UFH condensing oil boiler short cycling

[markmorrison1957]

Thanks so much for sharing all your knowledge - I've put Wilo pump at speed 2 and will monitor. I'll likely not get a chance for a few days as working in my retirement job for a large UK retailer selling Percy Pigs!!
Strangely the on and off times when set at 6.0 seemed to vary from 1 on/3 off to 1on/1off. UFH was the only demand on the boiler. Sometimes I can hear faint tapping noise from manifold but I don't know if always there!!

 

[markmorrison1957]

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 upstairs radiators come on in morning when I'm still in bed and come on at night when I'm in bed! They are on for 30 mins each time and heat up we'll all over thought just a fraction cooler at bottom! Hopefully at moment not using excessive amounts of oil (usually 5000L yearly) as no winter fuel allowance!

 

[Johntheo5]

Are you running a Hotel?.
5000L/year, 51,750kwh, seems enormous, my 4 b/room house only consumes 1400L/year, 14,500kwh.
Your HW demand alone, at 5L/day, is more than my total yearly consumption and should produce 840L/day at 60C.
Are you sure, maybe only using 500L/year and 0.5L/day for HW?.

 

[markmorrison1957]

I have kept a spreadsheet over the past 20 years noting oil purchases and how long until next one! Each year over the 20 years ranges
from 4500L to 5500L - your 1400 annual litres would just about last me 3 winter months. I'm on first name terms with oil tanker driver - his quickest delivery after my phone call order was 30 minutes! In depths of winter sometimes daily consumption has topped 29L a day.
I've turned boiler thermostat up. The thermostat just has a thin to thicker line instead of numbers so at a guess its currently 65c (I think the thermostat range us from 55c -75c).
The dhw cylinder is set lower than 65c so powers off once it's temperature is satisfied. Temp at manifold shows flow of 50c (mixer valve maximum).
I'll try to get a chance tomorrow to note cycling times but earlier today was 2 mins on/1 min off thou ufh had only been on for 45 minutes and Wilo pump was setting no 2!
It's not a hotel thou with 4 bathrooms and two daughters who take daily baths and 30 minute showers I thought a daily summer oil consumption of 5L was good! The boiler is in a separate garage with a 20 foot run for boiler flow and return pipes to the house. The pipes are buried and I assume are well lagged about 5 feet deep. TBC.

 

[Johntheo5]

2min on and 1min off = boiler output of 27.3kw.
Don't forget the Wilo power when taking readings.

 

[markmorrison1957]

Okey doke!

 

[markmorrison1957]

Ok, with Wilo set at pump speed II the watts range from 39w when not firing to 44w when firing.
Firing time on average over 3 cycles etc was 2'43'' off and 1'01'' minutes firing.
Thermostat was 65c I think.
What do you reckon next- should we reduce pump speed to I and monitor? The ufh pump is set to 3!

 

[Johntheo5]

The Wilo must serve CH, UFH, and HW either singly or together so must be mindful of that, the setting will be a compromise, is it still noisy on UFH only, if not would be inclined to leave it there for now, but would change the UFH manifold to speed 2 and monitor a complete set of readings again, ie Boiler cycle times, Wilo power, UFH manifold flow & return temps.

From the above numbers the boiler output/UFH demand was, 41*61/224, 11.2kw, the boiler flowrate was 15LPM (from pump curves) but cant calculate other readings without the manifolf flow/return temps because a boiler demand of 11.2kw with a flowrate of 15LPM means a boiler return temp of 54.3C (assuming a boiler flow temp of 65C), this would then imply that the manifold return temp is also 54.3C which is hardly likely and most unlikely. Can you post a photo of the Wilo while running sometime.
Overall you will have to run with the other heat demands as well but not sure now what your aim is, is it still to determine if you require/desire a buffer? which IMO might realise ~ 3% savings or less.

 

[markmorrison1957]

The manifold temps for the most recent cycles were 48c in and 40c return. Ive since reduced boiler temp to 60c and l'll drop the grundfos pump to speed 2.
Will report back!
Do you think dropping a nozzle size from 1.15 to 1.0 might help in reducing hopefully boiler firing run times. I wonder is that worth a punt?
One problem is standing in garage where boiler located is my wife keeps turning down ufh thermostats without saying and therefore the volume of water circulating in system is up and down!

 

[Johntheo5]

Yes, why not do that, you still have a 36kw?? boiler.
Below is some idea of the efficiency degradation vs cycling times

 

[markmorrison1957]

Whoops - previously I said floor area for ufh was 50m2! It's actually 147m2 or 1592ft2

 

[markmorrison1957]

With both pumps set at 2 - over a 20 minute period after boiler had been on for 3 hours - it fired up for 6 minutes out of those 20 and was off for 14 minutes. There was 6 on and 6 off cycles and pump at boiler showed 40 - 44 watts!
If run time is 30% ie 20 minutes per hour does that compare to boiler that doesn't short cycle? Perhaps manifold speed down to 1 for another monitoring session in the garage!!

 

[Johntheo5]

What were the manifold flow/return temperatures?

 

[markmorrison1957]

48 in - 42 return!

 

[Johntheo5]

Boiler demand, 41*30%, 12.3kw, boiler circ flow 13.0LPM.
Firing time average of 60 secs, much of a much, this really won't change much, its the off time that will change with ufh chaning demand, if the ufh demand increases then the off time decreases. You should carry out some tests on CH only and HW only.
In view of the fact that there is 26/46kw stamped on your boiler you might ask Grant what is the actual lowest boiler output achievable, otherwise you just have to live with the 36kw when you change the nozzle.