UFH condensing oil boiler short cycling

[Johntheo5]

From post #32
The HW was the only circuit on during summer months and used about 5L of oil a day. It comes on maybe 1 min in 10 but will monitor as well.


The 5L/day would suggest that there is a very high demand, because that oil usage (38.8kwh) should, theoretically produce ~ 740L of HW at a boiler efficiency of even 75%, alternatively, firing 1 minute in 10 suggests, 11mins/cycle, would suggest that the boiler is firing for a total of 2hrs 22mins per day for whatever reason?.
I cant't find any info on this cylinder but it sort of hints that it may be a thermal store (but just used for HW) as it states 93.2L Heating and 200L Dom Hot Water, it also seems to have a thermostatic mixing valve, TMV.

Can you see what is written on the blue TMV? on top of the HW cylinder?

 

[markmorrison1957]

Cylinder is a Smartline E300 thou seems bigger.

 

[snb1]

that cylinder is an ACV smart E plus 300 . it is a tank in tank design cylinder.

 

[markmorrison1957]

Spoke with heating engineer who installed boiler and will call next week to downsize nozzle. He seemed to be of the opinion that 1 min on/1 min off was typical and acceptable for ufh and a buffer wouldn't work! I'll ask him his opinion on reducing burner down to a Riello 15/26 which is a straight forward swap.
I did have a thought to run past everybody.
When manifold reaches temp, flow in is say 50c and return 44c. Put a 2 port stat valve on the return from the manifold but close to manifold as to not interfer with return from upstairs radiator. The return stat valve could be set to 44c ie it would stay open until the ufh is up to temp and then close. There would be no flow, the mixing valve wouldnt cool the return flow to the boiler down as quickly as it normally does. Would no demand for heat shut off boiler for some more minutes than it currently does? Any danger to boiler? I suppose the short cycling would still occur until manifold return comes back at 44c but when demand returns and port opens do we go back to short cycling?

 

[Johntheo5]

Spoke with heating engineer who installed boiler and will call next week to downsize nozzle. He seemed to be of the opinion that 1 min on/1 min off was typical and acceptable for ufh and a buffer wouldn't work! I'll ask him his opinion on reducing burner down to a Riello 15/26 which is a straight forward swap.

Is he actually downsizing the nozzle next week?, if so, to what boiler output.
How do you run your system, do you run the HW demand first/alone and the UFH/CH in a zonal fashion etc, its important that you don't fall between two stools, ie burner maybe undersized (to 26kw) from oversized at 41kw, can the boiler be derated to say 30kw without changing the whole burner.

 

[markmorrison1957]

Downsizing initially to 36kw. We run HW early on in morning then radiators as well for 30 mins. The ufh comes on for 4 hours later and runs with hot water but never with radiators!
I believe we can go down to 26kw without changing burner - just need an air adjustment disc from Grant which I have ordered from them.

 

[Johntheo5]

Downsizing initially to 36kw. We run HW early on in morning then radiators as well for 30 mins. The ufh comes on for 4 hours later and runs with hot water but never with radiators!
I believe we can go down to 26kw without changing burner - just need an air adjustment disc from Grant which I have ordered from them.

Your HW cylinder is, as pointed out, a tank in tank, so can have very high circulation rates, hence your Wilo Pump > 53LPM circ rate.
During the summer when you reckon the oil consumption is 5L/day, is the HW cylinder programmed on all day because even a HW usage of 200L at 60C, 360L at 40C, will only require 15.5kwh (@ 75% boiler effic.) 1.5L of oil so there is still 3.5L of oil, 36kwh of lost heat going somewhere. Is there a long un insulated run where the secondary circ pump circulates around?.

 

[markmorrison1957]

The hw is on a circuit loop from garage boiler to house and back constantly circulating from 6.00am to 10.30pm. The hw in summer is on 7.00am to 10.30pm but is supplemented by 2.5kw of solar heating thou only if it's a nice day!
My next try is to turn dhw thermostat up to beyond boiler temperature. I'll run dhw at same time as ufh. With both on at same time the ufh could take advantage of the 90 odd heating circuit litres in cylinder which may act as a buffer!!

 

[Johntheo5]

The hw is on a circuit loop from garage boiler to house and back constantly circulating from 6.00am to 10.30pm. The hw in summer is on 7.00am to 10.30pm but is supplemented by 2.5kw of solar heating thou only if it's a nice day!
My next try is to turn dhw thermostat up to beyond boiler temperature. I'll run dhw at same time as ufh. With both on at same time the ufh could take advantage of the 90 odd heating circuit litres in cylinder which may act as a buffer!!

If my numbers are even remotely correct then you will be just adding that ~ 36kwh/day to make up for the cyl/pipework losses, havn't looked it up but I think 22mm of un insulated copper piping can loose something like 60 watts/meter so 36kwh loss in 14.5hrs = 2.48kwh/hour equates to total pipework length (if this is where the heat is going) of 2480/60, 41M????.

