Pump connected after boiler thermostat ???

[Jonathan]

I recently moved into a house and I've successfully upgraded the CH/HW controls to an 'S-Plan' type system (see previous posts). My modifications only required me to replace a mechanical timeswitch with a programmer, thermostats and 2-port zone valves. In other words, I pretty much ignored what was in the external boiler housing and replaced the single mech timer with some more elaborate switching gubbins.

Over the weekend, I noticed something that I think is a bit odd. The external boiler housing contains the oil-fired boiler (warmflow, no pump overrun required) and a circulating pump on the feed. However, the pump seems to be wired _after_ the boiler thermostat (a small gray box with a knob from 1..5 that supplies mains power to the burnber unit). It strikes me that this is odd and potentially damaging. Surely the circulating pump should be circulating water when either the CH or HW zones require heat, not just when the boiler is in the part of its cycle that fires the burner.

Does what I've described sound normal? Or should I rewire the circulating pump?

Cheers, Jonathan

[Set Square]

The pump should be running whenever the *external* controls are telling the boiler to fire - even when it's own internal thermostat is telling it not to.

It you wire it up according to the S-Plan diagram in

it will be fine!

In brief, the timer and room stat control the CH zone valve. The timer and cylinder stat control the HW zone valve. Whenever either or both zone valves are open, power is supplied to the boiler and pump, in *parallel*.

[Jonathan]

Thanks. That confirms what I thought. I'll re-wire it the way it should be.

Jonathan

[Dave Plowman (News)]

The slight problem with this design is that if the boiler is firing when the room or cylinder stat are both satisfied, when shut down it may kettle. IN which case a simple pump over-run circuit is needed.

[Jonathan]

I don't believe kettling is a problem in this case, and my fix should only cause the pump to ru nmore than it does now (specifically, when the zone valves tell the boiler to fire but the boiler's thermostat is telling it not to). But I'll bear in mind what you've said .....

[Doctor Evil]

The senility has set in. He said the makers say it doesn't need a pump over-run.

[Dave Plowman (News)]

Good grief - you've actually fully read a post and understood it.

Google readers, this is a first.

But why aren't you recommending it's all replaced with some Japanese gismo?

[Dave Plowman (News)]

Apologies. Your boiler already must have some provision for this, if it says no over-run needed.

[Doctor Evil]

You obviously can't do that.

[Jonathan]

Your point is still relevant though for another reason. As an external boiler, there should really be a frost thermostat, which there doesn't seem to be (based on the wiring installed by the builders), but that's a whole other story!

Incidentally, the symptoms that led me to check the pump wiring were (i) what I felt were unusually long times between boiler fire cycles (due to hot water not being cleared from the boiler) and (ii) a need to have the boiler thermostst near its maximum setting (although this probably isn't the right time to test this in Ireland, given the recent weather!). Do these symptoms make sense, given the problem I described with the pump wiring?

[Dave Plowman (News)]

Well, neither the C or S plan allow any feedback from the boiler or provide any over-run, unless I'm missing something.

So there's no way the boiler can know in advance when its services are no longer required. So I presume it has a fast acting shut off to prevent kettling and a heat exchanger made out of a low thermal capacity material,

-ie not cast iron.

But I'm only surmising as I don't know anything about oil boilers.

It's simple enough to bring the temperature of the water stored in the boiler down - simply clamp a cylinder stat with changeover contacts wired to break at low rather than high - most do - to the output pipe so that it keeps the pump running until the desired lower temp in the boiler is reached. Although I simply built a timer which runs the pump for a couple of minutes after everything is shut down as it made for easier wiring in my case.

[Set Square]

If by (ii) you mean that your DHW didn't get hot unless you turned the boiler stat UP, I would agree. When heating the water, your hot cylinder probably can't absorb heat as fast as the boiler can produce it - with the result that the boiler needs to cycle on and off. BUT the pump must run until the cylinder stat is satisfied - even when the flame isn't on - otherwise the water won't continue to heat once the boiler stat has opened.

(i) sounds reasonable too - because with no pump the hot water *inside* the boiler won't get taken away - and will stay hot for a long time, causing the boiler stat to stay open.