Boiler cuts out before hot water temp satisfied.

[Deleted member 112076]

Hi all,

My problem is that my oil-fired boiler cuts out before the hot water has reached the thermostat temperature set on the hot water cylinder. At best I get luke-warm water.

I have a vented system and the heating side of the system looks like it is working fine. However, with the boiler set to push out 70 degrees C and the thermostat on the hot water cylinder set at 65 degrees C the boiler shuts down before the hot water has even reached a luke warm level. The immersion heater is doing the job at the moment.

At first, of course, I suspected the thermostat on the hot water cylinder but the Danfos heating/hot water controller is demanding heat and the hot water zone valve is open ('B' position) so I guess that the thermostat is not at fault(?).

Any ideas?

Jack

 

[SJB060685]

Are you sure you're constantly getting a feed from programmer, through cylinder stat and motorized valve and to boiler? You need to determine if the boiler is cycling on and off through boiler stat (which maybe at fault) or you're losing a feed somewhere.

 

[John.g]

AS above, oil boilers will always cycle on/off, particularly when on HW only, mine, with a cold cylinder (& 20 kw boiler) will cycle, ON ~ 2 mins...OFF ~ 4 mins & when cylinder temp is nearing its 63C SP, ON ~ 2/3 mins..OFF ~8/10 mins so with a lukewarm cylinder the off time should be only 3 to 5 mins if boiler burner is still enabled.
I wonder if the cylinder temp is increasing with both HW&CH on together?.

 

[SJB060685]

Another thing is what is your water area like? If this has been slowly getting worse you could have a scaled up cylinder if in a hard area

 

[Deleted member 112076]

Are you sure you're constantly getting a feed from programmer, through cylinder stat and motorized valve and to boiler? You need to determine if the boiler is cycling on and off through boiler stat (which maybe at fault) or you're losing a feed somewhere.

Thanks for your reply...on Christmas day!!

Yes, I have a constant signal from the controller and the hot water valve is 'open' in response to that signal. I'm not sure about the boiler stat but the heating is working fine so I don't think that that is faulty.
[automerge]1577378813[/automerge]

AS above, oil boilers will always cycle on/off, particularly when on HW only, mine, with a cold cylinder (& 20 kw boiler) will cycle, ON ~ 2 mins...OFF ~ 4 mins & when cylinder temp is nearing its 63C SP, ON ~ 2/3 mins..OFF ~8/10 mins so with a lukewarm cylinder the off time should be only 3 to 5 mins if boiler burner is still enabled.
I wonder if the cylinder temp is increasing with both HW&CH on together?.

Hi. Thanks for your reply.

If only the hot water is demanding heat then the boiler fires, runs for a while (the hot water cylinder gets luke warm) then shuts down. It then doesn't come back on for at least thirty minutes; it's not cycling at all.
If both hot water and heating are demanding heat then the boiler does cycle (I assume with the heating loop return temperature) and the hot water still only gets luke warm.
[automerge]1577378866[/automerge]

Another thing is what is your water area like? If this has been slowly getting worse you could have a scaled up cylinder if in a hard area

Soft as a baby's bottom!
[automerge]1577381748[/automerge]
I think I might have found it...could this be as simple as an air lock?

There's an automatic air bleed on the inlet to the hot water cylinder (this is the highest point of the whole system as it is in a bungalow). Like this one...

[7,62~10,01€ 8% OFF] Vanoeu automatique en laiton TMOK TK901avec soupape de sécurité et soupape de décharge de sécurité pour système de chauffage, ventilation et climatisation (HVAC) amélioration de l\'habitat from mobilier et rénovation on banggood.

TMOK TK901 Robinets en laiton avec soupape de vidange automatique pour eau chaude

m.banggood.com

I haven't had the place long and I've just found the air bleed fully screwed down. When I unscrewed it air rushed out for about ten seconds then water started coming out.
I'm guessing that the water coming out bit is not right and that it's broken. I also guess that the previous owner screwed it down to prevent it leaking and therefore inadvertently caused the air lock.

Since then I have bled it a few more times (it always leaks after the air has gone) and now I have hot water again.

 

[SJB060685]

Yes if there was an airlock flow would be completely restricted or partially, only allowing so much heated water through cylinder coil resulting in a less than desired temperature. Make sure the system is fully bled and all points tightened up and report back in 24 hours, you might be lucky and have found the problem.

 

[Deleted member 112076]

Update:
The air lock seems to haev been the problem. The hot water has been behaving itself. Thanks for all your replies.

P.S.
I have one of these drain cocks that is weeping (might be the cause of the air in the system) and I assume that the seal has perrished.

Hep2O Draincock Brass Spigot

Push-fit Hep2O includes unique features that enable quick and easy installation with a secure fitting. It is the only push-fit plumbing system with joint recognition, secure demounting and a 50 year guarantee.The system is compatible with both plastic and copper, making it ideal for new build...

www.toolstation.com

Can I just buy a new one and swap the plunger/screw-in bit to save having the drain the entire system and then get involved with soldering? I was hoping i might be able to screw it all the way out and just screw in the new piece?

 

[SJB060685]

You'll need to drain down but should be fine to change innards

 

[SimonG]

Unless your feeling lucky!

Tried tightening with spanner?

 

[Deleted member 112076]

Unless your feeling lucky!

Tried tightening with spanner?

Oh yes...until I thought I might screw the whole thing off! Yep...tried that. Defo the seal has gone.