HW stalls boiler

[mrtcotton]

Hello All,

I have a traditional vented system, Honeywell Sundial setup with Glowworm 50FF boiler.
CH works fine, Diverter valve appears to work ok (B is hot when HW on, A is hot when heating on). When call for HW, pump fires, boiler lights then goes out after a few minutes. Pump continues to run.

Cylinder thermostat measures 240v and seems to trigger a simple on/off when I vary the temperature. When "at heat" the pump stops. When calling for heat, the pump restarts, so does the boiler.

Boiler registers no fault on LED panel. CH works fine and boiler doesnt cutout.

Thank you in advance if you have any ideas or need more info.
Justin

 

[oldbutnotdead]

Have you got any bleed valves on the primaries into the cylinder? If so, use them & see if any air comes out.
Is the F & E tank full of water? Are the feed and return getting warm at the cylinder when the boiler fires up (on HW only)?
It sounds as if there is no or poor circulation in the hot water primary pipework- the boiler is reaching its internal set temperature and shutting down as it should do.
EDIT Presumably you have no hot water- or do you get hot water when you run CH + HW?

 

[mrtcotton]

@oldbutnotdead Thank you. Auto bleed valves and air free. F&E water level is ok. It could be poor circulation, although I do keep cleaner in there due to hard water.
The cylinder is of an age, i will try a flush through. I didnt think there was an issue with the controlling components.
Failing that I will call somebody out.

 

[oldbutnotdead]

Have you done a touch test on feed and return where they enter the cylinder? May be worth taking one of the auto bleed valves off- these things can get blocked or stop working and the water flow will tell you instantly if the feed from the tank is blocked. And no it doesn't sound like control failure or boiler failure. It can be a messy job to diagnose, you might even have a collapsed coil inside the cylinder (rare but happens)

 

[mrtcotton]

No, you mean to check for temperature? When the HW circuit kicks in, there is quite a wooshing sound which has been there for a while. CH doesnt do this. I suspect quite a limscale build up in the coil. Is there a way of flushing the coil easily, or is it a pro's job?

 

[oldbutnotdead]

The whooshing sounds more like air (& lots easier to clear than limescale), again don't trust the auto bleed valves, crack the top one & see what comes out. Checking for temperature on F & R will give you a good clue about circulation (or lack of)

 

[mrtcotton]

F&R pipes heated up in tandem.
I turned the HW on with the control and went upstairs to the cylinder and waited for heat. There wasnt any. I went downstairs into the garage to the boiler and the reset light was on. This is a cut-out scenario. I reset (turned dial off and back on to heat level) it kicked in. I went back upstairs to check the F&R pipes. Like I say, the flow felt immediate. After around 3 minutes I went back to the boiler and the boiler was off. No LED's, nothing. Pump runs (I suspect runs when the cylinder stat calls for heat). Not sure why the boiler is stalling though.
No whoosing air sound neither this time.

 

[oldbutnotdead]

Curiouser and curiouser. Silly question- there is DHW in the cylinder? (ie when you turn a hot tap on water comes out at normal rate). Do you get water if you set the programmer to CH + HW and run? Does the return get warm fairly quickly all the way back to where it joins the heating return (or as far as you can check it without ripping floorboards up)

 

[mrtcotton]

I will check. Not a silly question. DHW - yes. I am using the immersion heater to heat HW at the moment and I hear it re-fill. I will try both on from cold, might be tomorrow now and see what happens. I appreciate your time and suggestions.

 

[mrtcotton]

Woke up this morning to a boiler reset LED on permanently with CH enabled. I have disabled the HW as this was the previous issue. I reset it with the dial and it fired back up (pump running fine). Boiler cut again with reset LED on permanently and Burner LED flashing 2hz which I believe is a lockout.
The boiler is a Glowworm Micron 50FF. I replaced the PCB in February 2019 with a new thermistor so I will be disappointed if these have failed again. I have reset the boiler again and we have heat for now.
I will check the thermistor readings again later.

 

[oldbutnotdead]

That sounds dull and expensive. Does power cycling the boiler clear the lockout (don't do it too frequently)