Pump Over-run Timer

[Set Square]

My Baxi Solo (Mk I) boiler controls the pump in order to provide pump over-run. It achieves this via an additional set of contacts on the boiler stat - which connect the pump to switched live when the temperature is below the stat setting, and to permanent live when it rises above the stat setting.

This generally works ok, but I've had a couple of instances of the over-heat trip operating in the last few weeks.

When the boiler is in full flight, the outlet temp is below the stat setting by definition - otherwise the stat would cut the burner. When the room stat turns off, everything stops dead *until* the residual heat brings the temperature up to the stat setting (typically after a delay of 10 seconds or so) - whereupon the pump starts up again, as it is meant to do.

My feeling is that the boiler would less likely to trip if the pump stayed on during this delay period - this preventing the residual heat from getting its foot in the door. I reckon that it should be possible to achieve this by powering the pump via the type of delay timer which is used for bathroom extractor fans. The sort of thing I have in mind is this:

TLC site doesn't give much of a spec - but I assume that the load is turned on by switched live and kept on by permanent live for a certain time (adjustable from 1 to 45 minutes) after switched live is turned off. I would take my switched live from the existing boiler pump control terminal, so I would simply add a couple of minutes to the over-run period controlled by the boiler but - more importantly - would keep the pump running immediately after the room stat switched off.

Can anyone see any problem with this suggestion?

[Fentoozler]

Are you sure it IS overheating? I must have replaced hundreds of faulty OH stats on solos in the past.

Angus

[SimonJ]

Mine trips every time it switches off, if you have it turned onto full. If you leave it set at about 3/4 it is OK. Does that sound like a faulty stat to you? ( I have had the central heating 'engineer' out to it, and his method of 'repair' is to turn the stat down!)

[Fentoozler]

Hardly a repair!

The boiler stat should cut out before the OH stat does (obviously). You can bypass the OH stat (in fact Baxi have provided useful twin spade connections on the OH stat for this purpose) to see if the boiler stat does cut out at the maximum temp (or put the boiler stat phial into a mug of boiling water and see if the boiler cuts out (or fails to start)). NB: Officially, bypassing the OH stat is considered dangerous and shouldn't really be done (it is a safety device after all), but as long as you are supervising the 'test' with one hand on the power isolation in case the boiler starts banging, things will be ok. My guess is that it is a faulty OH stat - like I said, I have changed loads of these (an entire council estate we maintain has solos, and every one has had a new OH stat, in fact, when we took over the contract, most had been bypassed and left like it)! They're only around £15 to buy and a doddle to fit (as long at the old curly OH probe doesn't break off in the pocket - that can happen on Solos).

Angus

[SimonJ]

My thoughts exactly!

The boiler stat works fine when set to maximum, during normal use of the boiler. The overheat only trips when the boiler switches off.

The problem I have is that I live in a rented house, and the landlord (who lives abroad and never sees the results) insists on using a particular company for repairs.

The problem I had in the first place, was that the boiler kept tripping out, and the hot water cylinder was leaking. They replaced the motorised valve. Funnily enough, this did nothing to cure the problems!

On their next visit, they replaced the cylinder, and installed a cold water feed tank above it. They 'repaired' the boiler by turning it down. (this was in summer) Things were OK for a couple of months, when the radiators started making lots of gurgling noises, and the boiler started cutting out again. The 'engineer' bled the system, and this seemed to cure the problem. (though he said nothing about how the air got in there)

Last week, the same thing happened again. as it was Christmas, I investigated myself, and found a small leak on the pump.(badly made joint when they put the cylinder in) This explained how the system was loosing water, but not how the air was getting in there. further investigation found that the stop c*ck to the feed tank was switched off, so as the water was leaking out, there was no water in the tank to replenish it. So I turned the tap on, only to find that the brand new tank that they installed had a big split in it. They obviously knew about the problem, or they would have left the tank switched on.

In the new year I am going to go and play holy hell with the letting agent, whose stock response is always 'well the landlord insists that we use that same company'

[meow2222]

write to the landlord, tell him

NT

[Set Square]

I suppose it *could* indicate that the OH stat is tripping at too low a temperature. Turning the main stat down reduces the temperature at which pump over-run is invoked (because the same stat does both jobs) - thus increasing the margin between boiler controlled temp and OH temp.

[SimonJ]

Letting agent will not let me have his address!

[SimonJ]

I have never noticed pump over-run working on this boiler. Mentioned this to the 'engineer' and he said this boiler type does not need pump overrun, but it seems from the posts on here that perhaps it does.

10 mins later......

Have just read the installation instructions, and it does indeed need a pump overrun. Something else this 'engineer' has got wrong!!

[Dave Liquorice]

IANAL but I'm not convinced that that is legal. Worth investigating independantly and trying to see if there is any "connection" between the agent and repair company. Even if it's just that they (not on the landlords instruction) always use that company.