UFH BUFFER TANK SELECTION (no rads)

[John.g]

Definitely, heat would appear to be leaking away somewhere, I have a 3 bed (+converted Attic bed) and I am averaging 0ver the past few weeks, 5 ltrs/day of Kerosesne ~ 53kwh/day, allowing say 75% SE oil boiler then my house (+ very small HW usage) is comfortable with heat input of ~ 40kwh/day with heating enabled 15hrs/day, downstairs kept at 20/22c and upstairs 16/18C.

 

[dmitripopov1984]

summarise the key information about the ground floor UFH construction and operating temperatures

Hello,
Thanks for your response.
The ground UFH construction is the one which is indicated in my previous attachment (AmbiSolo).
Operating temperatures... 21c across kids bedrooms and 18 in our bedroom.
Cannot really understand where October 2021 came from. First of all, we did not run heating in October as it was fairly warm month, secondly we do not run UFH in the lounge as we do have a woodburner installed there now, which does it's trick.
The main question is how to overcome the issues when only bedrooms are running (around 2kWh), with the boiler being set to 10kWh output.

 

[dmitripopov1984]

100*25/860

John.g, please don't mind me asking but where the figure "860" came from?
I am trying to make calculations using your formulas on which tank (volume) is better to install. There are several options available, ranging from 30 litres to 350 litres (the ones I've looked at). Personally I think to stick to 100 - 200 litre range tank.
Would you be able to advise on the choice?
Thank you in advance!
Kind regards,
Dmitri

 

[John.g]

Hi Dmitri, aploligies for late reply but was up very (very) late last night.

860 is a conversion factor that I've used for years to calculate power in KW based on Flow in LPM X boiler dT in degC /60/860, and to calculate energy in KWH based on cylinder volume in litres X cylinder temperature in deg.C /860 and to calculate energy required to raise raise the cylinder temperature is cylinder volume in litres X dT /860. knowing any two allows the calculation for the other. 860 is (nearly) actually 3600/4.182. SO
Power = LPM*60*dT/860 KW and
Energy = Litres*Cylinder temperature degC/860 KWH and
Energy = Litres X cylinder dT/860 KWH.

I have no hands on knowledge of buffer sizing but others on here probably have but you can get a good feel for it by calculating the volume required based on two scenarios, based on 2kw minimum power required and perhaps 7.5kw based on minimum continuous power boiler output so as above.

100 Litre Buffer (2kw required), (boiler 10kw rated.)
Buffer available energy: 100*(75-25)/860. 2.91kwh.
Buffer supply time: 2.91/2*60, 87.3 minutes.
Buffer Recharging Time: 2.91/(10-2)*60, 21.82 minutes.

100 Litre Buffer (7.5kw required), (boiler 10kw rated.)
Buffer available energy: 100*(75-25)/860. 2.91kwh.
Buffer supply time: 2.91/7.5*60, 23.28 minutes.
Buffer Recharging Time: 2.91/(10-7.5)*60, 69.84 minutes

The above supply/recharging times will be directly proportional to the buffer volume chosen.

I wouldn't rush off just yet though, remember that nearly all smart?? room temperature controllers like Evohome continually fire the boiler for as little as one minute with 6 cycles/hour so the boilers are supposed to be able to deal with this, there is probably more wear and tear but figures I saw years ago suggest that there arn't huge efficiency losses with continuous cycling, oil fired boilers spend their whole lives doing this.

I would suggest taking a few numbers off your own boiler and maybe contact Vaillant of advice.
Can't remember if you said how many cycles or off times it takes to get the boiler to fire but next time id does fire up, note flow/return temps d.001/d.002 especially in the first minutes of firing as this is where most problems occur as the boiler fires at ~ 70% of max output (despite being range rated down to 10kw or whatever) for up to 60 secs before modulating down.
What can generally happen is the the boiler dT is > 30c and the boiler stops firing for any cycle time(s).
For example if the boiler fires at 70% of 38kw, 26.6kw then to keep the dt at say 25C requires a flowrate of 26.6*860/60/25, 15.25LPM, 915 LPH and if it fires at 70% of 30kw, 21kw then the required flow rate is 21*860/60/2512.04LPM, 722 LPH, if you give the numbers to Vaillant they may suggest increasing the pump head, another problem that was posted on here is that some models trip the burner when the boiler fow temperature reachhes target (flow) temperature +2 or3C, it should be target temperature+5C to give the boiler a chance to modulate down on reaching its target temperature.
When it does go to anti cycle mode check status S.53 & S.54 which are boiler waiting times

 

[dmitripopov1984]

Hi Dmitri, aploligies for late reply but was up very (very) late last night.

Hi John,

Thank for a such detailed response, muchly appreciated!

I also think by having the buffer installed it will give me more precise balancing of the zones. I mean the buffer will give constant temperature output, unlike boiler does (once additional zone(s) opens up the Flow temperature to manifold sometimes fluctuates in 5-10 C of the target temperature, feels like boiler is struggling to keep target temperature, or it starts cycling).

I will do some experiments / tries on my side now with your suggestions.
It's about right time now as it got colder since the weekend.
Stay safe, chat soon!
Kind regards,
Dmitri