Boiler gurgling and cutting out when on 'hot water'

[Alex Nicolson]

Glowworm Ultimate 100ff - Y system

Boiler when firing for Hot Water' only, cuts out after a few minutes, works fine on 'central Heating', does same when firing for both Hot Water and Central Heating together.

Motorised valves were replaced a few years back (I know that can make little difference) but The water is going down thro' the pipework and thro' the cylinder, it does get hot, then the boiler cuts out with water gurgling sounds within about 3-5 mins.

I think the issue is the air vent on the return pipe? From the cylinder, lots of corrosion around the top...causing an air lock and not being able to vent it?

Being an electrician (retired ) but competent enough to do certain things on the heating pipework etc. I will be draining the system shortly (got a cleanser in it) refilling and putting an inhibitor in..is it easy enough to replace the air vent and can it be done prior to draining without water pi**ing

 

[Alex Nicolson]

Looking to replace with this...

 

[Alex Nicolson]

Replaced air vent, system drained and refilled with an inhibitor added.
Central heating works fine, Hot Water still the same..... pipework going thro' the tank , you can feel the water being pumped thro' it and it does get hot, then the boiler will cut out before the water tank thermostat reaches temp and opens, the boiler cut out tends to operate after a 10mins or so, but it all works absolutely fine on CH.
Ideas anyone?

 

[Madrab]

Sounds like the boiler is overheating - the HW (primary) circuit tends to flow through the coil quicker, therefore it can slower to lose it's heat and the return can be hotter than the CH.

Basically the boiler is reaching it's set temp and shutting down, (maybe overheating a bit if you can hear it kettling), the pump keeps running and once the boiler cools down a bit it fires back up again. Is there a limiting gate valve on the system primary pipework to/from the cylinder?

 

[Johntheo5]

Replaced air vent, system drained and refilled with an inhibitor added.
Central heating works fine, Hot Water still the same..... pipework going thro' the tank , you can feel the water being pumped thro' it and it does get hot, then the boiler will cut out before the water tank thermostat reaches temp and opens, the boiler cut out tends to operate after a 10mins or so, but it all works absolutely fine on CH.
Ideas anyone?View attachment 358443

What is the minimum power output of the Glowworm?, is there another 2 port motorised valve on the cylinder coil?, are you sure the mid position valve is moving fully back with port B only open with HW only requested and that port A pipe to the CH remains cold?

That boiler looks as if its non modulating so, ~ 30kw would have to be absorbed by the cylinder coil to stop it cycling, not a hope IMO, it will cycle even more as the cylinder temperature increases but thats how all oil boilers work.

 

[Alex Nicolson]

Cheers for the reply... its a gas boiler, not oil fired..
There are 2x 2-port valves after the boiler (Y-system) 1 to HW and 1 to CH, I was assuming that with being able to feel the flow of water being pumped thro' the pipework for water tank coil, the 2-port valve was opening correctly?
Re: minimum power output, not sure but I have attached the label from the boiler..

 

[Johntheo5]

Cheers for the reply... its a gas boiler, not oil fired..
There are 2x 2-port valves after the boiler (Y-system) 1 to HW and 1 to CH, I was assuming that with being able to feel the flow of water being pumped thro' the pipework for water tank coil, the 2-port valve was opening correctly?
Re: minimum power output, not sure but I have attached the label from the boiler..

Your boiler, seems to only have 3 outputs, 23.45kw, 26.38kw & 29.31kw, most 30kw gas boilers can modulate down to at least a minimum output of say 6kw, I was just saying that this boiler is to all intents and purposes a non modulating boiler like all (or most) oil fired boilers so the coil must absorb 23.45kw to avoid it cycling, but yes, if the coil inlet is very hot and the coil return quite hot as well, (the hotter the better to give the greatest heat transfer) then the 2 port valve is opened, also, if you pull on the manual lever (with the valve open), you should feel no resistance, the lever should feel loose, you should feel a fairly stiff resistance immediately when you start pulling on the lever with the valve closed.

With HW only on, if you time the burner on (firing) time in secs and the burner off time in secs then we can calculate the actual coil input, suggest doing this for say 3 cycles and note the on/off times in seconds for each on/off cycle.

Also check the water level in the feed&expansion cistern (small tank) in the attic, it should be ~ 1/4 to 1/3rd full.

 

[Alex Nicolson]

Cheers...Will do that tomorrow

 

[Alex Nicolson]

Hi, sorry for the late reply....school holiday week in Scotland was away with the grandkids last week

On HW.... boiler fired for 4mins 44secs, then OFF for 6mins, ON for 1min 36secs, OFF for 5mins 7secs, ON for1min 40secs, OFF for 5mins 20secs, ON for 1min 40secs, OFF for 6mins the ON again for1min 40secs and so on the overheat cut-out never activated this time which it usually does, so that's got to be a bonus
This seems to be the pattern all the time...
We still have an immersion heater, hence we still have hot water at least, when needed.

Any advice is much appreciated...

 

[Johntheo5]

That looks fairly typical, ~1.64min burner on, 5.21mins off, cycle time, 6.85mins, burner duty, 1.64/6.85, 23.94%, assume boiler output at 23.45kw, coil absorbing 23.45kw*23.94%, 5.6kw. My 150L cylinder with a 0.62M2 coil absorbs ~ 8.0kw with a cold cylinder falling to ~ 2.5kw when the cyl is nearing its SP of 60C. (With 20kw oil fired boiler)

 

[Alex Nicolson]

Thanks for the reply..
It's only initiall, that it stays on for around the 5min mark, for the other cycles it roughly only fires for about 1min 40secs or so then off for around the 5/6mins...is this normal?

 

[Johntheo5]

Quite normal, yes, the very first 5min firing is probably because its heating up the system water, after that then it will fire as above, 1mn 40sec on say 5min 30sec off is a burner duty of 1.7/7.17, 23.71%, coil absorbing, 23.45kw x 23.71%, 5.56kw, but depending on the boiler set point/flow temperature then the coil may only absorb ~ 2.5kw as the cylinder approaches its setpoint temperature, so based on the above, you might get a burner on time of 1min 24sec & a burner Off time of 11min 43sec. The higher the boiler flow temperature, the greater the heat absorbed by the coil as the cylinder gets cose to its SP temp, normally 60C. Some boilers now have priority DHW where the flow temperature is automatically increased to 80C and then reduced to its CH set point temperature when the cylinder has been reheated.

 

[Alex Nicolson]

Thank you for your help...it is much appreciated