Hot water short cycling boiler

[The Woo]

Hello

New to the forum so please be gentle

I noticed a few days ago that our gas boiler was short cycling. Burner comes on for 1-2 mins, cycles down and shuts off.... Just for it to immediately get another call for heat and turn back on again. .. It does this over and over and never turns off long enough for the zone valve to close or pump to turn off.

I've narrowed this down to the Hot Water. When I turn off the HW at the programmer, the short cycling stops.

Checked CH independently of HW and CH is working fine.

At first I thought it could be the HW zone control valve staying stuck open (or at least the end circuit remaining closed), but if that was the case, switching off the HW at the programmer wouldn't have any affect - right? If its stuck open, the programmer wouldn't have any influence over it any more.

This now makes me think it's the cylinder stat. Stat not satisfied so it keeps calling the boiler. The boiler stat is quickly satisfied so shuts off, just for the cylinder thermostat to switch it back on.... And so on and on and on. Thoughts?

Take the cylinder stat out of the equation (by turning it off at the programmer) and the issue stops.

Tested the theory by turning up cylinder stat and letting it run until boiler turned off (so cylinder full of freshly heated water). Turned off HW at programmer and lowered temp on cylinder stat right down - so stat set lower than the water in the cylinder.

My view here being, if stat is working OK, the freshly heated water in the cylinder is hotter than the new stat level, so it shouldn't immediately demand heat........ Right?

Turned on HW at programmer again and low and behold, the boiler is met with an immediate demand for heat.

Stat issue?

Any other checks I could do?

Any other ideas of what it might be?

 

[ianmcd]

When you select HW at the programmer, that sends a live to the CYL thermostat, if that is satisfied nothing happens, when the stat is calling for heat it sends a live supply to the DHW zone valve which now motors open , when it is fully open the micro switch inside the valve is made and this sends a signal to the boiler to fire

 

[The Woo]

And that's my understand too.

The programmer sends live to CYL Stat. If the stat is satisfied, nothing should happen - no heat required. When stat registers need for heat, stat completes circuit to energise HW zone valve - valve opens (for water to flow) and micro switch closes end circuit to call boiler and pump.

This should run until the stat is satisfied. Stat then cuts power to zone valve, so valve closes (on its spring), thereby opening the end circuit and boiler/pump shut off until heat is needed again.

Issue I have is the boiler is being continually called for heat for hot water.

Boiler cycles down (after running for no longer than 30-60secs), just for it to be immediately met for new demand for heat and this happens over and over until I cut the live to CYL stat (by turning off at the programmer). When I kill the power to the stat, the valve closes and boiler/pump turn off as expected.

As per my original - any ideas what could be causing this continuous call for heat on hot water?

I'm still leaning towards faulty CYL stat.

 

[ianmcd]

More likely to be the microswitch sticking on one of the zone valves as these are fed by a permanent live so if the valve motor closes and the micro switch sticks the water cant circulate past the valve but the boiler is still getting a demand, if the HW is off at the programmer there isnt any power at the Cyl stat

 

[The Woo]

And that was my initial thought too, but if it was a sticking microswitch (as you say, fed by a permanent live feed), the switch would remain stuck, even after I cut the power to the stat.

So, programmer would show as HW off, but the stuck switch would mean the end circuit is still closed and continue trying to call the boiler. Right?

This isnt happening in my scenario - when I turn the HW off at the programmer, it turns off exactly as expected - no more calls to the boiler - valve closes and end circuit opens (not stuck closed).

It's only when I turn the HW on at the programmer and put the CYL stat back into the equation that at I see the cycling issue.

 

[dilalio]

Simple to test the cylinder stat... Just turn it down and power should cut. If it does then stat is fine. If not then programmer maybe not switching off on timer. Meter should reveal all.

 

[The Woo]

Thanks.

I don't actually use the timer for HW - at the programmer it's on 24/7 (or at least it was until this issue started - now I'm manually flicking it on and off as and when hot water is required) and its left to run from the CYL Stat.

Will try the stat test though - next time I switch heating on at the programmer, I'll lower the stat on the cylinder and see what happens.

 

[Harry Bloomfield]

Might it be that the cylinder stat is set at too high a temperature above the boiler's stat?

That would cause the cylinder stats demand to never be satisfied, so boiler would do its best to meet the demand - but limited by its own stat.

 

[The Woo]

Thanks Harry.

I've started to think the same thing.

Not ever touched the CYL Stat (until today) since we moved into the house several years ago, however the boiler was serviced shorty before I noticed this issue. Looking at the boiler temp control its definitely been lowered (was set nearly at max - at least about about 5/6 up the dial, but now is set at no more than 2/3). Turned it back to roughly where it was and monitoring.

 

[The Woo]

Thought we had it - alas not.

HW has been switched on at the programmer all evening with the higher temp set on boiler stat and no short cycling. Great!!

Boiler just fired up after about 3+ hours of silence and unfortunately it's doing the same as before!

Burner came on for 4-5 mins, burner shut off and boiler cycled down.... Just for it to immediately fire again for less than a minute, cycle down and come back on - this happened 3-4 times, before I turned HW off at the programmer again.

Whilst it was all behaving I also checked the CYL Stat. Slowly turned up the temp on the CYL stat and it clicked, opened the zone valve and activated the pump/boiler. Turned it down, valve closed and pump/boiler turned off.....all exactly as expected.

Now really at a loss of what the issue might be.

Any ideas?

 

[The Woo]

Can failing thermostats work intermittently?

I'm working on the assumption that thermostats are binary - they either work or they don't. However an intermittent fault on the CYL Stat would explain what I'm experiencing.

I really don't thinks its:
1) Faulty boiler stat as that would impact CH too and CH is fine.
2) Faulty zone valve or microswitch. Everytime I turn the HW off at the programmer, it turns everything off exactly as expected. I really don't think a stuck (open) valve or stuck (closed) microswitch would do this.

 

[fezster]

Set your boiler to be 10 degrees higher than your cylinder stat, to allow sufficient heat transfer without the boiler hitting its stat before the cylinder does.

 

[Harry Bloomfield]

I agree with fezster, there needs to be that 10C difference, to avoid the cycling as the HW nears the cylinder stat target temperature. Mechanical stats are also not that accurate in switching. I have just recently upgraded mine, so actual room and HW temperature values are fed back to the control system - much better control because the boiler can now modulate its output, aware of how near temperature is to target, rather than on/off.

 

[The Woo]

Thanks guys.....

Not sure of the exact temps as they are both mechanical knobs with no temp markings (helpful), but if the manuals are to be believed:
Cylinder = circa 60C
Boiler = circa 75C

I have heating engineer coming on Thursday who's convinced its the zone valve/microswitch, but that just doesn't stack up in my head. As noted above, stuck is stuck - turning off HW at programmer wouldn't magically unstick it each and every time.

I've watched the light on the boiler as well and it appears to be going through its correct sequence - no faults or lockouts:
1) Static Amber - awaiting external call for heat
2) Flashing Amber - call received ignition checks and ignition
3) Static Green - burner on
4) Flashing Green - boiler stat satisfied
5) Static Amber - awaiting external call for heat

Its just the Static Amber bit is 1-2 seconds at most when HW is on at the programmer and Static Green is less than a minute.

Is this just really poor modulation?

Bemused.

 

[dilalio]

Sounds like a hysteresis issue then.
Simplist solution would be to put your DHW on a timer so that this doesn't continue all day. Just time the DHW to come on for however long it takes your cylinder to recover after high demand (early morning and evening?).

What's boiler do you have and is the cylinder vented or unvented?