Pump overrun

[meow2222]

Whats the simplest way to add a pump overrun feature to an old CH system? I've nothing against a diyed overrun.

NT

[Andy Burns]

Pipestat relay-ORed (or whatever) to pump?

[dennis]

If its an old cast iron one it probably won't need it. If it does need it it would already be there.

The ones that need it have the pump connectors on the boiler where the overrun thermostat is.

You need a bypass somewhere unless its already in the boiler. Fit an autobypass valve if you need one.

[Bob Minchin]

You could possibly keep it simple and yet adequate by a simple timer/relay to hold the pump on for a fixed run on time 5 - 10 minutes perhaps?

[John Rumm]

[Roger Mills]

Yes, that's what I did - using one of these:

It's effectively connected in series with the pump, but also has a permanent live - which keeps the pump running for a pre-settable time after the demand is removed. Mine is set to about 2 minutes.

See

and scroll down to the bit about adding pump over-run using a Manrose fan timer.

My boiler does actually have (fairly crude) built-in pump over-run 'logic'[1] - but that can sometimes get caught out, allowing the boiler's over-heat stat to trip. I've not had that problem since fitting the timer.

[1] This uses an extra set of contacts of the boiler's main stat - the effect of which is that the pump runs whenever there is an external demand to the boiler OR when the boiler temperature is above the stat setting. The idea is that it will keep the pump running until the boiler has cooled to below the stat setting. Problem with this is that if the room stat cuts the demand when the boiler is just below the stat setting, the pump stops instantly - but the temperature continues to rise due to the residual heat. By the time the stat wakes up to this and turns the pump back on, it's too late - and the overheat stat trips before the pump can prevent it. Using a timer to keep the pump running for an extra couple of minutes whatever, avoids this problem.

[dennis]

That is more complicated than using a stat.

You probably need to wire it so the common goes to the pump, the live that is connected to "boiler on" to the below temp connector, and the above temp to permanent live. That way the pump will run as soon as boiler on goes live and will stay on when hot but the stat won't supply "boiler on" by mistake.

[Chris French]

True our CI boiler doesn't need one, but I did add one when I redid some stuff a couple of years ago. It may as well dump the heat in the water in the boiler into the house/HW tank rather than up the flue (passive vertical flue)

I just used a pipe stat to keep the pump running until the water cools enough

[Michael Chare]

A 'delay off relay' does this sort of thing.

[ARW]

Well there you go then. That's answered the OP question.

[meow2222]

I've nothing against a diyed overrun.

I like the idea of a fan overrun timer. There's a swiftair one for £17 de livered here

The system is old & unmaintained, lightweight exchanger & vented primary. I t kettles badly. At this point I dont know if its crud in the boiler or a f ailing pump. I gather its always kettled, but is now a lot worse.

I guess the way forward is to do all 4: flush system & inhibitor, new pump, clean out boiler HE and add an overrun timer. It looks like a Y plan, thou gh it might be a 'one or the other only' 3 way - either way it should be fi ne with overrun.

Anyone come across swiftair overruns before?

NT

[Roger Mills]

I've not come across that particular one, but it appears to do the same job as my Manrose timer, so I think it should work ok.

If the system is either a Y-Plan or a W-Plan, pump over-run should work fine without a by-pass because there should always be somewhere for the water to go - unless, of course, all the radiators have TRVs and they are all closed!

Does your boiler kettle all the time - or only immediately after the flame and pump have been cut? If the latter, keeping the pump running for a bit may help. If the former, you probably need to look for a different solution.

[John Rumm]

There is a difference between need and "would benefit from" though...

[Roger Mills]

Yes, indeed. Such boilers probably date from the time when it was usual to have a gravity HW and pumped CH system. In HW-only mode, the pump wasn't running - so the boiler *had* to be able to hold its own without any pump over-run. When the boiler stopped firing, the gravity circuit would continue to circulate - so any residual heat would be dissipated.

But when you use such a boiler in a fully-pumped system, it's a different story and, with smaller pipes, there's unlikely to be much gravity circulation even if there is a path.

So having pump over-run can be beneficial in making some use of the residual heat - even if the boiler isn't in danger of tripping if you don't have it.

[John Rumm]

I used to find my old Mexico would kettle a little all the time unless treated with a couple of litres of boiler noise reducer each year.

[Fredxxx]

Max 200W. I would have hope for that price it would have been more!

I would be worried about using a simple timer given heating pump power requirement.

[Roger Mills]

Why? My Grunfos Alpha+ pump has a maximum power requirement of 80 watts. What sort of pump, have you in mind?

[Fredxxx]

I had assumed that power consumption was around the 200W mark. Just looked up a common Grunfos and can confirm they are sub 100W.

[meow2222]

:

em? I've nothing against a diyed overrun.

7 delivered here

y. It kettles badly. At this point I dont know if its crud in the boiler or a failing pump. I gather its always kettled, but is now a lot worse.

ump, clean out boiler HE and add an overrun timer. It looks like a Y plan, though it might be a 'one or the other only' 3 way - either way it should b e fine with overrun.

This one kettles badly when the flame & pump go off. When burning its becom e noisy too, but not boiling, more like a kettle well before it boils. I'll fit the overrun.

I reckon the pump's working ok. It is making a bit of scraping noise at tim es but when that dies down its still doing the same speed.

Thanks everyone.

NT

[meow2222]

I've nothing against a diyed overrun.

Update... The swiftair fan overrun timer works perfectly. It rated 85w max load, the pump's 105w. The odd voltages of a Y plan system appear on a diff erent place to the pump & boiler, so maybe its some odd variant on a Y plan , I don't know. There's no way I'm going back ther to trace the system wiri ng, the way its done would make that a mare anyway. The minimum overrun set ting is 1 minute, which is fine for the boiler.

So pump overrun timers are now cheaper.

NT