Grant oil boiler hunting

[George J]

Hi All,

My Grant Vortex oil boiler is hunting, as in starts and stops, but if I switch my Honeywell clock to either Hot-water or Heating the boiler runs normally. Selecting both results in the boiler hunting.

This problem started when the system was used for the first time since previous winter. As far as I can remember nothing on the boiler, clock or cylinder has been altered.

George

 

[Johntheo5]

If a sealed system check boiler pressure, if a OV system check F&E cistern has waterin in it.

 

[George J]

Hi Johntheo5,

thanks but its not a pressurised system.

It is the fact that it runs OK when one or other of the Hot-water or Heating sides are selected, at the clock, that has me foxed.

Cyl. stat, and the immersion and its stat appear to be working fine.

 

[Johntheo5]

Just check that the small cistern in the attic is ~ 1/3rd full.
Where is the circ pump located, is it close to the boiler and is it pumping into the return to the boiler or away from the flow?
any idea where the cold feed is teed into the system?.
What is the make/mode/setting of the circ pump?.

 

[George J]

Pump - Grundfos UPS 15-50 130 set on 1, of 3. sited upstairs by cyl. Pumping into return to boiler via control board so solid fuel back boiler preheats system. This control board has not been used for over a year to include solid fuel back boiler in system.

Header tank is half full.

Cold feed, I think, feeds into boiler return and is sited near cyl.

What I am struggling with is why boiler works fine for either Hot-water Or Heating but not both.

 

[Johntheo5]

A strange one all right, presume by hunting you mean the boiler is cycling on and off very rapidly as distinct from its normal cycling. Speed 1 on a 5M pump seems quite low but one would think it should affect the cycling if either one of the systems is on which it isn't. Is there a manual or automatic by pass (ABV) installed anywhere? Have you drained/partially drained the system recently?, just wondering if there is a partial blockage where the cold feed is teed in but if the system refilled OK recently then no blockage.
Maybe just increase the pump speed to 2 or even 3 when CH + HW next on.

 

[George J]

yes seconds between cycles.

It runs normally on either but when I scroll the timeclock selector, and turn on the other, within a couple of seconds the hunting starts.

Have not done anything to the system since last winter. Only thing I've done is tried bleeding the oil thinking there might be air/dirt in it. But then discovered the problem, of the hunting, was resolved by turning one side off. Not ideal.

I think there is an auto by-pass next to the cyl but surely if there was an issue with it both sides of system would be affected.

I'll turn the pump up. Fingers crossed.

 

[Rodders53]

Hmm. My HW cylinder gets a 1 hour ON every day... I'd just time it on for a period I don't need heating as a work-round until one can sort out why.

You've a vented system.
What other controls?
Two 2-port motorised valves for HW and CH?
A 3-position motorised valve?
or what?

 

[Johntheo5]

My 19 year old Firebird very occasionally does this micro firing but never trips on combustion failure, I wouldn't, in a million years have associated it with the hydraulic side, if the power is being interrupted for whatever reason for a few seconds then the boiler should do a 12 sec purge before refiring each time, is it acting like this?., it certainly shouldn't be the boilerstat because this has a 8 to 10 deg hysteresis and a oil fired boiler HEX which is around 20L or so acts as a buffer so the burner should run for at least a few minutes before the stat is satisfied.

 

[George J]

Rodders53, good idea, So I have a plan B. I will try and list the system controls for both the heating and for the back boiler control board.

Johntheo5, what constitutes the "hydraulic" side. Does the "power interrupted" maybe point at the boiler or maybe the Honeywell clock ? I think it best to time the sequences the boiler goes through in the various options. I am not that familiar with the burner controls or the sequences such controls go through. Whats the HEX ? Heat exchanger ? I assume it is a small tank to provide a volume of water that delays the tripping of the stat.

Thanks to both of you. I'll post more details tomorrow morning.

 

[Johntheo5]

Rodders53, good idea, So I have a plan B. I will try and list the system controls for both the heating and for the back boiler control board.

Johntheo5, what constitutes the "hydraulic" side. Does the "power interrupted" maybe point at the boiler or maybe the Honeywell clock ? I think it best to time the sequences the boiler goes through in the various options. I am not that familiar with the burner controls or the sequences such controls go through. Whats the HEX ? Heat exchanger ? I assume it is a small tank to provide a volume of water that delays the tripping of the stat.

Thanks to both of you. I'll post more details tomorrow morning.

The hydraulic side refers to the heating side, CH, DHW, circ pump etc.
The power interruption could be anywhere.
Boiler controls, when there is a call for heat then the boiler fan just runs on its own for ~ 12 secs to purge any gases from the furnace, the burner then fires up, so every time the boiler is "switched off", or the power is interrupted, it will do this 12 sec (pre) purge when the power is restored. If the hunting you describe is happening at 12 to 15 sec intervals then possibly power interruption.
A oil fired boiler heat exchanger (HEX) is just a metal box filled with water, (the boiler itself), which contains ~ 20 litres of water.

 

[George J]

CH and DHW runs as usual. No apparent issues until I try turning on both together at the clock.. Then hunting starts within a couple of seconds and then continues to cycle off/on with roughly a second between,

When switching on boiler, from cold, it seems to purge correctly. Fan running and then a purge after 10+ secs.

Rodders53, think the attached would be wiser than me trying to explain board for oil and solid fuel control.

 

[Johntheo5]

Thats quite a complicated set up, I can't read the labels but can see that the top left motorized valve is a mid position valve, (MPV), there may be a problem with this in mid position, it may be hunting with both CH & HW requested, so suggest the following tests, with HW demand only and cylinder stat calling for heat (turn it up, if necessary) ensure boiler firing normally then request CH on as well, when both then calling does the boiler start hunting?, if so just turn down the cylinder stat (let it on at the programmer), see if the boiler then fires normally on CH only, if turn the cylinder stat back up and see if boiler starts hunting, if so, turn down the roomstat (let it on at the programmer), check that boiler returns to normal firing.
With both CH&HW selected and boiler hunting, listen closely to the MPV and see can you detect any noise.
If you can use a multimeter then a few readings at the boiler will tell a lot.

 

[Johntheo5]

Just carried out a test on my own Firebird.
The purge period on this boiler is 12 secs.
If I interrupt the power while firing normally then it will restart after doing this 12 sec purge.
If I interrupt the power towards the end of the purge period but before it fires then it will again do the full 12 sec purge. Based on that, this boiler/burner at any rate, will stay off for a minimum period of 12 secs no matter how often the interruption may be.

 

[Johntheo5]

Just had a look at the MPV schematics and a very easy test that might work, select HW only on, switch off the CH at the programmer, there is a manual lever on the side/end of the MPV, keep pulling this across to its mid position until you latch it in (push the lever up), the boiler should/may keep running, see if its running normally now.