Central heating Pump Overrun

[Malcolm H]

I have a Glow-worm Fuelsaver 40-50 conventional flue boiler which was installed in 1984.

Everything works fine except that the central heating circulating pump stops running as soon as the room thermostat reaches the desired temperature. This results in some noisy bubbling from the boiler as it overheats for a short time after the room thermostat switches the pump and boiler off.

Does anybody know if this boiler was designed to have a pump overrun feature to avoid this problem?

If the boiler was not designed to have a pump overrun feature is there a control available which can be added retrospectively. e.g. is there a timed control available which will allow the pump to run for a short time after each switch off?

TIA

[Ewan MacIntyre]

How bizarre: I've been wondering about exactly the same problem, with my Glow-worm Hide Away. It's slightly younger than yours, possibly 1986 or so.

I think it's quite wasteful to have all that recently-heated water suddenly come to a halt, rather than circulate it for a short time to spread it around. Also, as you mention, it causes kettling which must in turn reduce the life of the boiler by a small amount each time.

I've considered building my own overrun feature using a mains powered relay, a microcontroller and a solid-state relay, so that when the system turns the pump on, it is actually just indicating this to a microcontroller, which will delay the turning off. Like so many similar ideas, it's in my head rather than built...

Ewan

[Malcolm H]

I too have a design in my head but it only requires one power diode (rectifier), one resistor, one electrolytic capacitor and one relay.

[Set Square]

I don't know whether the boiler has a built-in pump over-run facility - but being 1984 vintage, probably doesn't.

In think that in your position, I would implement my own - using a mains relay and a pipe stat. Connect the coil of the relay to the boiler switched live. Connect the relay contacts to a permanent live and to the pump. Connect the pipe stat contacts in parallel with the relay contacts. Install the pipe stat on the boiler flow pipe and set it a similar temperature to the boiler's own stat (a bit of experimentation may be required!)

The pump will then run whenever the boiler is firing *and* whenever the boiler outlet temperature is above the pipe stat setting. This is, in effect, what a built-in over-run stat would do - and is better than an arbitrary time delay.

[Dave Plowman]

I added this feature to mine. ISTR I had to change the pump switching to a mains relay, as simply feeding mains to the pump after the thermostats were satisfied caused problems by feeding those volts elsewhere - although that might well not be the case with others. I made the timer circuit which is set for 30 seconds. Basically, a small low voltage power supply, a 555 timer and a relay. Cost nothing as I had all the bits lying around, but probably no more than a tenner to buy.

At the time, I investigated finding a ready made unit but couldn't. And, of course, most boilers have these built in now.

[Dave Plowman]

This will work, but you might have to add a second relay to the pump, as you'll be feeding volts to it which may then go somewhere else that they shouldn't, as it were, with the system 'off'. That's what happened with mine. I never bothered working out exactly why, but IIRC it caused the system to fire continuously.

[Andy Hall]

Yes it did. I used to have one until a year and a bit ago.

The pump over-run is provisioned by a separate set of contacts on the thermostat. Either the pump was never wired to them, or as on mine, the thermostat has failed.

When I enquired regarding the cost of a new one it was in the £70 range. Since this followed a failed thermocouple (that admittedly only cost £2 to replace) I decided that the boiler was at the end of its useful life. These things are a piece of junk anyway. I was never able to dismantle it without cutting myself on sharp metal somewhere.

Considering also that they run at 65% efficiency at best, I felt that spending money on a new thermostat was pointless and put it towards the cost of a new condensing boiler instead.

You could do this with a stairway lighting timer.

Have a look at rswww.com part number 277-3520

It has a mode whereby the output remains on for a time after the input drops.

.andy

To email, substitute .nospam with .gl

[Set Square]

I can't see why you'd need a second relay. I drew a circuit diagram before posting the above, and satisfied myself that, with the single relay as described, the boiler firing will cause the pump to run but *not* vice-versa.

[Malcolm H]

Thank you for your reply Set Square. Your solution looks very elegant to me and I see no reason why it should not work. I propose to implement it as soon as I can.

Malcolm

[Set Square]

Please let us know how you get on!

[geoff]

In message , Malcolm H writes

Having spent most of today poring over Glowworm manuals, I can say with a fair degree of certainty that it doesn't

Yes there are several, but if this has started happening recently, it would make sense to find out why it's happening rather than bodging it

[geoff]

A microcontroller just to give you a 2 minute delay - bit of an overkill isn't it?

[Andrew Gabriel]

Low power 555's handle minutes without problems, but this application probably only really needs around 10 seconds, and it doesn't need to be accurate.

[Andy Hall]

Mine definitely did, although thinking again it was installed in 1985.

From the Sedbuk web site, there was a Fuelsaver 40-50b (identity

4131583) which was last manufactured in 1983.

There was a Fuelsaver 50Mk2 (4131320) which IIRC is what I had. It was range rated from 11 to 14kW and I'm pretty sure said something like

40-50 on the outside case.

Perhaps the over-run was the Mk2 feature....

Either way it's time it was in the skip.

To email, substitute .nospam with .gl

[Steve]

Cheap enough these days, and about 30 seconds of programming effort. Why ponce about with 555 timers (flaky at long delays) ?

Steve·

[geoff]

In message , Andrew Gabriel writes

The commercial ones use shift registers (40154?)

There is no such thing as 30 seconds of programming

[Dave Plowman]

Maplin used to do a chip designed for car heated rear windows etc on which you could set the time from a few seconds to a day or so with just one R & C. It had enough drive current capability for a meaty relay, and could either toggle on or off, or on at power up. Much less susceptible to interference on the supply than a 555. Cheap, too, and I bought a load of them as it's an easy answer to many problems. One of them provides the 14 hour timer for my Ni-Cad charging area.

[Ewan MacIntyre]

Yes, unless microcontrollers are already being designed into the system.

My house has 4 zones + DHW, yet there is just one thermostat. The zones must be controlled by flicking switches behind a kitchen unit...! I think whoever installed it was good at plumbing but knew nothing about usability.

I'm currently working on controlling these in a more convenient manner, using one or more PICs.

Ewan

[Malcolm H]

It's just been pointed out to me that it will be necessary, of course, also to install a by-pass for the valve which controls the central heating water flow, otherwise the water will have nowhere to go!!

[Christian McArdle]

You should really be installing a programmable thermostat in each zone. Then each zone can be timed separately AND will hold better temperatures.

Christian.