UFH added, now boiler occasionally trips out

[arbitrary]

My central heating used to consist of a Potterton Profile 60e boiler with radiators on 2 floors and a hot water cylinder upstairs. The system is vented with a circulation pump. There was a three port valve that sent the boiler's output to either the hot water cylinder or the rads as requested. It worked fine.

Then I retrofitted underfloor heating on the ground floor. The three port valve was replaced with 3 two port valves (1 each for the UFH, rads upstairs and the hot water cylinder) and under the control of a Heatmiser wiring centre and thermostats. The whole system has generally worked fine for the last 3-4 months except that the overtemp "breaker" on the boiler occasionally trips out and requires a reset with a fingertip. This has happened perhaps 4 or 5 times since October.

I think I have identified the circumstances under which the boiler trips out. When the system is demanding heat but the heating period ends (such as at 07:15 on a weekday morning when I plan on leaving the house and reduce the daytime temp to 14 rather than 18) the Heatmiser system tells the boiler to turn off and it closes all of the 2 ports valves. If the boiler was still going full steam at that time its heat output has nowhere to go and the boiler trips out. The circulation pump does continue to run for perhaps 10 minutes but only the 2 towel radiators are on the bypass circuit and if they're good and hot there's a good possibility there's nowhere for enough heat to dissipate fast enough. I had this happen yesterday; I heard the Heatmiser relays click off and the boiler stop and perhaps 10 or 15 seconds later I heard the boiler trip.

I suspect this issue never happened with the old system as when the boiler shuts off the "excess" heat went to either the rads or the hot water cylinder as the three port valve was always open on one side or the other. But now the 2 port valves all shut off and then the boiler might trip.

I've asked Heatmiser if the wiring centre can be configured to delay the closing of the 2 port valves but they say no. I have turned turn the boiler's output temperature which should help but I would welcome any other suggestions on how to further reduce the odds of the boiler tripping out.

 

[picasso]

Google automatic bypass.

 

[arbitrary]

Thanks for that - are you suggesting that I need that automatic bypass in case the TRVs on the towel radiators are shut meaning that the towel radiator bypass circuit is effectively blocked?

 

[MrTherm]

No auto bypass, cut and pasted from manufacturers instructions -->

"Where a pair of two port valves are used, a by-pass is
necessary. The total length of the by-pass circuit taken
from the boiler connections should be greater than 4
metres of 22 mm pipe. It should be fitted with a
lockshield valve and be adjusted to maintain a minimum
flow through the boiler of 4.5 litres/min (1 gal/min)"

Of course a permanent on radiator (with no trv) can be used provided it bypasses any zone valves so that the built in pump overrun of your boiler can opperate.

 

[arbitrary]

and look what I found installed in the airing cupboard tucked away under the shelf! any idea how to set it? when the plumber fitted it the system was drained so at best he would have guessed at the setting.

 

[Daveydub]

Is the arrow pointing in the direction of flow or is it perhaps fitted the wrong way round?

 

[arbitrary]

It looks to be installed in the correct direction. That said the output pipe is very hot all the way to where it joins the rest of the returns to the boiler suggesting it's allowing some water to bypass the 2 port valves even when one (for the UFH) is open.

 

[Daveydub]

Although the zone valve to the UFH is open, it could be that the blending valve is moved all the way to full re-circulate therefore there is no flow through the zone valve.
The auto by-pass ought to be sufficient (assuming that you have at least 4m of pipe between boiler & airing cupboard.
You can not turn that valve much further than it already is but take it to the limit (0.1).
If that still has not cured it, turn one of your towel rails to max and see if that does the job.

 

[arbitrary]

At the moment the return from the UFH is surprisingly cool; I think the UFH is soaking up a lot of heat. Input temp is roughly 56 C and output perhaps 28.

 

[snb]

your profile boiler doesn't have pump overrun so an auto bypass will serve no purpose to stop the current issue. The only way round this would be to turn the boiler stat down lower, but this may still cause the odd trip or to fit a pump over run timer so that it continues to pump for a period after demand has ceased.

 

[arbitrary]

your profile boiler doesn't have pump overrun so an auto bypass will serve no purpose to stop the current issue.

Not sure I agree - the pump can be felt and heard running after the boiler has been told to stop by the controller.

 

[Wildcat1961]

Profile utilises pump overrun.

 

[Daveydub]

Was the system dirty? If so the maybe muck in pump impellor causing poor flow.
Are your pumps set up correctly?
Is flow & return at the boiler very hot when it overheats, are they much the same temperature, is pump still running or has it stopped?