Central Heating and Boiler Technical Questions _ Bit of an Essay

[TheScullster]

Hi all

Apologies in advance this could go on a bit!

Since installation a year ago, the blue warning light on my Worcester Bosch

24Ri had occasionally been flashing intermittently. This is in a "standard" s plan plus system - 2 heating and one HW zones - vented. As it was a rare occurrence, I procrastinated having loads of other projects to attack (you know how it is). Decided to phone WB to query the cause of this and the first thing he mentioned was pump over-run and to check that the system had been wired correctly to allow this. So I started investigating whether the pump did over-run and found that when HW demand was set, the pump did over-run by 3 minutes. Tried the same with the CH but the pump just continued to over-run with the piping around the pump staying hot.

After some deliberation it ocurred to me that the by-pass valve was probably not operating. So the call for heat is turned off, but, as the solenoid valves are closed and the by-pass isn't working, the boiler experiences no flow and overheats (presumably this causes the flashing light). As the boiler isn't being cooled by any water flow, it simply continues to run the pump which is effectively working against closed valves.

So finally the questions are:

Will the boiler have been damaged in any way by being allowed to sit with over-hot water content? I do not believe that it would have fired in this state, just sat there! Similarly will the pump have been damaged by being allowed to pump against a closed valve arrangement (I am fairly certain that the pump was actually generating heat when left running in this state)? The pump over-run that occurred on my CH test continued for over 20 mins. But this state would only exist at the end of a heating or hot water time period.

Any informed comment appreciated

TIA

Phil

[Roger Mills]

It's unlikely to have damaged either the boiler *or* the pump.

But it seems to me that the boiler's pump over-run logic needs to be looked at. Unless the by-pass circuit is extremely short - and well lagged - I can't see how it could possibly take 20 minutes to cool the boiler to a safe temperature after it's stopped firing. Mine (different boiler and setup) runs for 30 seconds max!

[TheScullster]

"Roger Mills" wrote

Thanks Roger,

The reason that it was over-running for this long was because the by-pass valve wasn't opening and therefore no water was being circulated through the boiler to allow it to cool! I haven't checked the over-run time accurately since re-setting the by-pass valve (correctly I hope). Out of interest, do you have (or have knowledge of fitting) a by-pass valve? Mine is a DU145 (honeywell I think) installed in a pretty standard 22mm airing cupboard configuration. If you have the same in a similar install, can you tell me the set point for operation in your system please? Mine was about 2.5 on the end cap dial.

One other question came up in a discussion on this subject: Do the zone valves simply stay open for the duration of each "on period", or do they close each time the relevant stat is satisfied?

TIA

Phil

[Roger Mills]

Hopefully it won't use a timer anyway, but will go on a stat when the boiler temperature is low enough. I assumed you'd already fixed the by-pass problem!

Sorry, I've got a Y-Plan system (and currently no TRVs) so there's always a flow path and no need for a by-pass. But the by-pass needs to be adjusted so it opens when the pump is running with the valves closed, but stays closed in normal operation.

In an S-Plan system, the valves close whenever the relevant demand is satisfied - like when a room stat or cylinder stat clicks off. It's secondary switches within the valve which operate the boiler and pump, so the valve has to close in order to turn the boiler off.

[geoff]

In message , Roger Mills writes

How? its in a PIC

20 mins is not uncommon - the potterton Suprima has a 20 min pump overrun

I don't remember what the WB overrun is

[Roger Mills]

Well, I meant he needs to check whether it's functioning as intended. What you do about it if it isn't, is another matter.

What's the rationale for that? The over-run on my MkI Baxi Solo just operates until the main boiler stat opens - which seems perfectly reasonable, and only runs for half a minute or so. Why would you need to run for a long time?

[geoff]

In message , Roger Mills writes

... temperature dependent old thing

The Solo 2 has a pump overrun of 10 minutes (unless it has a certain fault where it never switches off)

[Roger Mills]

OK, so I repeat *why*?

Surely the purpose of the pump over-run is to get rid of the residual heat after the boiler stops firing. And surely the way to determine that it has been successful is to measure the temperature, *not* to run it for an arbitrary fixed time and hope for the best! Where's the progress in *that*?

[Cicero]

================================== You might find the appropriate instructions here:

[Ed Sirett]

20 minutes is excessive, but...

One of the functions of the pump-overrun might be where you have an indirect coil. You want to keep the primary circulating so as to keep transferring heat. You might have an old cylinder and when there is only a small difference in temperature between the primary and the DHW, the effective heat transfer rate will be very small. However if the pump stop after only a minute the heat in the primary pipework would then not find its way to the cylinder. The boiler only re-firing when it cools down enough.

[geoff]

In message , Roger Mills writes

I don't have a definitive answer to either question - ask one of the manufacturers

[Roger Mills]

Yes, but in an S-Plan system - which is where this thread started - the boiler will only be turned off (apart from cycling on its own stat, with the pump running normally anyway) when the demands are all satisfied, and the zone valves all closed. That being the case, during the over-run cycle the water will only go round the by-pass circuit, and not through the coil. With any other 'plan' there's no guarantee of a flow path through the coil - for example, a Y-Plan's mid-position valve may well remain in the CH position when the boiler cuts.

So, although I hesitate to question your wisdom in such matters, I'm struggling to believe that boiler manufacturers would have this consideration uppermost in their minds when designing over-run logic.

In cases where there *is* a flow path through the coil, isn't there a danger that you'll actually *cool* the cylinder rather than heating it if you circulate water through the coil for a long time without running the boiler?

[<me9]

I found when I was using the immersion heater because of a boiler fault, and hadn't bothered to isolate the boiler, the pump would heat the radiators from the tank quite well, for a short time.

I must work out how to make the DHW override the heating (modified S plan) when a call for DHW. Back contacts of DHW programmer and thermostat perhaps.

[Ed Sirett]

yes the pump over run when all the zones are satisfied is OTT.

However in the case where there is a rather slowish indirect coil and there is HW only selected my argument stands. When the boiler has cut on its internal t/stat the pump must over run for a while.

Of course many (older) systems the pump will run when any zone is selected.

[Roger Mills]

Ed, you and I seem to have a rather different concept as to what pump over-run is all about!

My understanding is that it's purely about protecting the boiler from overheating after the burner has been turned off by an *external* control - such as a room stat or zone valve. The pump needs to keep going for a bit in order to carry away the residual heat - and the water must have somewhere to go - hence the need for a by-pass circuit (preferably automatic) in systems (e.g. S-Plan) which can have *all* their zone valves shut at the same time. So it's precisely when all zones are satisfied that it *does* need to work.

When the boiler cuts on its *internal* stat, the external controls will still be demanding heat - so the pump will be running anyway, and doesn't need to invoke the boiler's pump over-run logic.

[TheScullster]

"Roger Mills" wrote

OP here!

Yes that is also my understanding. And the fact that in my case the by-pass was incorrectly set meant that the call for over-run did not result in the objective of carrying off residual heat. Hence the alarm state on the boiler.

Phil

[Roger Mills]

Have you now sorted the by-pass, and fixed the problem?

[TheScullster]

"Roger Mills" wrote

I think so, Roger, thanks. With the alarm fault on the boiler being very intermittent, I can't be 100% sure, but the fault hasn't occurred since re-adjusting the by-pass. I will try and remember and re-test each ciruit and the resulting by-pass operation this weekend. Not easy to do this in the evening as all pipe work is already hot by the time I get home!

Phil