Oil Boiler Condenser flow temp?

musicman100

Hi,

trying to figure this out. All the internet advice seem to relate to gas boilers? Will it still apply to oil boilers?

My Bosch one will not let you set it lower the 50 oC.

Thanks

ThisIsWeird

Hi MM100.

If your boiler is a condensing type, then I will hazard that, yes, it will equally apply as it would to gas.

For the combustion/flue gases to be 'condensed' to the point that the products of combustion (mainly water) are condensed out, and all that latent heat (not sure that's the correct scient term) is pinched by the system water, the surface temp of the collector will need to be below a certain temp, governed by the return temp. That's all relative, and even 'high' surface temps will still condense out to some degree when there are lots of condie being produced.

You ask about your Bosch, and this 50oC temp. Is this a 'flow' temp? If so, then the return will almost certainly be below 40oC - which in my books means pretty much 100% cond. 

How cold do you want to go, man?

Albermarle

On the other hand you normally need the flow temp to be higher than 50 to keep the house warm. Unless you have a very well insulated house and/or big radiators. So a compromise on the temperature is needed. Also if the boiler is heating a hot water tank as well, it needs to be higher to prevent any bugs spreading. Officially your hot water should be heated to 65, but unlikely to be a problem if it was 60.

ComicGeek

It's all well and good lowering the flow temp to save money, but none of the online advice points out that this reduces the heating provided!

If it's an older heating system designed to run at say 75 degrees, then that's a temperature differential between the mean heating water temperature and the design room temperature of 50 degrees.

If you lower the flow temperature to 50 degrees, then that's a temperature differential of just 25 degrees. You've just at least halved the amount of heat output from the radiators. Not great on a cold January day, might be ok on a March day.

Rodders53

ComicGeek said:

If you lower the flow temperature to 50 degrees, then that's a temperature differential of just 25 degrees. You've just at least halved the amount of heat output from the radiators. Not great on a cold January day, might be ok on a March day.

Running a system at lower circulating temps, but for far longer, may work equally well and the efficiency gain over very high temperatures may equate to less fuel burned (and less likelihood of radiator burns to elderly or children that a 75C temperature offers).
There may be more electric consumed due to the circulation pump running time as a downside, though?.

If the OP finds things OK at a low flow temp it likely bodes well for a heat pump replacement at a future date.

FreeBear

Most oil boilers do not modulate, so there is a limit to how much efficiency can be gained by turning the flow temperature down. With a gas boiler, turning the flow temperature down results in the boiler modulating the heat output down to suit which can lead to greater efficiency. Your typical oil boiler isn't smart enough. Instead, I would adjust the flow temperature to around 60-65°C, aiming for a return temperature of no more than 50°C (it is the low return temperature that is responsible for condensing). This should get you 86-88% efficiency which is about as good as you can expect.

Turn the flow temperature down too low, and the boiler will start to short cycle - This burns more oil and reduces the working life of various components if the short cycling is continuous.

Veteransaver

Oil boilers aren't as good at condensing as gas ones. Our 2 year old one doesn't really condense at all except from a cold start. And yes can only turn it down to 50 flow temp, but it seems that once return temp gets above about 30 there is very little condensing happening.