Adding a pump overrun

[ARWadsworth]

Is it possible to add a pump overrun to a boiler that does not have this facility built in. I was hoping that there was some sort of control box for this but I have been unable to find one. The reason I ask is that the plumber thinks the heat left standing in the pipes is causing the boiler's thermostat to stop the boiler re firing after it has stood for a while after being used. The boiler is about 15 metres from the pump, rads and HW tank and placed in an outhouse. Any ideas or help please?

-- Adam

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[Andrew Gabriel]

You need a circuit which provides a delayed switchoff, like the overrun timer for a bathroom fan, which are available separately. You would probably need it operating the pump via a relay, as they only contain a small non-heatsinked triac for handling a 20W fan. It may be that dedicated boiler pump versions are available too. The circuit simply by virtue of additional complexity obviously introduces additional risk of failure to operate pump, and unless the boiler has a water flow detector, I would worry about that -- personally I wouldn't want to rely on over temp detector only.

[fred]

In article , Andrew Gabriel writes

Using a bit of parallel thinking, how about using a pipe stat to keep the pump running until the flow temp drops below a certain value, say

50degrees? Normal pump demand and normally open contact of pipe stat connected in parallel. Certainly a cheap option, 6-10quid.

[IMM]

That is usually the accepted method. Set the pipe stat on the flow near the boiler and set it slight less than the boiler stat.

[Andy Hall]

The right one would need to be chosen. Generally pipe thermostats are for preventing freezing.

However, Honeywell make one (L641B-1012) which is adjustable from 50 to 95 degrees and has a changeover contact. Discounted Heating have them as I am sure others do, but price is in the £23 area.

.andy

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[Grunff]

How about a capillary stat, e.g. BES 9780, with the bulb strapped to the pipe?

[Andy Hall]

Could do. However, the catalogue doesn't say what contact arrangements there are. For this application it needs to have a pair of contacts which are closed above the set point rather than open as is normally the case. Changeover would be OK of course.

The other thing would be making certain that there is good contact between the phial and the pipe. Normally these things go into a pocket in the heated item. Otherwise, the thermostat is likely to be a bit inaccurate, although that probably doesn't matter.

Something like a short piece of appropriate size microbore pipe, soldered to the water pipe being monitored would probably be a reasonable solution. The phial would slide into the piece of tube.

.andy

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[Dave Plowman]

When I fitted an over-run, I had a play with this but discovered it not to be quite so simple - the volts from the thermostat went the 'wrong' way as well and caused problems with my 3 port valve. Can't remember exactly what as it's a long time ago - something to do with wanting it to work after the entire system had shut down as well. The way round would of course have been to add a relay which isolated the valve switching etc. But having concluded that, I simply made an electronic timer using bits I had lying around based on a U6047 car heated rear window timer chip and a relay, as I didn't have any suitable mains coil relays 'in stock' at the time.

[Ed Sirett]

A way to do this as I understand it: Connect the pump to the common pole of the changeover switch, connect the boiler demand to the pole which is closed when the pipes are cool, connect a permanent live feed to the pole which is closed when the switch is hot.

However none of this adds up as far as the OP is concerned when the boiler is demanded the existing arangements should start the pump and very soon cooler return water will bring the boiler temp down and make it fire.

Perhaps the existing boiler has an overrun thermostat and there is a wiring failure somewhere and/or the overrun thermostat has gone out of spec?

[geoff]

In message , ARWadsworth writes

What's the boiler?

[geoff]

In message , Andrew Gabriel writes

CH Pump overrun pcbs are readily available (maybe not the cheapest option though)

[Bob Smith]

Before you spend anything (money or time) you could wire the pump to be on permanently, to see if it actually fixes the problem.

Bob

[fred]

In article , Andy Hall writes

The cheapie BES cyl stat has a pipe fitting option and has changeover contacts:

item 9777 The coily spring thing in the picture is the pipe fitting spring. £6.73 + vat & post.

>

[fred]

Nice idea, simple is best . . . .

[fred]

In article , Ed Sirett writes

The impression I am getting is that the boiler is firing, heating for a while, then boiler stat clicks off as expected, but residual temp in the boiler is causing it remain off for longer than is desired. Nice that it is not short cycling ;-), but not what is desired. The pump run on will hopefully sort that out, but hopefully not result in short cycling before the room stat is satisfied.

Makes me glad mine is modulating . . .

[Bob Smith]

Well I was thinking short term, just to see if it would work, but you could do a permanent job by having the pump switched on by the CH timer.

Incidentally, my dad has his on permanently to stop the sludge building up in the pump during the summer months, because his pump is low in the system.

Bob

[IMM]

If there is sludge in the system something wrong with the system and it should be rectified. Also the sludge should be removed and correct inhibitor used.

He could have saved a ton of money in electricity by using a simple timer to turn the pump one a day for 30 seconds.

[ARWadsworth]

I have not seen it. It is a friends house some mile away, but he says it is old.

-- Adam

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[ARWadsworth]

Just a bit too obvious and simple. This is definitely what I will do to see if it cures the problem and if it does, I like the idea of adding a pipe stat to allow the overrun (Thanks Fred for that one)

-- Adam

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[ARWadsworth]

That would seem to be precicely the problem. I have not seen the boiler and I have only spoken to my friend and his plumber who cannot fix this on the phone. It is the plumber who thinks an overrun will cure the problem.

I will get the make and no of the boiler tommorow. But I am told that it is old so I do not expect an overrun to be part of it's design

-- Adam

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