Header tank over flowing

andy-man · 25 Apr 2026

I have an issue with our open vented heating system.
The system was fitted around 7 years ago and has worked with no issues.
I have just fitted 4 new designer rads and a heated towel rail. Re filled and bled the system, and balanced the rads. The issue i have is, when the boiler turns off, water comes out of the header tank.
I have tried turning the pump down as I thought it was drawing air in via the vents pipe. But it's made no difference. Live in a bungalow so the header tank is only about 3 foot above the boiler.
Never had an issue before fitting the vertical rads.
Could they be the issue.

Andy

CountryFan · Sunday at 3:43 AM

Have you checked if the float valve is operating correctly? Could be as simple as that.

Johntheo5 · Sunday at 8:29 AM

andy-man said:
I have an issue with our open vented heating system.
The system was fitted around 7 years ago and has worked with no issues.
I have just fitted 4 new designer rads and a heated towel rail. Re filled and bled the system, and balanced the rads. The issue i have is, when the boiler turns off, water comes out of the header tank.
I have tried turning the pump down as I thought it was drawing air in via the vents pipe. But it's made no difference. Live in a bungalow so the header tank is only about 3 foot above the boiler.
Never had an issue before fitting the vertical rads.
Could they be the issue.

A

Can you see what type of vent & cold feed arrangement you have, you don't say whether oil or gas fired with external circ pump?, your system may look something like the attached (close coupled) where from the boiler flow, you have the Vent then the Cold feed (no more than 150mm from the vent) and then the circ pump, also post make/model/setting of the circ pump.

andy-man · Sunday at 4:13 PM

Gas boiler, and close coupled as above.
it's a Grunfoss UPS3 set a speed 1.
Never had an issue in 7 / 8 years. Its only happened since I fitted some vertical tube designer radiators.
The boiler is an Ideal heat 24. I have noticed it only happens if the boiler is on and the timer turns off. The pump stops, as far as I can see from the wiring diagram. There is no way the pump can run, as it takes its feed from the orange wires on the 2 zone valves.

Johntheo5 · Sunday at 5:08 PM

It is very unusual with a gas fired boiler not to have pump overrun + a ABV (automatic bypass valve) for a a few minutes when the boiler stops, the pump would then be fed from the boiler, most of which hve the facility to provide power to the pump for this overrun time, however you have lived without it for years.

You say water comes out of the header tank, is this through the overflow?, does it only happen for a minute or so after the pump stops, is there any pump over through the vent?. Get someone to start the boiler via CH timer/roomstat while you watch the vent and see does it vent water through the vent for a few seconds and again keep viewing it while getting them to switch off the boiler.

A few other things you might try is, first, with the system shut down, just manually latch open the cylinder zone valve and ensure that the boiler does not start, I know that Honeywell zone valves do not close the auxiliary switch which fires the boiler but EPH do, others may do so as well, if the boiler does not start, then ensure HW programmed off...then latch its zone valve open, run the boiler for say 15 minutes or so on CH, then switch off and see do you still get overflow.
Secondly, you might change the pump speed to CP1, (constant pressure), which means the pump will run with a 3.0M head vs 4.2M and see does it make any difference, 3.0M should probably still be OK to circulate your system but see what happens anyway if you do those tests.

danfre · Sunday at 6:14 PM

S plan, no bypass

andy-man · Sunday at 8:10 PM

danfre said:
S plan, no bypass

S plan, and bypass.

andy-man · Sunday at 8:19 PM

Johntheo5 said:
It is very unusual with a gas fired boiler not to have pump overrun + a ABV (automatic bypass valve) for a a few minutes when the boiler stops, the pump would then be fed from the boiler, most of which hve the facility to provide power to the pump for this overrun time, however you have lived without it for years.

You say water comes out of the header tank, is this through the overflow?, does it only happen for a minute or so after the pump stops, is there any pump over through the vent?. Get someone to start the boiler via CH timer/roomstat while you watch the vent and see does it vent water through the vent for a few seconds and again keep viewing it while getting them to switch off the boiler.

A few other things you might try is, first, with the system shut down, just manually latch open the cylinder zone valve and ensure that the boiler does not start, I know that Honeywell zone valves do not close the auxiliary switch which fires the boiler but EPH do, others may do so as well, if the boiler does not start, then ensure HW programmed off...then latch its zone valve open, run the boiler for say 15 minutes or so on CH, then switch off and see do you still get overflow.
Secondly, you might change the pump speed to CP1, (constant pressure), which means the pump will run with a 3.0M head vs 4.2M and see does it make any difference, 3.0M should probably still be OK to circulate your system but see what happens anyway if you do those tests.

