Gas boiler cycling on and off with underfloor heating

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Hi Everyone

I have a Baxi Duo Tech Combi and a wet underfloor system downstairs and radiators upstairs. I'm coming into my 2nd winter with this installation.

I have noticed that when the Underfloor zones are demanding heat from the boiler, the burner light goes on for about 5 seconds, then off for about 5 seconds, then on again, repeating this pattern while the zone is demanding heat. It could be doing this for hours. Is this normal?

I can't recall this happening last winter but may not have noticed it. I had the boiler serviced for the first time in July and had to get the company straight back as the boiler was left not working properly. Something to do with the gas pressure not being set right.

When the radiators upstairs are switched on the burner runs continuously.

Cheers, Andy
 
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It sounds as if there could be a fault with the boiler.

How is the hot water produced? How does the boiler operate when a demand is placed for water?

I would be suspicious of a firm who left the boiler not operating correctly in respect of gas pressures. Having said that some models are particularly sensitive.

Tony
 
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Thanks Tony,

The boiler seems to be delivering hot water as normal with showers etc. The only issue seems to be the endless cycling of the burner with the UFH.

I contacted Baxi for the service and used their 'approved' service company. The boiler was running fine up to this.

I'd like to get the opinion of the forum before going back Baxi in case they try to fob me off.

Cheers, Andy
 
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I have a similar system, except that I also have radiators downstairs as well as wet underflor heating.
I have experienced the same effects as you, with the boiler powering up and down. I think this is called modulation and its where there is a greater difference in water temperature between the outlet and return at the boiler than the system can handle. By shutting down the system allows the water temperature to cool to a more acceptable temperature before firing again because there is a call for heat from the thermostats.
On my system I managed to track the cause of this imbalance in temperatures to a faulty pump. I have 2 pumps. On on the underfloor heating manifiold with a by pass/return valve and one in my airing cupboard next to the hot water cylinder also installed with a by pass/return valve. This one which was quite elderly, was the cause of the problem. It wasn't always pumping when it should be, and it was making horrendous noises. It would take ages for the rads to get any heat and the boiler would be modulating all day.
I replaced this pump (Grundfoss) last week and everything is now as it should be. Nice hot rads, no noise and the boiler doesn't modulate anymore.

I hope this helps.

Fenders
 
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The fact that the boiler is happy when the radiators are on suggests there isn't enough flow through the underfloor pipework.

Is there just one pump in the system and are there any valves fitted which might not be opening properly on this circuit?
 
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Hi Everyone

I have a Baxi Duo Tech Combi and a wet underfloor system downstairs and radiators upstairs. I'm coming into my 2nd winter with this installation.

I have noticed that when the Underfloor zones are demanding heat from the boiler, the burner light goes on for about 5 seconds, then off for about 5 seconds, then on again, repeating this pattern while the zone is demanding heat. It could be doing this for hours. Is this normal?

I can't recall this happening last winter but may not have noticed it. I had the boiler serviced for the first time in July and had to get the company straight back as the boiler was left not working properly. Something to do with the gas pressure not being set right.

When the radiators upstairs are switched on the burner runs continuously.

Cheers, Andy

If it works OK with the upstairs radiator zone on, then I'd hazard a remote guess (i.e., not accurate at long range ) that it's something to do with inadequate flow through the boiler.

When the downstairs is warmed up, the UFH mixing valve could be re-circulating, say, 80% of the UFH return and mixing this with 20% of the water from the boiler. The (hypothetical) 20% flow through the boiler is probably inadequate to stop the boiler cycling or tripping a flow switch or other safeguard device.

You could test this by turning the UFH off, or down, and then starting it from cold, or turning the temperature up. This should cause a demand for a large amount of hot water from the boiler, so it should operate normally for a time until it warms up.

You could fix this, if it is the problem, with a thermal store. Boiler primary flow through store is 100%, the secondary circulation is whatever is required. The heat store increases the time for the boiler to go through a heat/ cool cycle.
 
D

DeltaT2

Not sure how well that combi modulates?? But regardless, if you've got enough heat in your home via the UFH, in other words; Is it working OK at heating your home? Then, I'd say your control system is very basic when running the UFH. If your boiler can't modulate low enough for the limited demand of the UFH, then the UFH temperature controller/room thermostat should be wired as an interlink directly with the boiler.

Has the boiler been set up correctly? How is your system piped up & what controls have you got?
 

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