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      01-15-2013, 06:34 PM   #1
briers
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Central heating boiler cycling

Boiler seems to be cycling a lot. It will run from cold for say 15min then stop, boiler stat on max. It will then stop for about 1 min and come on for a further 3min or so and do this until the the wall stat clicks off.

However if I move from pump speed 2 to 3 it stays on for practically the entire time it takes to heat the house. But, it's very noisy and I seem to get a lot more air in the closed system doing this. Trv whistle, circulation whurr etc.

Pump is 15/60 to 20 radiators and 90% have trvs, does the cycling sound normal or should I try to balance the radiators? I tired to balance by hand recently, maybe I didn't let enough water in to escape and loose enough heat before returning?

That or the flow is too slow through the boiler?
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      01-15-2013, 07:24 PM   #2
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not an expert but perhaps your boiler is putting too much heat into the system too quickly, then shutting off as the return temperature has been reached. Reduce the stat on the boiler and see what this does?

Did you balance with a thermometer of some kind otherwise with 20 radiators could be tricky. Again, just live in an old house with many radiators but not an expert.
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      01-16-2013, 03:36 AM   #3
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Sounds like kettling where the water in the boiler boils, have you got thermostatic radiator valves? Are they all 'satisified' (as in closed due to there being enough heat) Do you have any radiators without TRVs?
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      01-16-2013, 04:05 AM   #4
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What boiler and system type is it?

And where's it trying to cycle to?
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      01-16-2013, 05:55 AM   #5
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I thought the boiler stat control just turned the boiler off sooner, didn't think it reduced gas input.

It's an ideal classic ff boiler, sealed system with grundfos 15/60 pump.

All up stairs radiators have trvs and might be satisfied but all downstairs radiators don't have trvs and the one near the wall stat is fully open.

The boiler is indeed knocking/kettling slightly, don't know if this is air still in system or scale. Due to add some inhibitor. But it still knocks but runs without cutting out on pump speed 3 even when trvs are closed and there is knocking.

Just so noisy and system generates so much air on pump 3.
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      01-16-2013, 06:11 AM   #6
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I suggest you fully open the valves on the two rads which do not have TRVs fitted. Then open all the other rad valves fully, and balance each rad to have a 20c drop. do this two times. Next, adjust the TRVs to your personal preference. Let us know how you get on.
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      01-16-2013, 06:21 AM   #7
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Thanks John I will try balancing again but with thermometers this time

I assume then the boiler shouldn't be cycling like this after its initial warm up phase.

Btw it isn't a condensing boiler and its non modulating too.
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      01-16-2013, 06:29 AM   #8
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Further to my previous reply,some boilers do have modulating gas valves. Now you say that possibly 50% of your rads have no TRVs? It is also possible your boiler stat is out of sync. Try turning it down. Boiler flow temp. should be 80-85c.
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      01-16-2013, 06:34 AM   #9
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Just checked and 7 of the 20 radiators don't have trvs.

Ok I will turn it down a bit too

Silly question but I also read somewhere modern systems don't need balancing precisely as the pipe work installed is smaller diameter restricting flow this way to each rad. When we moved in all lockshields were fully open. I still think this is wrong but wanted to check. After attempting a by hand balance we do get much better warmth in a room that was previously too cold.
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      01-16-2013, 12:32 PM   #10
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Briers mate, a 20 rad sealed system should run on pump speed 3. In reality any size of sealed system the pump should be at max. There is no risk of it pumping over into a header tank or drawing air. I know your saying its noisier but its probably due to scale. A cleanser should be added and run for a while then drain the system and add inhibitor and silencer. It may benefit from a powerflush but if all the rads are heating then a cleanser will do. The boiler cycling is due to the pump speed being too low and not clearing the heat away. The boiler stat controls the temperature of the radiators to touch and trv's are controling air temperature in each room. So the higher the boiler stat the hotter the rads will get but the boiler will be on for longer periods and increase gas consumption. Also if the boiler stat is up high and the system is scaled you will get the kettling which is obviously more noise.
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      01-16-2013, 12:45 PM   #11
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Thanks, great reply

I will sort the descaler and inhibitor out.

I switched it back to pump speed 3 and i'm pretty convinced now that is where it needs to be, despite the TRV whistle, air and noise. It fired up earlier on 2, ran for about 20mins and cut out when hot. Then began cycling. Immediately switching the pump to 3 it came back on and stayed on until wall stat clicked off.

My only issue now is the air it creates on 3. I think the water is moving so fast it hits the impeller and creates air. My AAV doesnt seem effective so along with sorting the inhibitor i will ask my gas man to fit a new AAV too, maybe it will help purge the air out.
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