I can answer a few questions. We are away now for a few days, so will have to try the others when we are back.
Unfortunately an Ideal heat only boiler doesn't have the facility for pump overrun. Unless you by a pump overrun module.
Yes, water flows out of the overflow for 30 seconds or so after the boiler stops. But it only happens when the programmer switches the boiler off at the set heating off time.
If I manually latch the valve open, the boiler doesn't start.
A couple of things re the pump. Its always been set at speed 3. Now if it's on 3 there's a noticeable know when the boiler turns off.

Thanks for all the advice

denso13 · Sunday at 9:55 PM

If it is pumping over, just as it shuts down, then probably a restriction/blockage at the feed/expansion connection. A common problem.

Johntheo5 · Monday at 7:35 AM

andy-man said:
I can answer a few questions. We are away now for a few days, so will have to try the others when we are back.
Unfortunately an Ideal heat only boiler doesn't have the facility for pump overrun. Unless you by a pump overrun module.
Yes, water flows out of the overflow for 30 seconds or so after the boiler stops. But it only happens when the programmer switches the boiler off at the set heating off time.
If I manually latch the valve open, the boiler doesn't start.
A couple of things re the pump. Its always been set at speed 3. Now if it's on 3 there's a noticeable know when the boiler turns off.

Thanks for all the advice

Was that UPS3 installed with the original system ~ 7 years ago?, if so, and has been running on speed 3 since, then its simply incredible that you havn't had all sorts of pump over/air ingress problems in your OV system as this is a very powerful pump and will still run at a 6.4M head at 1.15m3/hr, over 19LPM. (see link below to a whole company that never seems to have realised this). I'm still not clear as to whether the F&E cistern is actually overflowing with no vent pumpover or pumpover with no overflow or both. Obviously though if still getting problems on speed 1 or (constant pressure) CP1 then possibe restriction as suggested in post #9

https://www.diynot.com/diy/threads/...e-does-installers-liability-end.649196/page-3

1

andy-man · Monday at 9:19 AM

Johntheo5 said:
Was that UPS3 installed with the original system ~ 7 years ago?, if so, and has been running on speed 3 since, then its simply incredible that you havn't had all sorts of pump over/air ingress problems in your OV system as this is a very powerful pump and will still run at a 6.4M head at 1.15m3/hr, over 19LPM. (see link below to a whole company that never seems to have realised this). I'm still not clear as to whether the F&E cistern is actually overflowing with no vent pumpover or pumpover with no overflow or both. Obviously though if still getting problems on speed 1 or (constant pressure) CP1 then possibe restriction as suggested in post #9

https://www.diynot.com/diy/threads/...e-does-installers-liability-end.649196/page-3

View attachment 413499
1

The system only overflow, when the timer turns off the boiler and the boiler is in heat mode.
If the room stat turns if the boiler, the pump continues to run, and no overflow. If the programmer turns of the boiler when it's in heat mode, 20 seconds or so later the header tank overflows.
When the pump was on speed 3, the system shut down with a thump.

Johntheo5 · Monday at 12:27 PM

andy-man said:
The system only overflow, when the timer turns off the boiler and the boiler is in heat mode.
If the room stat turns if the boiler, the pump continues to run, and no overflow. If the programmer turns of the boiler when it's in heat mode, 20 seconds or so later the header tank overflows.
When the pump was on speed 3, the system shut down with a thump.

If you mean that "the room stat turns if OFF the boiler" and the pump continues to run continuously then something wrong if wired like you say it is, see schematic below, my oil fired boiler (no pump overrun) is wired similarly, then when the roostat is satisfied or programmed off, the zone valve is de enrgised and closes, its end switch opens and switces off the 230V supply on the orange wire to both the boiler and the pump, both then stop.

andy-man · Monday at 1:13 PM

Johntheo5 said:
If you mean that "the room stat turns if OFF the boiler" and the pump continues to run continuously then something wrong if wired like you say it is, see schematic below, my oil fired boiler (no pump overrun) is wired similarly, then when the roostat is satisfied or programmed off, the zone valve is de enrgised and closes, its end switch opens and switces off the 230V supply on the orange wire to both the boiler and the pump, both then stop.

No the pump stops after about a minute, when the stat turns off the boiler.
Spoke to Ideal who confirmed that there's no over run when the programmer shuts of the boiler. They do sell an over run kit though.

75pete · Monday at 1:18 PM

Did you drain the system down to fit the new radiator? Was the header tank clean and empty before draining ?
So as denso 13 said, poss blockage.
Simple trick that I’ve used: see FAQ 12 for a blocked header tank ( posted by Chris )

Johntheo5 · Monday at 2:04 PM

andy-man said:
No the pump stops after about a minute, when the stat turns off the boiler.
Spoke to Ideal who confirmed that there's no over run when the programmer shuts of the boiler. They do sell an over run kit though.

Well. Something must be telling the pump to run on for a minute, if the cylinder heating is on and the cylinder stat is satisfied what does the pump do?

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Header tank over flowing

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