Advice on oil boiler thermostats

[SJB060685]

Would one of these TapStats (I think they are called) be a cheap(er) option if the HW temperature is the only concern, these connect directly via the strapped on cylinder stat to a valve fitted in the cylinder coil and have no other (electrical or otherwise) requirements. Drayton controls make them.

Looking at the specs John i can't see how this cuts power to burner? Although this will govern the HW temperature if its not cutting power to burner then you will still get cycling with nowhere for the heat to go, unless the 3 way also acts as a bypass?

 

[John.g]

No, it won't cut power to the burner but I was under the impression that there are no controls on the boiler apart from a on/off switch and maybe a timed on/off so the whole system, CH+HW), runs as one?.

 

[SJB060685]

His current setup is gravity HW and pumped CH. At present when the programmer comes on for HW that's a direct live to burner and no temperature control other than boiler stat governing HW temperature. When the programmer comes on for CH it brings on HW as well for live to burner and the roomstat just controls the pump, a very primitive and dated way of doing this. In the back of the programmer there will be a plug for fully pumped or gravity.
He needs to consider updating to fully pumped with independent control or at least a C plan as I mentioned above, both get rid of the short cycling

 

[Steve-T]

I think you're going to be disappointed with that high limit stat clipped to the boiler flow. Depends how quick it is to react. May be okay if if has a mercury phial.

I once tried to govern a boiler, that had a very high hysterisis internal thermostat, using the Honeywell pipe stat (which was a basic bimetallic cylinder stat which clips to a pipe). The problem was that the boiler raised the water temperature faster than the Honeywell stat could react. So the boiler would overshoot before the stat became aware of it. If it set the Honeywell stat lower, it would prevent overheating, but then the boiler had to run almost stone cold before the stat would allow it to refire. Eventually it turned out that the boiler internal thermostat, thought to be obsolete, was available, and replacement solved the problem.

I would agree with others that the solution would be to fit a thermostat to your water cylinder and convert to fully-pumped. It'll be of the cheap and nasty bimetallic strip design, no doubt, but the quantity of water involved means that the stat will have time to gauge the temperature. You'll find a cylinder stat may not be especially accurate to the marked settings, but once you have got the setting where you want it to be, it should be surprisingly consistent.

Thanks for all the replies thus far.

It appears the only replacement thermostat is another firebird unit because of the length of wires to the probes:

My understanding is that most of these dual thermostat units have a leeway of +/- 0.5c so I would imagine fitting a new unit would solve the problem.......alas you have had experience of the boiler heating up the water faster than the thermostat can react to, so this fix might not work.

A understand that the system is old in design so a new cylinder with separate thermostat valves for the hot water would be the answer, but the is not an easy fix during Covid-19 and I'd imagine I'd be throwing £1000 at the problem too.

 

[John.g]

His current setup is gravity HW and pumped CH. At present when the programmer comes on for HW that's a direct live to burner and no temperature control other than boiler stat governing HW temperature. When the programmer comes on for CH it brings on HW as well for live to burner and the roomstat just controls the pump, a very primitive and dated way of doing this. In the back of the programmer there will be a plug for fully pumped or gravity.
He needs to consider updating to fully pumped with independent control or at least a C plan as I mentioned above, both get rid of the short cycling

I must be missing something but where does it show the above set up?

 

[SJB060685]

I'm assuming from comments above that's the case

 

[John.g]

Oh, fair enough, as I said above, even though I have a motorized valve controlling my cylinder temperature via a spare output on my solar controller and a PT1000 probe in the cylinder which gives pinpoint temperature control I don't use the motorized valve end contacts to switch the boiler, I just use the programmer to switch the boiler on/off wired through one (main living room) roomstat, the other 10 rads have TRVs and this gives excellent results. If the OP's system is fully pumped and he wants to keep it simple, he could also just install a cylinder coil motorized valve & a cylinder thermostat.

 

[Ric2013]

My understanding is that most of these dual thermostat units have a leeway of +/- 0.5c so I would imagine fitting a new unit would solve the problem.....alas you have had experience of the boiler heating up the water faster than the thermostat can react to, so this fix might not work.

Not what I meant, Steve. I have experience of a boiler heating up faster than a Honeywell Pipe Stat can react. Purpose-designed boiler stats are probably fast enough to react - provided, that is, that the boiler isn't massively oversized for the number of rads in use.

A understand that the system is old in design so a new cylinder with separate thermostat valves for the hot water would be the answer, but the is not an easy fix during Covid-19 and I'd imagine I'd be throwing £1000 at the problem too.

You almost certainly will be able to keep your existing cylinder. You'll benefit from faster cylinder heatup times if you switch to a fully-pumped design. Depending on how accessible your system is and where you live, I think we're talking less money than you think. If not a Tapstat or Cyltrol may be a cheaper option if converting to fully-pumped proves difficult for some reason.

May be worth getting some prices from local plumbers when possible.