Oil burner not functioning properly

[Martin 1]

Hi,

I have an oil fired heating system, approx 17 years old.
Its a Warmflow boiler with a Riello oil burner. The unit feeds the radiators plus a Santon pressurised storage water tank.

I have 2 issues.
Occasionally the high level trip activates meaning I have to reset the button.
I have adjusted the dial to about 2 o'clock, then 3 o'clock and 4 o'clock. The boiler obviously cuts out when the thermostat temp is reached, but at these lower settings it seems the boiler is cycling on and off well before the desired house temp is reached. Surely the boiler should stay on until the temperature I set in the house thermostat is reached and then cycle on and off to maintain that heat?
I understand the boiler thermostat is there to prevent the boiler overheating, but why is it cycling on and off way before the house temp is reached, and on occasion, tripping off completely needing a reset? In other words, I suppose, why is it overheating?


Recently, I have had replaced the red expansion tank, had a new Wilo Sonos Pico pump fitted and had a Fernox TF1 filter fitted.
The system was drained down a few weeks ago, cleaner added, run for a few days then drained again, and then inhibitor added and refilled..

None of the radiators are cool, and all heat to very hot when allowed too, they also bled correctly and there is no sound of air in the system so I don't think there are any issues there.

The expansion tank shows just under 1bar when cold, and about 2 bar when hot.
Ive checked every valve etc and everything is functioning externally as it should, yet my boiler still seems to be overheating?

Can anyone advise what might be causing the boiler overheat?

 

[Martin 1]

anyone?

 

[oilhead]

Lack of circulation, which also explains the cycling. You must have an airlock somewhere that impedes the flow.

 

[Martin 1]

The more I try and look at this systematically, the more I tend to agree. Its a new pump, so I don't see an issue there. I did notice that the rads return valve on most of the radiators was screwed way down, in some cases, barely on. I am wondering is this over restricting the flow back to the boiler. I have now opened every rads outlet valve by 2 turns to see if this helps the flow. Ill monitor how the furthest away rads react, but I'll try this. Other than that, it can only be an airlock or faulty pump, but the pump seems to be functioning fine, I took it off and checked last night. I also bought a new boiler stat and wired this up earlier and there was no change, so the stats not at fault, in fact its doing its job correctly as far as I can see. If I have no luck with all the rads stop valves opened more, then the next thing Ill try is to drain and refill the system again.

 

[ericmark]

I have the same problem getting to grips with oil fired boilers, with gas because they can modulate, they can run continuously. But will oil there is a balance, my boiler, on switch on, even with no radiators being heated, can run for 20 minutes before it cuts out. We want to heat the occupied rooms fast, so if the boiler was much smaller then the time before the radiators are hot would be a lot longer. By selecting the sequence at which rooms are heated we can reduce the time to heat occupied rooms, but 17 years ago the idea of programmable TRV heads was in its infancy, the idea of heating kitchen and living room first, then dinning room, and finally bedrooms was not even considered.

However to heat rooms fast has a inherent problem, they can over shoot and we then have a hysteresis where the temperature takes a long time to settle down. So in my house the wall thermostat is set in 0.5°C increments, one every hour, to reduce the speed and so also the amount it over shoots, it also allows the TRV heads to do their job, and get each individual room to the temperature set, yes it would be nice if my smart thermostat could work out if it runs the boiler for 30 minutes the house temperature will raise by 3°C so to move from 17°C to 20°C it needs a 30 minute burn, but to do that, doors would always need to be the same, and the TRV heads would need to be linked to the wall thermostat so they also allowed heating for full 30 minutes, in real terms that is not going to happen, even if the thermostat on the wall says 45 minutes to reach temperature, as mine does, that does not mean that will happen.

So the only way to control the output of an oil boiler is to alter the mark/space ratio, so there is little alternative but for it to cycle off/on. I fitted my own central heating in my first house, I worked out the BTU of radiators and boiler so radiator size was slightly more than boiler, it didn't work, I had to add two more radiators down stairs, the boiler is about half the BTU output of the radiators, so radiators can be trimmed back to balance the system.

So what size are all the radiators compared with boiler size? Today with electronic TRV heads, the boiler can be quarter of the size of the radiators, all it needs is for furniture to stop air circulating and the radiators can't deliver the energy to the room, unless using fan assisted radiators of course.

