boiler overheating only when UFH mis running

[Peppertack]

Good afternoon

I have a Vokera Mynute 25 HE which runs very well. HW, CH, and UFH all run no problem. Pressure is good cold and hot. Problem is, if only the UFH is on, it makes a demand for heat and gets it but the boiler not only delivers to the UFH set temp at 45 but gets a little higher, shuts off and the temp continues to rise.
I have the boiler set at 56 but with only the UFH on, after a demand cycle, the temp can raise to 92- 93 and of course it will shut down around this. The boiler itself shuts down the flame around 58 but then the temp rises very quickly after that.

This could be a result of the water recirculating in the boiler and rising too quickly but why?

Any ideas would be very gratefully received.

 

[PullerGas]

Good afternoon

I have a Vokera Mynute 25 HE which runs very well. HW, CH, and UFH all run no problem. Pressure is good cold and hot. Problem is, if only the UFH is on, it makes a demand for heat and gets it but the boiler not only delivers to the UFH set temp at 45 but gets a little higher, shuts off and the temp continues to rise.
I have the boiler set at 56 but with only the UFH on, after a demand cycle, the temp can raise to 92- 93 and of course it will shut down around this. The boiler itself shuts down the flame around 58 but then the temp rises very quickly after that.

This could be a result of the water recirculating in the boiler and rising too quickly but why?

Any ideas would be very gratefully received.

Is there a bypass valve fitted PT & what is it set at?

 

[Peppertack]

Thank you very much for that, PullerGas. I not sure if that is one on the end of the top line. That is on 40.

Kind regards P

 

[PullerGas]

Blimey is there only two loops on the UFH??

Auto bypass valve;

http://www.jtmplumbing.co.uk/heatin...Z8pjQwIhi1vD-2PEJaWcZx7f20SQ9PpptQaAmaS8P8HAQ

 

[Peppertack]

Thank you for that, PG - Yeah, only one for a very small kitchen and one for the hall. I am looking for the bypass now.

Thanks P

 

[Peppertack]

Well, I cant seem to find a bypass PG. Funny thing is it has been working just fine until last night. It did do this about a year ago but that was a result of the expansion vessel loosing its charge. Been fine ever since that was replaced.

 

[Agile]

I seems that you have a blending valve above the pump and controlled by the white sensor.

However, I am not sure that it is correctly arranged based on what we see in the photo.

If there was an auto bypass then it would usually be located close to the boiler between flow and return pipes.

 

[PullerGas]

While there will be a small bypass on the appliance(internal), it's unlikely to be big enough.

 

[simond]

The bypass fitted to the Minute is, according to the manufacturer, fit for purpose.

Also, as it worked OK before, something has changed. Boiler might have a partial airlock in it, or sludge could have blocked the internal bypass.

One of the other RGis on here is a Vokera specialist, so lets hope he picks up on it.

Vokeras normally have an aluminium HEX, so water flow is paramount to stop overheating.

 

[Peppertack]

Thank you very much for the replies Agile, PG and Simon. I understand the manifold has been supplied 'as is' TRV to the lockshield by Uponor and the plumber has only installed the air vents and gate valves. So I cant see why anything there might be wrong. I'll study the manual on the Vokera and see if I can see the bypass. I have spoken to them in the past when the Expansion V was finished. They told me there is an auto discharge for air. Sludge could certainly be in there even though the system is only about 5 years old. I think I should also flush out the two loops of UFH. Thank you again,P

 

[simond]

I don't think there would be any benefit in flushing the UFH.

Are you saying all the system, everything, was new 5yrs ago?

 

[Peppertack]

Yes. We renovated the house starting 7 years ago and the boiler and UFH went in new about 5. As I say it all works a treat but goes wrong when the UFH is on alone. The UFH on its own is fine if I manually open one of the motorised port valves but that is just turing the CH on isn't it?

 

[PullerGas]

I don't think there would be any benefit in flushing the UFH.

Are you saying all the system, everything, was new 5yrs ago?

Well, the right hand flow gauge on the UFH manifold looks a bit manky, but hey you'll know. Considering you know everything about the bypass in a 'minute'.

 

[PullerGas]

Most combi/system boiler central heating systems we now install are fitted with Auto Bypass valves also.
Boiler heat exchangers are getting smaller & smaller, requiring higher flow rates. Both Baxi/Potterton & Ideal are now recommending ABVs, with smaller radiator systems & S plans.

 

[Peppertack]

Thank you for that, PG - I'll get an ABV looked into. I had mentioned to the plumber that we would be extending the system to 4 loops in time when we build the extension. Perhaps he thought this set up would do for now.