LPG Boiler cutting in/out too frequently when heating water only

[rhino666]

I have a Glow.Worm 30HXI boiler running on LPG. It has been excellent and no apparent problems or fault codes.
Currently not running heating; just used to heat up water for an hour, a couple of times a day.
I noticed a bit of a gas smell from the external vent and the boiler firing up and then cutting out after a short time when it was heating water earlier today.
On inspection it repeatedly fires up when temperature on boiler shows a little over 30 degrees, runs for perhaps 10 seconds that raises the temperature to over 70 degrees and then cuts out.
The temperature shown on the boiler display very quickly drops back to the 30 or so degree level where the boiler cuts in again.
This fast cycling indicates that there is a problem to me that I hope someone can identify.

Thanks

Rob

 

[Chuck]

This fast cycling indicates that there is a problem to me that I hope someone can identify.

It suggests that the circulation of system water around the loop that serves the domestic hot water tank is not fast enough. Could be a faulty pump, faulty valve or some sort of silt build up (particularly if it is a 'gravity circulation' system without a pump.

Another, less likely, possibility is that the tank thermostat is set too high or has failed and is calling for heat when the tank is too hot for heat to be transferred in effectively.

A few more details about they system and maybe a couple of pictures might help us be more specific.

 

[rhino666]

It suggests that the circulation of system water around the loop that serves the domestic hot water tank is not fast enough. Could be a faulty pump, faulty valve or some sort of silt build up (particularly if it is a 'gravity circulation' system without a pump.

Another, less likely, possibility is that the tank thermostat is set too high or has failed and is calling for heat when the tank is too hot for heat to be transferred in effectively.

A few more details about they system and maybe a couple of pictures might help us be more specific.

Thank you for your reply.
I hope the attached pics help to clarify the system.
It is closed with a hot water tank that seems to provide hot water for baths, etc without issue.
No fault codes.
No expert on these systems but second photo shows a pump and diverter valve? that I believe indicates pumped flow to the hot water tank. Boiler is ground floor and tank on first floor.
Just checked central heating as not run since last test a month or so ago. Switched on for half an hour and temperature at boiler display gently climbs from 24 degrees to somewhere around 70 degrees. Cuts out and back in as expected.
Hot water tank is only 114 litres, heated up twice a day for an hour. Perhaps this heating time needs reducing?
Tank stat set at 55 degrees; not touched since we moved in over 9 years ago.

Thanks

 

[Chuck]

Just checked central heating as not run since last test a month or so ago. Switched on for half an hour and temperature at boiler display gently climbs from 24 degrees to somewhere around 70 degrees. Cuts out and back in as expected.

That suggests to me that the pump is okay. How does it behave when the hot water is rapid cycling? Also, what's the diverter valve doing?

Is there another zone valve for the hot water? It may be near the tank controlled by the thermostat.

 

[rhino666]

That suggests to me that the pump is okay. How does it behave when the hot water is rapid cycling? Also, what's the diverter valve doing?

Is there another zone valve for the hot water? It may be near the tank controlled by the thermostat.no

There is no other zone valve, just the Honeywell V4073A1039 3 port mid position valve. I found a test writeup on it and it appear to work perfectly. The pump runs very quietly, so checked that in operation with a finger on the casing to pick up vibration.
It's been cold here today so a good test for the hour long hot water routine this evening. Pump and boiler straight on as scheduled and boiler temp up from less than 30 degrees to probably 70 degrees in under 15 minutes. Pump continued running when max temp reached and boiler cut in after quite a long time when temperature dropped back. All seemed perfectly normal. Strangely at the end of the hour program temp dropping at around 50 degrees, no boiler as expected but also no pump! I would have expected the pump to run for the whole time. Even without the pump running the temperature shown dropping very slowly.
Can't understand what could possibly have caused the boiler temperature to fly up and down in seconds with just the hot water program the other night.
The 30HXI is quite a powerful boiler and the 114 litre tank is a very small cylinder I think. Have reduced the hot water program to half an hour, twice a day to see if that works for us and hopefully strange recycling was a one-off.

 

[Chuck]

Strangely at the end of the hour program temp dropping at around 50 degrees, no boiler as expected but also no pump! I would have expected the pump to run for the whole time.

What I'd expect to happen is for the boiler to shutdown but the pump to continue running for a short time, say 30s. (This is the 'overrun' time, which ensures the boiler heat exchanger doesn't surge in temperature.) During this period the water in the loop equilibrates with the temperature in the cylinder, so the boiler thermometer ends up at ca 50°C.

Can't understand what could possibly have caused the boiler temperature to fly up and down in seconds with just the hot water program the other night.

Evidently the pump failed to start. Most likely cause if you have a conventional pump is a failing 'starter capacitor'. This problem would explain intermittent starting.

 

[rhino666]

Evidently the pump failed to start. Most likely cause if you have a conventional pump is a failing 'starter capacitor'. This problem would explain intermittent starting.

Makes a lot of sense. Does not appear to be any kind of fault code that would be generated by this.
I imagine the starter capacitor is on the main PCB, part number 2000802731.
Appreciate the advice; thanks very much.

 

[Chuck]

I imagine the starter capacitor is on the main PCB, part number 2000802731.

No, it would be part of the motor.

 

[rhino666]

No, it would be part of the motor.

That's a shame; I thought I may have found it - see blowing capacitor top in first photo.
Circulating pump is a Grundfos 96281473 super electric 15-60 130. Please see second attached photo. Any way of testing the capacitor without blowing my multimetre?

 

[Chuck]

That's a shame; I thought I may have found it - see blowing capacitor top in first photo.
Circulating pump is a Grundfos 96281473 super electric 15-60 130. Please see second attached photo. Any way of testing the capacitor without blowing my multimetre?

Bulging capacitors are not good but since the PCB is part of the the boiler control circuitry we're not allowed to give advice about repairs of this type on this forum. You should contact a Gas Safe Registered Engineer if you think that's where the problem lies.

For the pump motor, which is not part of the boiler and so we can discuss here, I'd monitor the mains feed to it during the fault condition. If the feed is steady but the motor is not turning something's up with the pump. To be more specific, if it takes a while to start, sometimes works and sometimes doesn't, hums more than you'd expect, etc. it's probably the capacitor. A simple test of the capacitor would involve isolating the pump, ensuring the capacitor is safely discharged, disconnecting it and then using a capacitance meter (or scale if your multimeter has one) to see if it's in spec or not. Capacitors are not expensive it might be easiest check by substitution if you don't have the necessary test gear.

For safety, capacitors to be properly sized and rated. It's ensential to get the right replacement. If you're not sure just replace the pump, it's not worth risking a fire.

P.S. It may not be the pump/capacitor. It's just the most probable cause that seems to me fits the symptoms you've described. Others may have alternative suggestions.