 

[markmorrison1957]

The dhw (flow and return) are lagged and as the garage is 4ft below house level likely to be buried under 6ft of blindings/soil. The garage is about 7 metres from house and including internal run there could be about 30 metres of pipework. The ufh and radiators are plastic pipes from garage to house and surrounded with proper lining. I'll compensate the extra 4 hours run time for dhw by cutting down runtime elsewhere in the day. There are solar panels with excess power not used in house going to cylinder immersion heater. Mostly on a sunny day the cylinder can take 9-10kwh from panels.
I'm still at a loss to figure out how the previous boiler (Warmflow Bluebird 150-200) with 44-58kwh didn't short cycle. It was a straight swap after Bluebird started to leak!
I've adjusted stats and will report any change in on/off. The stats for each ufh room are all set to 20c but because it's mild some are reaching temp and drop out thus of course reducing volume.

 

[Johntheo5]

The dhw (flow and return) are lagged and as the garage is 4ft below house level likely to be buried under 6ft of blindings/soil. The garage is about 7 metres from house and including internal run there could be about 30 metres of pipework. The ufh and radiators are plastic pipes from garage to house and surrounded with proper lining. I'll compensate the extra 4 hours run time for dhw by cutting down runtime elsewhere in the day. There are solar panels with excess power not used in house going to cylinder immersion heater. Mostly on a sunny day the cylinder can take 9-10kwh from panels.
I'm still at a loss to figure out how the previous boiler (Warmflow Bluebird 150-200) with 44-58kwh didn't short cycle. It was a straight swap after Bluebird started to leak!
I've adjusted stats and will report any change in on/off. The stats for each ufh room are all set to 20c but because it's mild some are reaching temp and drop out thus of course reducing volume.

Don't want to keep on harping about it but a oil consumption of 5L/day in the summer is outrageous especially with that solar PV divert.

The Warmflow may have been jetted down to give 44kw or may hve had a 50kw jet with the pump pressure at minimum to give ~ 45kw, not hugely different to your Grant, the big difference is the warmflow HEX buffering effect, 50L vs 21L, if you had a fairly modest demand of say 12kw then the warmflow burner ONtime/Offtime/Cycletime (minutes), should have been, 2.11/5.81/7.93 vs 1.01/2.44/3.45 for the Grant, meaning the Grant is cycling at over twice the Warmflow's, is this something like you were seeing?.

 

[markmorrison1957]

Johntheo5
I think you have hit the nail on the head!
I do recall now the Warmflow was jetted down probably to 44kw but the Warmflows bigger buffering effect with 50L rather than 21L in the Grant meant I wasn't concerned about short cycling as your calculations for the Warmflow burner are spot on!
Now - to achieve a similar downtime with the Grant by only reducing nozzle size - what nozzle would be appropriate given we can down either to 36, 31 or 26kw?
Additionally - at extra cost we could also add a buffer or thermal store but would 25L be grand?

 

[markmorrison1957]

The nozzle on the Warmflow could have been reduced to 1.35 but not certain.

 

[Johntheo5]

Johntheo5
I think you have hit the nail on the head!
I do recall now the Warmflow was jetted down probably to 44kw but the Warmflows bigger buffering effect with 50L rather than 21L in the Grant meant I wasn't concerned about short cycling as your calculations for the Warmflow burner are spot on!
Now - to achieve a similar downtime with the Grant by only reducing nozzle size - what nozzle would be appropriate given we can down either to 36, 31 or 26kw?
Additionally - at extra cost we could also add a buffer or thermal store but would 25L be grand?

For any boiler output (assuming the same HEX) the OFF time will not change for any given heat demand, its the ON time that will change.
Again assuming a 21L PEX with a 12kw heat demand.
36kw nozzle. burner ONtime/Offtime/Cycletime (minutes) 1.22/2.44/3.66. 31kw nozzle 1.54/2.44/3.98. 26kw nozzle 2.09/2.44/4.53. so only gaining < 1 minute (0.87) in burner on time and cycle time by reducing from a 36kw nozzle to a 26kw nozzle assuming 12kw demand.

Installing a thermal store entails installing at least one more circ pump, the gist of how this might work then is that the Wilo circ pump would be dedicated to just circulating the store water through the boiler, there would be a thermostat located fairly well down in the tank set to say 65C, which will switch the boiler on and off, the boiler stat would be wound up fully to ensure the boiler runs continuously until the store is recharged.

There may be some smart way of installing a buffer with just a few bits of piping and a UV cylinder but I'm don't know how to achieve this.

 

[markmorrison1957]

I've looked at buffer tanks and a lot of advice for a 25L or 50L is to run a two pipe system and by doing that no extra pumps are needed. A T pipe connecting buffer to flow and a T pipe connecting return to Buffer Tank.