WB Greenstar 30i to run rads and UFH....??

[Rob99]

My builders plumber is recommending a WB Greenstar 30i as a new boiler to replace my (very old but very reliable) Potterton Prima 50F.

 

In terms of output the WB is about twice that of the Potterton (30kw compared to 15) so presumably should easily cope with the existing upstairs rads (4 beds, 2 baths & landing) and the new UFH we're installing downstairs (around 110m2, approx 7kw total output).

 

I notice its a system boiler with built in pump so presumably my existing circulation pump will be redundant.

 

Any advice on this boiler or things to look out for?

 

Thanks

 

[PeterW]

With a mix of UFH and rads are you having a buffer tank ..?? What’s the DHW via ..? 

[Rob99]

No, am not intending to have a buffer tank, DHW is via the existing cylinder. Essentially its just a conventional system where we're replacing the downstairs rads with UFH. The boiler has to be changed as it needs to be relocated during the alteration works.

 

[PeterW]

Beware that the boiler will short cycle then and won’t modulate down far enough to go direct to the UFH. Is this being done as a number of zones ..? 

[Nickfromwales]

IIRC the 30kW WB modulates down to around 7kW so if some of the rads were on at the same time youd prob be ok, but is the 7kW figure peak or average? eg could it be less than 7 because if it routinely goes under that then your into short cycling. The manufacturers literature will often tell you to design that out.

[AliMcLeod]

13 hours ago, Nickfromwales said:

IRC the 30kW WB modulates down to around 7kW so if some of the rads were on at the same time youd prob be ok, but is the 7kW figure peak or average? eg could it be less than 7 because if it routinely goes under that then your into short cycling. The manufacturers literature will often tell you to design that out.

 

Is it preferable to use more fuel than rather short-cycling? I've a similar setup to what @Rob99 is proposing here - UFH downstairs (3 reception rooms, 2 bedrooms, 2 bathrooms), rads upstairs (2 bedrooms, 2 bathrooms) and no thermal store, but we have a Greenstar 42CDi Classic (modulates down to ~10KW). We only really use our upstairs when we have visitors, so i have the rads (and upstairs towel rails) set low, and my boiler does short cycle when heating up the UFH.

 

I'm now wondering whether i should turns the upstairs rads up so as to consume more of the heat and reduce the short cycling. From looking at our gas bills over the past year, we use between around 20-100KW per day (summer/winter) for Gas. This includes DHW.

 

@Rob99, based on my reading here, if I were to move my boiler now, i'd try to make room for a thermal store if at all possible.

Edited October 15, 2018 by AliMcLeod

[CC45]

We've got that system boiler - UFH downstairs, rads upstairs + UVC for DHW.  We've got a heating buffer tank to eliminate short cycling as the boiler does not modulate down low enough for our heating demand - so had to do something.

 

Why hasn't the plumber actually worked out what you need?

[Nickfromwales]

1 hour ago, AliMcLeod said:

 

Is it preferable to use more fuel than rather short-cycling? I've a similar setup to what @Rob99 is proposing here - UFH downstairs (3 reception rooms, 2 bedrooms, 2 bathrooms), rads upstairs (2 bedrooms, 2 bathrooms) and no thermal store, but we have a Greenstar 42CDi Classic (modulates down to ~10KW). We only really use our upstairs when we have visitors, so i have the rads (and upstairs towel rails) set low, and my boiler does short cycle when heating up the UFH.

 

I'm now wondering whether i should turns the upstairs rads up so as to consume more of the heat and reduce the short cycling. From looking at our gas bills over the past year, we use between around 20-100KW per day (summer/winter) for Gas. This includes DHW.

 

@Rob99, based on my reading here, if I were to move my boiler now, i'd try to make room for a thermal store if at all possible.

You can only use as much heat as the dwelling will take at any one given time.

 

If you turn the upstairs TRVs up to setting 1 or 2 you’ll still probably see them either staying shut or only opening fractionally to acclimatise with the ambient temperature upstairs, generated by the heat rising from downstairs, and then they will close automatically anyway. At that point you’ll have used more heat and be back to square one, albeit with a slightly warmer house. 

The issue is when the system is idling, after raising the house temp to the set point on the stat, it needs to shut off, but with multiple zones you have to leave the boiler idling to give heat even if just a single loop in a single room is still calling for heat. 

Thats the part that you or your plumber should be designing out, it’s just most plumbers are preprogrammed to give you heat in abundance and they don’t often cater for low heating loads. Hence the 30kW boiler when it’s likely you could get away a 24kW with ease.

[AliMcLeod]

18 minutes ago, Nickfromwales said:

The issue is when the system is idling, after raising the house temp to the set point on the stat, it needs to shut off, but with multiple zones you have to leave the boiler idling to give heat even if just a single loop in a single room is still calling for heat. 

 

My setup is rather "interesting" in that their is no ability for my upstairs rads to call for heat - there's only stats in the 4 downstairs UFH zones, and if none of those are calling for heat, the upstairs will never get any hot flow, nomatter what the TRVs are set at. I was considering adding an upstairs stat into the mix but since we hardly use that part of the house, i'm now thinking it best to leave as is.

 

BTW, I'm not the original poster (just a thread-highjacker). I agree @Rob99 should be challenging his plumber to come up with a different solution.

Edited October 15, 2018 by AliMcLeod

[Rob99]

So, after further discussion with the plumber, and discovering the existing HW cylinder is weeping on the bottom!, we are now having a heatstore cylinder fitted too which should alleviate any issues with the UFH/Rads/DHW mix.

 

Cheers