2008 7:00 pm

A thermosiphon system uses mains cold water which is slowly pumped through a solar array to heat it and is stored in a gravity tank to supply domestic hot water. They are mainly used in warmer countries, see here

Do you mean a standard domestic immersion tank?

A thermostat should be fitted to an immersion tank to control water temp. and to make more efficent. No point in paying twice to heat the same water.

The tank system you describe should have a y valve fitted and be electrically connected to a y plan wiring system. The valve will give priority to the tank until it is satisfied and then divert heated water to the central heating system if a demand is made via the room thermostat. Because this type of control has not been fitted is the reason why the boiler is cycling.

Method one seems to be the best option, but i'm surprised that a boiler system like this has been installed without any controls. Or has a new boiler been added to an old plumbing system.

Sorry Eric, did post a link to a site with Y plan wiring and plumbing diagram, but the good doctor deleted it.
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Postby ericmark » Sat Jan 12, 2008 9:20 pm

I looked on the glow-worm web site before posting and this shows a single motorised valve in the supply to hot coil controlled by a thermostat on the cistern which is not connected electrically with any boiler controls. This would stop the water getting too hot. But the boiler would still cycle. I understand thermo siphon to be where one uses the fact that water above 4°C expands as it gets hotter so become lighter and will naturally rise which is why water at the bottom of deep lakes is always 4°C not sure what that has to do with a solar array? Y valves are used on fully pumped systems this system is not fully pumped. I am conversant with Y and S plan but to convert to that system would require re-piping and I don't want to lift floor boards.
My hope was to switch off hot water so boiler only came on when central heating required it but as wired that option is not available.
I am sure there is somewhere a standard wiring system like the Y or S to do what I want and I want to follow standard system so if it goes wrong any plumber or electrician will be able to repair in the future. I know the way I have considered wiring will work but in the future I may not be the person repairing so if possible I want to follow a standard way.

Postby ericmark » Sat Jan 12, 2008 9:39 pm

I think I have found what I was looking for seems its called the "C" plan. now feel happier following a standard system.

Postby ericmark » Sun Jan 13, 2008 1:37 pm

Having now worked out how to wire one further question. I have two ways to proceed one is run a twin and earth and three and earth from airing cupboard to kitchen and put the 2 way valve 5 wire type and thermostat in airing cupboard next to cistern the other is to fit valve in kitchen and use radio controlled thermostat more expensive but easy to wire. Question is the building regs do I need Part L or Part P the latter in my area is free since my parents are disabled I expect the same with former Part P forms are easy and I have 2391 so no qualifications problem but is it really a new circuit? Part L not a clue on that one the last time a building firm ran off the job so it was very involved never done it with no complications. And do I really need to? The wiring will be tested and I will fill in a minor works for their records is just if I need to inform council?

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