New Oil Boiler is Short Cycling.

[HandmadeMatt]

Hi guys.
We have a new external Worcester Danesmoor 18/25 kw System Oil Boiler.
The whole system is brand new, radiators and a un-vented 210 litre domestic hot water cylinder.

With all the radiators on full (the hot water cylinder valve is turned off for the moment. The kitchen and bathroom are not finished.)
After about 25 minutes the return flow on the heating circuit is hot enough to turn off the boiler's internal stat.
So the burner stops heating even though the room stat is still calling for heat. (I know this is normal.)
The central heating pump keeps running and then two minutes later once the return flow has cooled enough the boiler fires up again.
It then burns for two minutes until the return flow is hot again and then it stops.
On, off, on, off in two minute intervals.

So, it's all working properly but the cycle is too short, am I right?

Am I right in thinking that I want it to be at least a ten minute cycle?

The engineer who commissioned the boiler for me put in the middle sized nozzle.
I think it needs the smaller sized to run at the minimum of 18kw. Am I right?
I think currently the heat source is oversized for the radiator output. (This will be different when I start heating the cylinder obviously.)
(The engineer did do all the heat loss calculations and specified the radiators for me to buy.)

He came over yesterday and adjusted the bypass valve pressure and bled some air out of the heat exchanger in the boiler. He was reluctant to change the nozzle for some reason.

It didn't change the short cycling at all.
Any thoughts guys?

(All my radiators have TRV's currently set to maximum. Once we move in though and adjust them I'm hoping that our wood burner in the open plan living area will cause a lot of the valves to close down meaning that the boilers heat will be being sent mostly to the back rooms only in which case the radiator output will be even less. Will this exasperate any short cycle problems?)

 

[Terrywookfit]

The boiler is firing to replace the dissipated heat. An external boiler has a greater heat loss for obvious reasons.
If it did not do so , the radiators would never get hot.

 

[Agile]

You don't seem to realise that its relatively mild outside and so the heat requirements are fairly low.

Lowering the heat output will mean the heat up time will be longer. So you will then complain about that!

But most of all you will be complaining when its really cold outside and the boiler fires continuously and you are still cold because there is not enough heat being produced.

It is probably possible for the engineer to increase the anti cycle delay and I would suggest about 6 minutes would be a good compromise.

Tony

 

[oilhead]

Once you have the hot water connected, and the bathroom circuit in use, ten things may change. If this is likely to be soon, then do not worry about it.
Tony needs to stick to gas. You don't have an anti cycle delay function. Wait until you are using it properly, when you do so, you need to have the correct heat input. If you measure the return temperature, you may find it is to cool to utilise the condensing properties of the appliance, and if it is returning too cold, you may cause corrosion to the boiler because of this.

 

[HandmadeMatt]

OK, thanks.
A longer warm up time is not a problem if it means that it's all set up right Tony.

Is the current two minute short cycling OK for now then? There's not excessive wear on boiler components? Suffering efficiency?
I'm keen to learn about all this and deal with the engineer well so that the whole system is working as harmoniously as possible.
Thanks.

 

[oilhead]

How long is it until you we will be fully functional? It is unreasonable to expect full efficiency until then. Give your engineer a little leeway as complaining before the job is finished will alienate him.

 

[HandmadeMatt]

I'm not complaining and have a good working relationship with him, I just like being involved in the process so I can learn.
It will be some weeks before I intend to heat the cylinder but the commissioning is being done at the moment.
Besides the cylinder, there's no difference between now and the future in regards to heating demand as far as I can see. Everything is installed and all the pipework is lagged. It should be working properly.

I'm not living in the house yet, I just have the heating programmed for the early morning to frost protect and minimise damp, to take the edge off the cold so that whilst I'm working I'm not totally freezing. I have the hous stat set to ten degrees.
(I'm here full time doing the renovation living in a caravan.)

Is the current two minute short cycling OK for now then? There's not excessive wear on boiler components? Suffering efficiency?
What's bad about short cycling? (I'm keen to learn.)

Thanks again guys.

 

[oilhead]

Whilst you have the stat to such a low level, I would think short cycling is inevitable. Your input requirement will be too low for the minimum your boiler will produce even if reduced. For such a short period, then I would not think you will do any damage, but just for interest, why not see how it performs at normal levels for a day or two.

 

[HandmadeMatt]

The house stat is still calling for heat, the ten degrees isn't reached by the time the boiler starts short cycling.

It's short cycling on it's internal return flow stat, not the house stat.

What's bad about short cycling? (I'm keen to learn.)​

 

[Agile]

It lowers the operating efficiency of the boiler.

But as explained, because domestic oil boilers do not modulate they can only adjust their heat output by on off modulation.

In your case the makers have preset the anticycle at your two minutes and its not adjustable. They have decided on that from their consideration of efficiency, start up wear and condensation and corrosion aspects. You will have to accept it.

You need a certain heat output to give adequate output for when its -1 C outside.

Tony

 

[HandmadeMatt]

In your case the makers have preset the anticycle at your two minutes and its not adjustable. They have decided on that from their consideration of efficiency, start up wear and condensation and corrosion aspects. You will have to accept it.

OK, that's interesting. I didn't realise that there might be a preset anticycle function limiting the boiler to two minute cycles, without which the cycling could be much less?
If that's the case then I imagine everything is fine as it is.
It doesn't mention anything about that in the literature. Does anyone know that for sure?

This is all it says in the manual about short cycling:

 

[oilhead]

What temperature setting have you got for the boiler thermostat?
What setting have you got your TRV's on?
Is the return achieving the high temperature?
Is the boiler flue pluming?
Forget the 2 minute cycle, it does not exist on your boiler, Tony needs to stop applying gas technology to oil boilers.

 

[HandmadeMatt]

What temperature setting have you got for the boiler thermostat?

It's set mid way on the knob. The engineer measured this as 75 degrees celsius output in the log. The radiators achieve 60 degrees + with an infrared thermometer.

What setting have you got your TRV's on?

They are all on maximum for now but I will be adjusting them and trimming the system once we move in.

Is the return achieving the high temperature?

Yes, that's why the boiler turns off.

Is the boiler flue pluming?

Yes

Forget the 2 minute cycle, it does not exist on your boiler, Tony needs to stop applying gas technology to oil boilers.

Understood.

 

[HandmadeMatt]

There are ten radiators as follows:

1200 x 600 K2 single convector
1200 x 600 K2 single convector
600 x 600 K2 single convector
600 x 600 K2 single convector
600 x 600 K2 single convector
1200 x 300 K2 double convector
1200 x 400 K2 double convector
300 x 500 K1 single convector
300 x 400 K1 single convector
600 x 1800 chrome towel rail

In the future I may wish to add one more in the bathroom.

 

[oilhead]

As long as the circuits are all up to temperature, then you will do no harm. I suppose that the thermostats and relays have an optimum number of switches, but given that many oil boilers have thermostats that last 25+ years and replacing control PCB's with faulty relays is very rare, as these are the components being overworked, you should not worry. Just think of all the oil combi's that spark into life every time a hot tap is opened.
You may find if it were an internal boiler, that it would not cycle quite so frequently. The fact that your boiler is outside, despite the insulation, the unit will be subjected to low temperatures that will cause faster temperature loss than an internal boiler. Once everything is up and running, then you may wish to have the output adjusted, but while downgrading the output may increase warm up time, once it has warmed up, you will probably find it cycles just as often.