UFH condensing oil boiler short cycling

[markmorrison1957]

In the summer I replaced a massive Warmflow Bluebird 200 oil boiler holding 50L with a Grant Vortex 36-46 holding 21L.
As its colder I have now switched UFH on but boiler short cycles every minute once up to heat. There was no short cycling with the Warmflow boiler.
The UFH is set to 45c flow and return after 1 hour is 40c - 45c.
Floor area is 50m2 and UFH manifold is 6 zones and 12 loops.
Boiler is set to 65c.
Flow pipe from boiler goes to upstairs radiators (15 and gate valve) and T'd to UFH manifold where on demand pump pushes flow through.
Pump on return brings water back to boiler.
Should I put in 2 port buffer tank and what size or is there a simpler solution?
Wracking my brains to stop short cycling!

 

[Johntheo5]

What kw output is the Grant jetted to and what was the Warmflow's output?
are you saying the dT through the UFH is, 45-40, 5C?. if you add up all tĥe UFH flowmeters what is the total flowrate?
What is this on demand pump?, is it the UFH manifold pump with a thermostatic mixing valve?
What is boiler stat cutting back in at?, does it appear to be 55C, 10C hysteresis.

 

[markmorrison1957]

Thanks for your reply. I'm not technically aware but in response I think the Grant Vortex is 36kwh whilst redundant Warmflow was 52kwh at least. Somehow the Warmflow didnt short cycle but was that because it held more water? The UFH flow is mixed down to 40c returning out at 38c when up to heat. The manifold is 16 years old with a circulating pump and thermostatic mixing valve but doesn't have flowmeters!! Boiler stat doesn't let me know when it cuts back in and no stat on return.
No short cycling when running upstairs rads (ufh only downstairs). I wonder is short cycling down to smaller amounts of water in newer boilers compared to those of 16 years ago - the boiler shuts down before it completes a full cycle at 65c because the ufh doesn't need it and there's nowhere else for the cycled water to go?

 

[Johntheo5]

A temperature drop of only 2.0C seems very low, would have expected 6C to 8C at least, you might only be extracting 4kw or so, however even so and while the Grant may only fire for 1 minute then I would expect 21L to suppy that 4kw for approx 4mins before refiring, I would expect the warmflow to fire for around the same period or a bit longer but remain off for 8mins or so.

EDIT: The Grant does a post purge each time it shuts down so this will probably reduce the buffering effect.
"Floor area is 50m2 and UFH manifold is 6 zones and 12 loops", if you have 150mm spacing then 50M2 should only require say 4 loops, 12 loops should do a requirement of around 150M2.

 

[muggles]

Trying to push 36-46kW into a system that's only capable of absorbing 4-5kW I'm not surprised it cycles. An EOGB Sapphire would probably have been a better option for you. Did you even need a 36-46kW boiler? Must be an enormous house...

 

[Johntheo5]

Trying to push 36-46kW into a system that's only capable of absorbing 4-5kW I'm not surprised it cycles. An EOGB Sapphire would probably have been a better option for you. Did you even need a 36-46kW boiler? Must be an enormous house...

I have a 21kw 19 year old SE Firebird and the rad heating demand is often as low as 3kw, the boiler may only fire for a minute or two but it does not refire again for 8 minutes or more, the big difference rad/ufh ith the same demand is that the boiler circ flow (with UFH) is often only a 1/4 or 1/3 of that with rads, IE, if one had a UFH demand of 5kw with manifold flow/return temps of say 45C/38C, then the manifold circ flow is 10.24LPM but tthe boiler circ flow rate (assuming boiler temp 65C), is only 2.66LPM, 10.24*(45-38)/(65-38), resulting in a very big boiler dT, a automatic/manual bypass between the boiler flow/return should help.

 

[markmorrison1957]

I spoke with Grant and advice is to reduce nozzle size from 1.25 to 1.00 which reduces btu's from 140000 to 122000. They said boiler is too big but there's 15 upstairs radiators to satisfy!! The boiler is new from April this year and was sized by heating engineer!!
If smaller nozzle has little impact an automatic bypass on flow return but forgive my very basis knowledge - what is advantage? Oh, by the way, thank you all so far for replies - grateful!

 

[muggles]

15 rads, rough rule of thumb 1kW per rad is 15kW, plus about 4.5kW of UFH means 19.5kW total. Smaller nozzle will reduce the boiler's power. Sounds like they're recommending that you drop it to its minimum approved output of 36kW, very likely still massively oversized for your home but better than it is. You can't go any lower on that boiler without invalidating the warranty though.

 

[markmorrison1957]

It's bewildering why no short cycling with bigger Warmflow boiler compared to now!
I'll ask engineer to check ufh re flow in case of sludge but he did put something in system when he installed the Vortex.
If engineer and Grant eventually recommend a buffer is 50L OK?

 

[Johntheo5]

I spoke with Grant and advice is to reduce nozzle size from 1.25 to 1.00 which reduces btu's from 140000 to 122000. They said boiler is too big but there's 15 upstairs radiators to satisfy!! The boiler is new from April this year and was sized by heating engineer!!
If smaller nozzle has little impact an automatic bypass on flow return but forgive my very basis knowledge - what is advantage? Oh, by the way, thank you all so far for replies - grateful!

Before replying to the above, what is OFF time before the boilwr refires?

 

[Johntheo5]

It's bewildering why no short cycling with bigger Warmflow boiler compared to now!
I'll ask engineer to check ufh re flow in case of sludge but he did put something in system when he installed the Vortex.
If engineer and Grant eventually recommend a buffer is 50L OK?

IF the manifold dT is only 2C after the system heats up, then IMO something seriously amiss, maybe sludge build up after this chemical treatment, before the Grant & chemicals, you may have breen getting something like a 6C to 8C dT which means the heat output was well over 3 times greater, if you have 12 loops circulating say 2.0LPM/loop with a dT of 7C then the heat demand is, 12*2*60*7/860, 11.72kw but is only 3.35kw at a dT of 2.0C. But even at 3.35kw then the grant's 21L should still supply enoughy heat for 4 minutes or so before refiring.

As a practical test, you could run the boiler to the rads only but shut them all off apart from one or two that combine to give say 4kw output and once up to temperature, time the boiler burner run time and the boiler off time for say 3 or 4 cycles, this will give a good idea of whats happening.

 

[markmorrison1957]

UFH has been on for 3 hours. On time for ufh is 1 min on and 4 mins 14 secs off! Could it be sludge? Manifold shows 50c flow and 44c return.

 

[markmorrison1957]

Additionally the water circulating in the pipes around the boiler is noisy! You can certainly hear it!

 

[Johntheo5]

UFH has been on for 3 hours. On time for ufh is 1 min on and 4 mins 14 secs off! Could it be sludge? Manifold shows 50c flow and 44c return.

Don't think you have anything to worry about really now that you have a manifold dT of 6C (you said dT of 2C after1 hour a few posts back?)
you had a total cycle time of 314secs of which 60secs was burner on time so boiler output was 41*60/314, 7.83kw, this gives a total loop circulation of 18.71LPM or 1.56LPM/Loop, pretty par for the course IMO. The smaller jet sized to give 35.8kw will improve the cycling somewhat but not alot IMO. So boiler/system performing almost exactly as I would expect.

 

[Johntheo5]

Additionally the water circulating in the pipes around the boiler is noisy! You can certainly hear it!

The noisy pump operation is due to the pump settings as the boiler is only circulating 18.71*(50-44)/65-44), 5.35LPM, post the pump make/model/mode/setting and can suggest a more appropriate setting.