Central heating controls

16 Oct 2005
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United Kingdom
Last year I had our oil fired central heating boiler replaced with a Grant outdoor type. As it lives outdoors this boiler has a built in froststat to protect it in cold conditions. I suppose Grant err on the safe side & I found that the boiler ran whenever it was about 5deg C or colder - last year this was almost continuously from October to March.

The problem is that as far as I can see from the wiring diagrams, electrically the froststat calling for heat has exactly the same effect as the programmer and roomstat calling for heat i.e. whenever it's cold ouotdoors the whole central heating system operates.

This means that the house is lovely & warm all the time (even when nobody is here or when I'm in bed) but my oil consumption more than doubled !

I am told that it is possible to install a short circuit which is opened up by an actuated valve. In this way when the room stat calls for heat the circuit operates normally but when it's only the boiler's frost stat (ie if I'm on 2 weeks ski-ing holiday) the water is circulated around a much shorter loop & doesn't heat the whole house.

Is anybody able to advise me how this can be done or where I can find somebody that can design it for me to wire up as I can find nobody locally that knows the answer.
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boiler has a built in frost stat
It sounds as if this frost stat is on the return pipe into the boiler, so the boiler has to fire up until the heated water finds it's way back along the return, having meanwhile heated up all the rads. All the following advice is based on that assumption, so it needs to be checked!

You could fit a by-pass near the boiler with a control valve which would only open when the temperature fell to the level at which the boiler frost stat operates. You would not really want the by-pass opening during normal heating operations since it would reduce the efficiency of the system.

A by-pass would result in the return pipe getting hot much sooner, so reducing the period of heating. You would still want a certain amount of heat to pass into the CH system to protect against other parts of the house freezing, so setting this up is quite critical.

The simplest form of control on the by-pass might be an ordinary radiator TRV (£10) carefully adjusted to open at the appropriate temp. This would respond to ambient temperature rather than return pipe temperature, so a better arrangement might use a Thermostatic Valve with a remote sensor (see Danfoss site for examples), in which case the sensor could be attached to the return pipe.

Neither of the above would require any electrical connections, but setting up the temperatures at which the valves open and shut would be critical. A more complicated solution is to use a motorised valve electrically controlled by a thermostat (again probably best fitted to the return pipe).

The advantage of electrical control is that opening and shutting of the valve can be set up and monitored more easily, and the control can be linked into the system controls in such a way as to avoid operation when normal heating is on, for example by feeding it from the CH off terminal on the programmer.

I don't think this can be correct. The froststat in the boiler housing senses ambient temperature - not water temperature.

Once the froststat calls for heat it overrides the programmer and roomstat and the boiler and pump run continuously with control being undertaken by the boiler's own limit and control stats.

I don't think a mechanical solution is the answer since the TRV would have to be sensing ambient temperature. Also as you point out it would mean that the heating system would work at reduced efficiency or not at all whenever the outdoor temperature drops.

I need somehow to be able to differentiate between the froststat calling for heat and the programmer calling for heat. At the moment this cannot be done as they both in common supply 240V to the burner & pump firing terminal.

Your comment about a CH off terminal on the programmer is interesting - do all programmers have such a terminal ? I'll try & check mine - it's a Potteron EP2002
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2sheds said:
The frost stat in the boiler housing senses ambient temperature - not water temperature.
This doesn't change the general principle of what I was saying. The mechanical thermostatic valve with a sensor on the return pipe would still work.

During normal heating operations when return temperatures were high, the sensor would shut the by-pass. When frost stat was on but heating off, the sensor would partially open the by-pass to maintain a return temperature in accordance with the setting of the control (say 40 C?). This would reduce circulation around main circuits and speed up increase in ambient temperature (in boiler casing) so reducing heating period.

It would be rather neat if the boiler frost stat operated at mains voltage, since you might then use the output of this to operate a motorised valve, but I suspect that it is low voltage. Even so, there are low voltage valve actuators so it might be worth checking this option too.
2 sheds, Chris has tried hard to give you some (free) information based on your initial post, if he'd known the Grant Froststat sensed ambient temp he could have saved a lot of keystrokes!

Given the information to hand I suggest the following:-

1. Install a pipestat on the return pipework, interlock this to remove the froststat signal to the boiler and pump when return water temp reaches, say 40C. As the froststat runs directly off the mains this will be unaffected by programmer action.

2. Install an automatic bypass valve (about £25) after the pump, and usually in the airing cupboard. The final setting will be determined by local conditions. This will have other advantages too.

3. Convert to 2-port valves (assuming you may have a 3-port HW/HTG valve)

4. Install a house protection froststat and second pipestat wired to operate the heating and locate it in the most exposed place in the house, set it at about 5C/40C.

5. Insulate the boilerhouse and all exposed pipework, insulate the loft, draughtproof the house. Consult OFTEC registered fitter regarding ducting the combustion air to the boiler rather than running it through the outhouse to the boiler (keeping the outhouse warmer without compromising safety)

In this way the boiler in the outhouse protects itself but doesn't run for long periods, the house does likewise if it gets cold, even when you're off in Cloisters for the week.

As for the wiring, a competant (and registered) electrician should easily be able to make it work properly and safely, and issue you with the correct documentation to meet Part P of the building regs, and stay legal.

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