Oil Boiler - best way to use

malc_b

Cardew wrote: »

The other important factor is that the output of the boiler modulates. i.e. it turns the output up or down as required.

No it doesn't at least according to Sedbuk and if I have the right model which sedbuk calls Grant [FONT=Arial, Helvetica, sans-serif]Vortex Utility 15-21.

I'd be interested to know which, if any, oil boilers are modulating?[/FONT] Gas yes, but oil boilers, AFAIK, are all very similar. A pump pressurises the oil and it sprays out of a small nozzle as mist. A fan blasts this mist with air into the firing chamber. A spark ignites it and you get a wall of flame. The only way to change the oil flow rate is to change the nozzle. You can't just drop the pump pressure like you can reduce gas flow because then the oil wouldn't spray out as a fine mist.

And clearly this boiler isn't modulating otherwise the OP wouldn't report it short cycling.

malc_b

rosscouk wrote: »

again, thanks for all replies.

I have just been in the kitchen and timed what the boiler was doing. It seems to run in 3 minutes cycles with 1.45mins on and 1.15 mins off. Temperature has gone up to 17c at the thermostat since turning boiler up with radiators definitely hotter to the touch.

What controller do you have? It sounds like it might be similar to what I use which is a clever combined time clock and thermostat. You program it with set point such as 20C @ 8am. It then decides when to start firing the boiler to ramp the temperature up so that it is 20C at 8am. On cold days it starts earlier. One the settings on my controller is the duty cycle which for oil you set at 15 min, for gas at 3 min (gas doesn't have a short cycle issue). So I suspect your controller may not have been set right.

You can usually find the controller manual on the net if you go to the makers web site. There is often a users manual and an installers manual with the stuff like the duty cycle only in the installers manual.

malc_b

Of course, for efficiency lower water temperature is best so you might want to try heating longer rather than hotter. Heating for longer will mean the house loses more heat but that could be as little as 5%. But longer heating will also mean that the CH works less hard and so you can get by with cooler radiators which allow condensing to happen.

The only way to know which is best to work out your heat loss in each room (there are programs around that help with that). Then estimate rad outputs by comparing with rads from screwfix, wickes, etc. of similar size. And see how they stack up. AFAIR rule of thumb for condensing is 30% oversized and even that means you don't condense in mid winter. You need rads 60% oversized get the same output at 60C (55C return water, 65C output, 60C rad average, 40C above ambient, rad output ~62%)

smallblueplanet

malc_b wrote: »

... Short cycling is said to be inefficient because the oil pump runs then the spark occurs so you lose some unburnt oil at the start and while the flame establishes itself. I've no figure for how much that is. My CH controller has an oil boiler mode with a 15min cycle, i.e. for 50% power it burns 7.5min every 15min, so it is a known effect.

Is this just condensing boilers or all oil boilers? Can I just get you to clarify, if you have the central heating water temperature set below maximum then it's inefficient as the boiler short cycles (by which do you mean fires and heats the water in short bursts?)?

How much difference does it make and why then do people lower the water temperature?

Thanks.

Cardew

malc_b wrote: »

No it doesn't at least according to Sedbuk and if I have the right model which sedbuk calls Grant [FONT=Arial, Helvetica, sans-serif]Vortex Utility 15-21.[/FONT]

[FONT=Arial, Helvetica, sans-serif]I'd be interested to know which, if any, oil boilers are modulating?[/FONT] .

Yes you are correct - I had forgotten we were talking about oil and was was in 'gas mode'!

Regarding your question, I found this article by a director of Worcester Bosch.

http://www.worcester-bosch.co.uk/installer/our-company/news/erp-regulation-update

The major stumbling block is on the modulation of an oil-fired boiler. Generally speaking, certainly in the UK, a domestic oil-fired boiler typically has a fixed burner, a non-modulated burner.

It simply cycles on and off when it reaches its required temperature.

However in the current proposals, manufacturers would incur a penalty of 7% on the overall efficiency of the boiler.

We consider this notion to be very unfair as we don’t see any great difference in efficiency between an on/ off burner and a modulating burner.


As a result we believe, a penalty which takes 7% off the efficiency of, for example, one of our oil-fired boiler will make it difficult, if not impossible, for oil-fired boiler to continue in the UK without changing to a modulating burner.


A modulating burner would cost almost double the price of the present burner used in oil burners and also require an increase in controls complexity.

However the main point for the OP is that if his CH system* cannot get his house to 20.5C, he needs to increase water temperature - or get bigger/more radiators.

* assuming that the system is working correctly.

malc_b

smallblueplanet wrote: »

Is this just condensing boilers or all oil boilers? Can I just get you to clarify, if you have the central heating water temperature set below maximum then it's inefficient as the boiler short cycles (by which do you mean fires and heats the water in short bursts?)?