 

[Martin 1]

thanks ericmark, appreciate the time you took to explain that. I don't know what's happening in my system at the moment, frankly Im exasperated. It has been fine for years, no noticeable cycling, certainly never tripping the high level trip and doing a great job warming the radiators to whatever temp I set on the hall and bedroom stats. I could if I desired heated the rooms until was like a sauna and the boiler ticked merrily away with the boiler stat set at 3 or 4 (not that I did that normally).
But this last 2 weeks I have had this problem. I thought I was on to something with the return valves as in my post above, but Ive now opened the return valves on all the rads at least a turn and a half which is more than enough, and the boiler is still turning itself on and off every 5 mins.
3 new boiler stats tells me there is no issue with a faulty stat. A new Wilo pump (Yonos Pico with a 6m head, exact same spec as my older Wilo pump), new expansion cylinder although fitted a month ago.
Other than that the only changes to the system from when it was working fine is the fitting of a Fernox TF1 filter (easy job and it is connected with the arrow pointing in the right direction, and the subsequent draining of the pressurised system to fit TRV valves on all the rads (except where the house stats are fitted). I then refilled the system including cleaner for a week, then drained down again, added inhibitor and refilled, bleeding all the rads accordingly. The AAV at the boiler was hissing away as I was refilling the boiler for about 10 seconds, suggesting the air was being expelled, and the rads were all bled until only water was coming out the bleed valves. The rads are all hot with no problem rads, and the hot water is as expected.
I have ball valves each side of the pump, I took the pump casing off and opened the top and bottom valves and water is gushing out. I did the same on the ball valves fitted above and below the Fernox TF1 filter, then opened the fernox TF1 drain valve and water is gushing out under pressure, so flow seems fine.
I'm really at a loss now.

 

[ericmark]

The rads are all hot with no problem rads, and the hot water is as expected.

So what is the problem? The jets with oil do wear, and do need changing from time to time, but I assume with so much work done, the boiler will have been serviced.

For the boiler to run for a long time all radiators need to be taking their share of the water, the normal way that the lock shield valve is set, is all TRV heads removed so no restriction due to them closing and the temperature difference on each radiator is set the same, easy enough with a gas boiler that modulates as the water heats up so never turns off, but with oil, you get half way through, and then boiler switches off, so one would have to do it over many days, after 15 minutes of running measure one radiator and we all know that is not going to happen.

So I cheat, turn off a radiator at the lock shield, then slowly ¼ turn at a time with around 4 minutes between each adjustment turn it back on until I feel the feed pipe warm up, then just a little more. Idea is there should be around 15°C differential between feed and return on each radiator with all TRV heads removed.

If one radiator is wide open, it will send hot water back to boiler before all are hot, I have seen it with a gas modulating boiler where the guy said the boiler was too small as it took around 3 hours to heat the house.

What was happening was the returning hot water was turning the boiler down, so although a 28 kW boiler, it was only running at 8 kW. It would heat room 1 then the TRV would start to close, then it would heat room 2, until that TRV started to close, then it heated room 3, and so on. Once the house was hot the TRV heads would control the flow so all rooms stayed hot, but on start up from cold when all TRV heads wide open it heated one room at a time. Cure was simple adjust the lock shield valves, a by-pass valve stuck open would do the same thing.

I found once the lock shield roughly set, with a gas boiler and an electronic TRV head, I would close the lock shield until the target and the current temperature matched, not so easy with oil, as when the boiler stops the TRV opens so there is always some hysteresis.

 

[Martin 1]

So what is the problem? The jets with oil do wear, and do need changing from time to time, but I assume with so much work done, the boiler will have been serviced.

No, the boiler hasn't been serviced lately. Do you think the jets may be a potential issue? I haven't even considered the burner side of things because I presume a boiler overheat would be more to do with the 'water' side, ie flow, pump, thermostat related. The problem is that my boiler is overheating. The rads may well hat up and I have hot water, but the rads are only heating up to a point, and cant get any hotter due to the temperature on the boiler stat rising and cutting power to the burner. It doesn't matter whether the stat is on setting 1 or setting 6, the temperature of the stat will eventually rise to the position of the dial, and we are then off on a merrygoround of cycling. It never done this before, at least before the last draining down of the system and fitting of the TRVs.

 

[ericmark]

I am an electrician not a oil fired boiler man, as part of my job, we had to repair space heaters, I was on Falklands so there were no demarcation lines, I was getting complaints that they were irritating users eyes due to fumes, space heaters don't have a flue like a central heating boiler, I found fitting new jets into the small heaters cured them, and fitting the jets that had come out of the small heaters in the large ones also cured them. It seems the jets wear.

Back in the UK I did not want to play, as getting it wrong costs money in wasted oil, so I got an oil boiler repair man to service it, and it seems they often need to fit new jets, and adjust the pressures. Since you say

Occasionally the high level trip activates meaning I have to reset the button.

then it would seem likely your boiler needs new jets, in spite of having worked on many similar units in the Falklands, I would not DIY it in my house, I am not in the room with the boiler when running so would not see any faults, and getting it wrong costs money with wasted oil, so paid £100 plus for a full service, can't give an exact price, as photo cell changed, jet changed, filters changed and cleaned, plus two new motorised valves fitted. But boiler runs sweeter now, no smell of oil when I walk past the flue, where there was before.

I think maybe also some fault with lock shield valve settings, but if getting it serviced most guys doing that job will advise on how to set up lock shield valves.

