Efficiency loss with oil boiler short cycling, is it worth doing anything?

[Gary_Funwick]

I have a Grant Vortex 36/46. Its derated to about 32kw but even so in normal weather it spends its time bouncing off the boiler stat, burning for about 2 mins, off for 2 mins, burning for 2 mins etc (I've got temperature loggers on the flow and return). Even with a modulating pump the return sometimes creeps up above 55 degrees at which point I presume its dropping out of condensing mode.

I've always thought that it was inevitable and the cost of sorting it (by installing a thermal store?) would be hugely more than the fuel savings. The "Beast from the East" and now the "Mini Beast from the East" have given the boiler the chance to burn continuously all day in full condensing mode (slightly underfired in fact, flow temp maxxed out around 65, boiler stat set at 70). From eyeballing the oil level it doesn't seem to be using that much more oil on "Beast days" than it does normally.

Which make me wonder how much below optimal efficiency short cycling is actually taking me. I asked Grant and they couldn't give me a hard number, does anyone have any figures.

And is there a way of fixing it (or at least making it better) other than putting in a thermal/buffer tank which I'm guesstimating would be 3 grand plus.

Thx

 

[oilhead]

You seem to have covered the alternatives. Why was an oversized boiler fitted in the first place?

 

[Gary_Funwick]

@oilhead
Client (ie me) with a bigger is better mentality.

Though I'd question whether its oversized (notional radiator size totals to 31kw, we've 2 unvented cyclinders and we're adding around 6kw of extra radiators to improve the heat up time of two rooms).

The boiler does burn continuously in the morning when the house is coming up to temperature (and the flow temp only gradually rises to 70). But as various rooms hit set point the load drops, surely that's inevitable; its a converted barn and the rooms have very different characteristics plus we keep unused bedrooms cool. If the boiler was sized for the load when the house was up to temp (on a normal day) it wouldn't be able to cope with the starting demand for heat.

I guess its a compromise I'm just looking for an indication of how big a compromise and whether it would be economic to try and mitigate it.

 

[oilhead]

Seeing the logic now.
In days gone by, where mechanical thermostats were used, it would have been possible to rewire the boiler stat with one that had an adjustable differential, but in these days of electronics, that is not possible. The buffer or thermal store seems a bit of a big job. You could try putting your pump into fixed mode, and experimenting with the speeds to see if that helps with cycling.

 

[snb]

You should be able to drop it down to a 0.75/80/eh
With a t5 head. That should solve the problem

 

[oilboffin]

Put in a wall stat and trvs and time the boiler also as snb said drop the nozzle size then the boiler will run longer you can go down as far as 0.65x80 EH on that model.Bob

 

[oilhead]

In cold conditions, the boiler runs properly. Derating will mean it may be ineffective in cold weather. A delicate balance.

 

[Gary_Funwick]

@oilhead
It is a delicate balance.

Thanks, in fact I've fiddled with the pump settings. None make a difference to the short cycling issue and what its on now (PP3, its a Grundfoss magna) seems to keep the return as low as possible most of the time.

What you comment about thermostats is interesting. I use Tado with eTRVs on all radiators so the house is split into eight zones/rooms. Obviously heat demand can vary massively. I've noticed that Tado handles small heat demands well and instigates a long duty cycle TPI-like algorithm (burns for 3-4 mins, off for 16-17 mins, so three cycles an hour). However mid-sized heat demands catch it out, I can speak to Tado to see if they can doing anything "in software".

In the same vein I was wondering if a boiler controller existed that would take an opentherm signal and convert it into an optimal on/off burn cycle by monitoring the flow and return temps.

The original question still stands though. If burning 2-mins on, 2-min off uses 50% of the oil of a continuous burn then there's no problem; its just how the boiler modulates its output. If it uses 75% of the oil of a continuous burn to produce 50% of the heat then it's becoming an issue. I need to put a flow meter on the oil line.

 

[oilhead]

A flow meter will do little apart from relieve you of a couple of hundred pounds.
The burner does not modulate, just on/off, so the amount of oil used is directly proportional to the length of burn, whether this is continuous, or in spurts.

 

[oilboffin]

You don't need a meter but you need a return temp over 45c to prevent dew point in the combustion chamber which will cause corrosion.Bob

 

[Gary_Funwick]

@oilhead
I understand that its on-off firing. The internet tells me that it has to prime the oil nozzle (wasting fuel), reheat bits of the boiler that have cooled (wasting fuel) and even that the flame takes time to stabilise (wasting fuel). I can understand that all these things *might* be true in theory but I don't know how important they are in real life. Trying to put a number on that was the purpose of my question. Your comment implies that its inconsequential and I should ignore it.

@oilboffin
timely that you mention that as when Tado is firing the boiler on a 20 minute cycle the return is typically only 40 degrees (flow 55-60 degrees). And speaking to Grant that was what they were most bothered about. I note also that when the boiler was serviced there was a fine patina of rust on the combustion chamber. Not enough to call pitted but there nonetheless.

So next question, how do I fix that? I presume that 3-4 grand's worth of thermal store/buffer tank would do it, but hits my original issue that fixing the problem costs more than the problem. Grant suggested a low loss header.

Thanks

 

[oilboffin]

If you get dew point in the combustion chamber that dew is acidic and will take out the boiler in a few years this is why they are doing away with condensing boilers in Germany and some other countries plus the fact the condesate buggers the drains.Bob

 

[JohnD]

they are doing away with condensing boilers in Germany

Link?

 

[oilboffin]

No link I was told it by a boiler inspector whilst doing a commercial boiler.Bob

 

[dilalio]

Grant suggested a low loss header.

I was thinking same.