How much difference does it make and why then do people lower the water temperature?

Thanks.

OK, so you get more energy from the flame to the water if the water is cooler. That's true for all boilers but is not that significant except with condensing boilers. Old boilers don't condense as they then rust, etc.. Condensing boilers are designed to do this as you then get a lot more energy out of the flame, say 8%. Expensive fuel is why we now have condensing boilers.

For a condensing boiler to work the return water temperature has to be cool enough (56C I think is the usual). On a modulating gas boiler you can set an output limit of say 68C and with a properly designed system you'll have 12C difference to the return water, i.e. 56C. The gas boiler will reduce the flame giving a lower output power.

But oil boilers don't modulate. A lower boiler thermostat will result in the boiler short cycling (i.e. hall thermostat demands heat, boiler thermostat is giving 2min on, 2 min off behaviour). That's not good for efficiency as you waste oil at each start and stop.

Now whether it better to short cycle and condense or long cycle and not condense I don't know as I don't have any figures for the loss due to short cycling. And also you will always condense at the start when the water in the rads is cool, it's only when the return water is above 56C you drop out of condensing mode. So it's not condensing or not, it's short cycling vs some non-condensing.

The other issue is that unless the radiators are 60% oversized then you have to run up and into non condensing mode in order to get enough heat into the house at mid winter. So mid winter you have to turn the thermostat up.

Update: I see Cardew has posted while I was composing this. The 7% figure is interesting. I guess that is from sedbuk and presumably has some basis even if Worcester Bosch don't agree. As it is similar to the difference between condensing and non condensing then I'd say avoiding short cycling is better so boiler thermostat set to max. The rationale behind that is you still get some condensing operation with the boiler thermostat at max, just not for all of the burn. So say 50% of the time you take an 8% hit, that's a 4% hit not 7%.

smallblueplanet

malc_b wrote: »

OK, so you get more energy from the flame to the water if the water is cooler. That's true for all boilers but is not that significant except with condensing boilers....

Okay, I'm getting a bit confused. I know the OP has a condensing boiler and so what you're saying relates to that. But does it hold for a non-condensing oil boiler?

As you say 'expensive fuel'...

So in trying to be efficient we have a lowish thermostat temperature of c18C and I had the water temperature lower than max as I was worried about getting 'burned' if I bumped into very hot pipes/rads. So in this circumstance are you saying it is more efficient to have hotter water to stop the boiler short cycling, or as it isn't a condenser it doesn't make much difference?

rosscouk

Wow that was flurry of activity!!

Anyway just come back in from the cold and the boiler (with its new Heat Temperature) has heated the house to a point where my thermostat is reading 18c. Things feel pretty comfortable so I have reduced the requirement to 18c now. I will see how the boiler copes with keeping the temperature there. I will know as I am about to go and cook dinner and I will keep an ear out for whether it is short cycling.

As for malc_b's question in regard to my system controller, I doubt it is anything as advanced. After a quick look, the thermostat is a wireless Siemens RDH10RF, the transmitter a Simens RCR10/433-GB and the controller is a Drayton LP522 (split hot water and CH).

malc_b

smallblueplanet wrote: »

Okay, I'm getting a bit confused. I know the OP has a condensing boiler and so what you're saying relates to that. But does it hold for a non-condensing oil boiler?

As you say 'expensive fuel'...

So in trying to be efficient we have a lowish thermostat temperature of c18C and I had the water temperature lower than max as I was worried about getting 'burned' if I bumped into very hot pipes/rads. So in this circumstance are you saying it is more efficient to have hotter water to stop the boiler short cycling, or as it isn't a condenser it doesn't make much difference?

Sorry TMI I guess. If you don't have a condensing boiler then set boiler temp to max to stop short cycling. That is most efficient for that type of boiler.

However, if you don't have a hot water cylinder stat then you'll get very hot water and you'll have very hot rads which might be a concern if you have children.

malc_b

rosscouk wrote: »

As for malc_b's question in regard to my system controller, I doubt it is anything as advanced. After a quick look, the thermostat is a wireless Siemens RDH10RF, the transmitter a Simens RCR10/433-GB and the controller is a Drayton LP522 (split hot water and CH).

That's just a normal thermostat and timer set up so the oil boiler will turn on or off when the thermostat says so. The heating on/off cycle will thus depend on how fast the room it is in cools/warms. A smarter controller would be more efficient. I have CM67 and I think my son has the CM907 (CM67 is discontinued). You can get wireless versions as well. For all boilers these adjust the turn time in the morning, starting later if it is warmer. For oil they have a minimum burn time and a fixed cycle time so you don't have short burns.