Boiler keeps short cycling with only UFH is on

[ShaunCorbs]

Unfortunately, I don’t have any spec as the UFH was installed by the builder’s heating guy and there was no design.

Do I need to turn down the flow meters to 2.5L when I only have the UFH on? Will that reduce the boiler cycling?

Need to get some temp clamps and get the individual flow and return temp differential on each set of white pipes to 5-7 dc on start up before the slab is hot eg when it’s had a day or two to cool

Won’t stop the boiler cycling due to ufh being let’s say 2kw and the min boiler kw to 6 etc

 

[Kurt Stone]

I had this same problem after installing my UFH. With radiator circuit on no problem but with ufh only the boiler was short cycling. I opened the manual bypass valve about 1 turn and the problem has gone away.

 

[Knappers]

Could you not just fit an auto bypass across flow and return to UFH manifold (or wherever there is most pipework/water available to increase themal mass cycling through boiler)?
This boiler has a very effective one fitted internally which is prob what is contributing to the kettling.
An external bypass at a lower resistance would bring the thermal mass of water in the flow and return pipes into play and automatically compensate for different heating demands?

If its not too difficult you could maybe even consider increasing size of primary pipework?

 

[Ben-gee]

If you’re considering replacing the boiler anyway, I would just do that.
38kW is way too big - get it sized to your actual heat loss and choose one which can modulate down to the size of your underfloor loss and there will be no problems…. if such a beast exists.

 

[windsor-tg]

I had this same problem after installing my UFH. With radiator circuit on no problem but with ufh only the boiler was short cycling. I opened the manual bypass valve about 1 turn and the problem has gone away.

@Kurt Stone - thanks. I have an automatic bypass value fitted along with a gate value (photo attached with yellow line showing bypass valve). If I were to close the gate valve and then open it one turn, would that be similar to what you did?

 

[windsor-tg]

If you’re considering replacing the boiler anyway, I would just do that.
38kW is way too big - get it sized to your actual heat loss and choose one which can modulate down to the size of your underfloor loss and there will be no problems…. if such a beast exists.

Even though I have a 38kW boiler, it has been capped at 24kW as I was getting an S.53 display message on the boiler (flow & return temp differential too large) so reducing the power of the boiler (d.0) to 24kW eliminated the message. A 30kW boiler is what is being suggested but I’m going to find it hard to find a boiler that can modulate down to around 1.5kW when I just have the UFH on.

 

[John.g]

@Kurt Stone - thanks. I have an automatic bypass value fitted along with a gate value (photo attached with yellow line showing bypass valve). If I were to close the gate valve and then open it one turn, would that be similar to what you did?

Where is the photo of the above?.

Re your tests:
At T+15, with both CH&UFH on, the UFH shows flow/returns of 48C/27C with a combined flowrate of 5LPM, this is a UFH output of 7.33kw, at T+40, UFH only on, UFH flow/returns of 49C/34C, combined flowrate 10LPM = UFH output of 10.5kw, in both cases, dts of 21C & 15C seem way higher than normal of ~ 7C to 10C.
Also, at T+20 & T+25, CH only on, the UFH still shows a combined flow of 10LPM? can you check your labelling again?

The UPS2 8M pump at any "fixed speed" setting will pump at a head of 6M at even the lowest setting., might be able to suggest a more suitable setting even though there is no constant pressure mode on that pump.
THe ABV at any index setting will have a very high flowrate at speeds 2&3 (7M &M) and will cause rapid cycling and might even still pass at speed 1 (6M).

So,
Check the pump setting.
post a photo of the bypass with the manual valve & the ABV and its make and setting.
what is/was the boiler flow target temperature at the above tests?

 

[windsor-tg]

Thanks @John.g
I will look to take a fresh set of readings over the weekend in case I made a mistake.
I have attached photos of the ABV. Hopefully this has come through.

Where you say ‘check the pump settings’ - do you mean the main pump or the UFH pump?
The boiler flow temp is set to 65 but I will double check before I take the new set

 

[John.g]

No, check the main circ pump near the zone valves or just take a photo of the LED display while running, if the manual by pass valve isn't well throttled then the ABV at that setting could well be by passing ~ 8 LPM or more as the minimum fixed speed pump head is 6M.

 

[ShaunCorbs]

The red pump