 

[oilhead]

With respect, I feel you are both inexperienced with heating. Irrespective of the heat source, a system will take time to heat up the area it serves. Gas boilers tend to heat up a small amount of water in a low water content heat exchanger, and move it quickly. The nature of an oil boiler is that the flame is only suited to a large heat exchanger, so initially it takes longer to get a hot flow. Once there, the boiler thermostat will maintain the set temperature. If the pump or system is not able to dissipate the heat produced, then the burner is switched off. When the boiler temperature is reduced, the burner fires again, or cycles. Generally the boiler thermostat will maintain a temperature around a 5 to 10 degree differential. The pump will move a consistent amount of water through the system. This means the boiler will cycle whilst the area is still cool, whereas a gas boiler may modulate.
If the temperature is not reached as quickly as it used to, then the problem iswithin the system not the boiler. Whilst oil nozzles wear, excessive wear will produce problems other than slow heating. If the boiler has not been serviced regularly, then this may be a problem, but with the fact that you have changed components and altered the balance, this would be my preferred point of problem. I suspect if you had a gas boiler, it would be operating in a permanently low mode.

 

[ericmark]

I would agree likely main problem is the balancing of the system. However before balancing all air locks need removing. I will admit I have not used the correct method in the past of turning lock shield and bleeding then TRV and bleeding to ensure air both in feed and return removed.

Until this house never needed to do that, simply opening one radiator at a time cleared all air locks.

So to reduce a 15/15 minute to 5/5 minute some way most of the circulating water is bypassing most of the radiators, either some radiators open more then the should be, a bypass valve has opened or some radiators are part blocked.

I would guess at first some part blocked with an air lock, and now some are opened up too far.

 

[oilhead]

Tripping the high limit is also indicative of poor circulation/air locks.

 

[Martin 1]

With respect, I feel you are both inexperienced with heating. Irrespective of the heat source, a system will take time to heat up the area it serves. Gas boilers tend to heat up a small amount of water in a low water content heat exchanger, and move it quickly. The nature of an oil boiler is that the flame is only suited to a large heat exchanger, so initially it takes longer to get a hot flow. Once there, the boiler thermostat will maintain the set temperature. If the pump or system is not able to dissipate the heat produced, then the burner is switched off. When the boiler temperature is reduced, the burner fires again, or cycles. Generally the boiler thermostat will maintain a temperature around a 5 to 10 degree differential. The pump will move a consistent amount of water through the system. This means the boiler will cycle whilst the area is still cool, whereas a gas boiler may modulate.
If the temperature is not reached as quickly as it used to, then the problem iswithin the system not the boiler. Whilst oil nozzles wear, excessive wear will produce problems other than slow heating. If the boiler has not been serviced regularly, then this may be a problem, but with the fact that you have changed components and altered the balance, this would be my preferred point of problem. I suspect if you had a gas boiler, it would be operating in a permanently low mode.

Thanks Oilhead. I actually do understand all that and I do understand the basic functioning of how an oil boiler works. My boiler always cycled as I believe most do, but not every 3 minutes on, 3 minutes off as it is doing now. It took almost 5 hours to bring the hall (where the zone 1 nest stat is located), up to 20 deg. It normally would have taken less than an hour. So I believe all the electrical components and mechanical components are functioning correctly, there is just a factor preventing them running normally, due to the boiler stat cycling regularly to prevent the tank overheating.
The general consensus seems to be a flow issue which makes a lot of sense to me. But I am perplexed as to what has caused it. The only things materially that have changed from the system was working normally are:

New like for like Wilo pump and two ball valves fitted from feed and correctly orientated (since taken off and checked, and functioning normally with strong resistance when trying to stop it spinning. Ball valves opening and closing fully )

Fernox filter and two ball valves fitted on return and correctly orientated) (simple fit, again, fernox filter drain opened and each ball valve opened and closed with strong water pressure from flow and return sides.)

New red cylinder expansion tank fitted a while ago, no issues and holding pressure with no drop over time.

System drained fully, cleaner added, refilled and run for a few days, then drain valve at boiler opened with hose outside, flushed with mains pressure from top up, then drained and refilled including inhibitor, and bled at each rad.

There are no cold radiators, they all heat up until too hot to touch with no cold spots, so I don't believe there are air locks in the rads. The hot water is hot. But of course I can only get the rads so hot due to the boiler overheat issue.

I am going to try draining the system and refilling again today.
I have 3 zones on the house (its a bungalow and all the 13 rads are on the same level) Zone 1 is main house, zone 2 is bedrooms and ensuite, zone 3 hot water.

I am thinking of draining then refilling and testing each zone independently? ie, draining with only zone 2 bedroom rads open, then refilling and testing heating with zone 2 only. This might help me isolate an issue?

My thinking is that a normally functioning system should work normally whether there are 3 radiators switched on, or the whole 12. So if I run with only 3 rads on zone 2 on and see how it effects the boiler function then I can begin narrowing down the source of the problem?

 

[oilhead]

Are you bleeding the rads through both the flow and return?

 

[Martin 1]

Sorry oilhead could you elaborate? there are two bleed screws on each double rad, I am bleeding through them. I don't know what you mean by bleeding both flow and return?