Cheap way to prevent boiler cycling

[Crunchynut]

My oil fired CH boiler was cycling. That is, it was cutting out having reached temperature but then re-firing again within a minute. I think it is a common problem, and given that it’s inefficient for the boiler to be stop/starting all the time, I wanted to fix it.

 

I researched on-line and there are some really expensive products (£400+) that allegedly solve the problem. Electronics that introduce hysteresis into the control mechanism etc. Sledge hammer to crack a nut, I thought.

 

Anyway, I solved the problem by wiring-in a time delay relay to the main supply. These relays - about £10 each - close their contacts after an amount of time has passed since their coil was energised, as determined by the adjustable setting on the relay body. I set mine to 6 minutes. What this means is, every time the boiler is off, or cuts out having hit the temperature limit, it will take a minimum of 6 minutes before it will restart. Cycling problem solved.

 

FYI in case others have the same problem.

[Temp]

Some but not all thermostats have adjustable hysteresis. I think some also have anti cycling time delay built in. The overall effect is the same, you get slightly less precise temperature control. A thermal store or buffer tank stops the boiler short cycling but costs £.

 

 

Another possibility is fitting a smaller jet in the burner. We did this after discovering our boiler was bit too big even in very cold weather. According to Grant the smaller jet is a bit more efficient as well. We haven't noticed issues with the smaller jet blocking. 

 

 

 

 

..

[Crunchynut]

Ah, you mean a smaller nozzle?

[JohnMo]

Turn down flow temp open all your TRV's, then balance rads. Don't zone your system to death.  The bigger the open flow system the less likely short cycling is.

[Crunchynut]

I agree the system should be set up correctly in the first place, but I never did get the idea of balancing rads if TRVs are installed, because if TRVs close then the whole balance of the system changes. Also, if the stat is a simple, not very accurate on/off with no built in hysteresis then you will get cycling regardless.

Edited July 23, 2023 by Crunchynut

[JohnMo]

You wind the TRV out the way set to max temp. Then bring the flow temp down so the coolest room is the temp you want, then use the lock shield valve to trim the flow across the radiator to get each room close to where you want. Then set the TRV a degree or so higher than target room temp.

 

All stats have built in hysterisis, the issue is most the hysterisis to just to big so you get wild temperature swings.

 

All the other magic fixes are just magic, not real.

[Temp]

19 hours ago, Crunchynut said:

Ah, you mean a smaller nozzle?

 

Yes the nozzle/jet in the burner. As I recall Grant boiler model numbers are the power in kW with different nozzles. So a 50/70 is 50kW with a small jet and 70kW with a big one. Most of the specs list power out and efficiency figures for three different size nozzles.

[Crunchynut]

Ah I didn’t know that. I have a Grant boiler with Reillo burner too. I just replaced my nozzle (like for like) as part of a routine service. Never crossed my mind to try a different one - though the boiler and CH runs fine (now I have my delay relay 😛).

 

Out of interest, how often do you replace your nozzle?

[Temp]

4 hours ago, Crunchynut said:

Out of interest, how often do you replace your nozzle?

 

 

Only the once. I just asked the guy doing the annual service if he could fit a smaller one for me. Showed him the table in the installation manual. That was quite a few years ago now. 

 

 

 

 

[Nickfromwales]

9 hours ago, JohnMo said:

You wind the TRV out the way set to max temp. Then bring the flow temp down so the coolest room is the temp you want, then use the lock shield valve to trim the flow across the radiator to get each room close to where you want. Then set the TRV a degree or so higher than target room temp.

Good luck attempting that in a real world situation  The second any TRV responds, the balance is gone. With oil, @Temp's solution, or adding a thermal store, is the only actual solution.

The kW demand will be identical, after the OP has gone round the twist trying to balance the TRV's / boiler temp, and then the lock-shields.

Oil boilers need to come on at 100% output, burn as required, and shut off again. Heat loss of the dwelling will dictate this, and nothing else.  

[Crunchynut]

37 minutes ago, Temp said:

 

 

Only the once. I just asked the guy doing the annual service if he could fit a smaller one for me. Showed him the table in the installation manual. That was quite a few years ago now. 

 

 

 

 

I tend to replace the nozzle and the braided flexi hose to the burner every 4 years. I read somewhere in the past that this was good practice but can’t now find that guidance, but it seems to make sense. The concern with the flexi hose is the rubber perishing and starting to deposit debris that block the nozzle (or maybe the filter bundle before the nozzle). However out of interest today I cut the old hose in half and looked at its condition and it seemed fine. 

[Nickfromwales]

5 minutes ago, Crunchynut said:

I tend to replace the nozzle and the braided flexi hose to the burner every 4 years. I read somewhere in the past that this was good practice but can’t now find that guidance, but it seems to make sense. The concern with the flexi hose is the rubber perishing and starting to deposit debris that block the nozzle (or maybe the filter bundle before the nozzle). However out of interest today I cut the old hose in half and looked at its condition and it seemed fine. 

If this was the case, boiler MI's would stipulate this as a requisite at periodic service events. The nozzle will very likely never 'clog' due to the massive pressure of the oil entering it.

[Crunchynut]

Ok. I need to dig out where I read this since my only source of intel in the past was the manufacture’s bumf.

 

Are you suggesting that neither need replacing at service intervals?

[dpmiller]

8 hours ago, Nickfromwales said:

The nozzle will very likely never 'clog' due to the massive pressure of the oil entering it.

Say you've never worked with an oil boiler without saying it.

 

Nozzles block. With particulates, with varnish (inside and outside), with... water. The orifice size you're talking about here is tiny- smaller than a human hair sized- and it doesn't take much. Only 7bar doesn't help...

 

Manufacturers don't need to mandate replacement as the engineer should know both by visual inspection and combustion analyser readings what the jet's like. Most just opt for annual replacement as a tenner on a nozzle saves callbacks...

 

Again hoses are a liability thing. Do you want to deal with the aftermath of 1000 litres of oil in your kitchen/ founds/ back garden? Any kinks or rusting to the braid, or UV damage to a rubber outer, and I'll change it. Moreso if the user is dabbling in WVO/ bio-D/ sump oil mixes as all are more harmful to the rubber.