Reducing Nozzel Size Oil Boiler ( Down Firing) outside manufactor specs.

[Fly100]

Does anyone have any experience of down firing an Oil Boiler ? Mine was oversizedfor the house, but ive spoken to the Tech helpline and they have given me 3 nozzle and pump settings. Going down upto 3 nozzle sizes below the lowest suggested in the manual. They did say however the lowest nozzle would probably be no good. I wonder has anyone done this themselves or has any experience.

 

Thanks,

Fly

Edited September 20, 2022 by Fly100

[ProDave]

You might find you can't get the airflow correspondingly lower so might not be able to set the flue gasses correctly.

[Fly100]

1 hour ago, ProDave said:

You might find you can't get the airflow correspondingly lower so might not be able to set the flue gasses correctly.

 

That was the suggestion about the lowest nozzle setting they gave me. I’ve looked at the parts list and the only significant difference is the  length of the blast tube.

[markc]

How are you determining it is oversized? Is it constantly firing and then turning off again pretty quickly?  Or are you calculating it is oversized 

Edited September 20, 2022 by markc

[Fly100]

19 minutes ago, markc said:

How are you determining it is oversized? Is it constantly firing and then turning off again pretty quickly?  Or are you calculating it is oversized 

Both, my calcs bring it out at approx 16 Kw boiler is reqd while the Airsource heat pump company who did a quote got it to 18Kw ( including DHW). And it short cycles sadly. The boilers lowest setting is 25kw.

Edited September 20, 2022 by Fly100

[markc]

53 minutes ago, Fly100 said:

Both, my calcs bring it out at approx 16 Kw boiler is reqd while the Airsource heat pump company who did a quote got it to 18Kw ( including DHW). And it short cycles sadly. The boilers lowest setting is 25kw.

If it’s short cycling what temp do you have water and heating (boiler temp) set at. A week or so ago a couple of us were discussing this as we had both found that upping the temps to stop short cycling resulted in a big fuel saving, these are gas boilers not oil but principal is the same.

a big boiler will not be very efficient or effective if it isn’t running hard enough so reducing jets/burners can often be false economy.

[Temp]

5 hours ago, Fly100 said:

Does anyone have any experience of down firing an Oil Boiler ? Mine was oversizedfor the house, but ive spoken to the Tech helpline and they have given me 3 nozzle and pump settings. Going down upto 3 nozzle sizes below the lowest suggested in the manual. They did say however the lowest nozzle would probably be no good. I wonder has anyone done this themselves or has any experience.

 

Thanks,

Fly

 

Not that far. We only dropped our Grant down to the minimum recommend in the manual. The manual says this actually improves efficiency slightly. It's been fine for 15 years.

 

Our Grant heats a thermal store and we still got cycling on the standard jet. Reducing the jet size wasn't enough to eliminate short cycling. I ended up fitting a bigger water pump so that with the boiler dial set on maximum the flow temperature never gets hot enough to shut the burner off. The burner now runs until the thermal store is satisfied.

 

This probably wont work if you don't have a thermal store/buffer.  What will probably happen is the return will get hotter and that will cause the flow temperature to increase causing cycling again. It might also drop out of condensing mode. 

 

Do you have a lot of small UFH zones? You could try configuring them so you have one stat controlling multiple or all zones. Less control but might help reduce cycling a bit.

 

 

 

 

 

 

[JohnMo]

Big buffer will sort it out.

 

As said upping flow temp will help.

 

Your cylinder, assuming you have one, will have coil in it designed for flows in excess of 70 deg.  If the flow temp is below that, the coil cannot transfer enough heat in to the cylinder, so the boiler will trip on excess return temp.  Then keep repeating until cylinder thermostat is satisfied.

 

CH, if you have UFH you could batch charge the floor, so run the flow temp a couple of degrees hotter for 4 or 5 hours, then timer the flow off.  Or fit a something like a 100 litre buffer (you would need to size it correctly).

[Fly100]

Upping the Flow temp push my return temp up and out of the Condesing range (47c on oil ?). Probaly going to drop a size as a trial and see how I get on. Time will tell.

Edited September 22, 2022 by Fly100

[JohnMo]

You may need to drop a size or two, plus a buffer.  My boiler turns down to 6.7kW feeding a max of 3kW and I have a 180l buffer to stop it short cycling.