System boiler kw sizing and expansion vessel

[meakinsl]

Hi,

After a bit of a saga with my plumber, I need to chose boiler size as soon as possible, the plumber is not going to do any heat loss calculations or do anything to work out accurately so I am on my own. Appreciate this is not what should be doing but after some level of finger in the air guidance. The plumber had said he thinks the baxi 824 (24kw) system boiler is fine for property, annoyingly they don't do a 28 in that range. Could go for a 28kw baxi megaflow but an older boiler with less of a guarantee. There is a Baxi platinum but that only comes in a 32kw model. My preference is for the 824 24kw boiler if can cater for the house (there will be no further additions once renovation is complete).

Here is summary of the house:

1955 detached house with cavity wall insulation, it is 4 bedroom two of which added to new building regs so well insulated. House is not particularly cold in that there is only one room where radiator is on full in winter.

At the end we will have 13 radiators, many of them quite small, have done some calcs and KW of them all seems to add up to on 12kw - seems low as guidance seems to be 1.5kw per rad but a few of them are towel rails and small radiators. Worse case guess would be 16kw.

We will also have 2 zones of under floor heating first zone 1 is 13m squared and second zone 2 is 24m squared using 16mm pipe.

We have 3 showers but 2 electric so only one will be running off tank, did consider combi but would be long way from where water needed and decided on the system approach with unvented tank in centre of house.

We will be going for a 210 litre indirect unvented tank from gledhill - StainlessLite System Ready Indirect | Gledhill - https://www.gledhill.net/products/unvented-cylinders/stainlesslite-system-ready-indirect/, the manual lists the 210 tank having a primary coil rating of 20.5 kw - not sure if reading that correctly as cylinder calculations normally allow 3-5kw.

Another thing that has been mentioned is that the 824 has an integral expansion vessel of 7L - is that enough for 13 radiators of varying sizes (6 are doubles) and the approx 37m squared under floor heating with 16mm pipe ? Seems a lot of people mention fitting a separate expansion vessel but never come across that before with friends systems. The 28 kw and upwards boiler comes with a 10L integral tank.

Any thoughts greatly appreciated.....

Lawrence

 

[ShaunCorbs]

Bin the plumber

It’s his job to advise you not the other way round

Baxi 818 system boiler with 18l of external expansion (extra)

 

[meakinsl]

Bin the plumber

It’s his job to advise you not the other way round

The plumber is connected with the builder and part of the quote, he is half way through the plumbing and the next thing is the boiler, I have been left wandering why it is me doing all the research rather than him but that is where I am. He has said the 824 is more than powerful enough and the 7L expansion vessel fine but guess just after some reassurance especially on the expansion vessel size as not much info out there and doesn't seem to quite add up.

Many thanks,

Lawrence

 

[ShaunCorbs]

1l per rad normally I allow for expansion it’s a tad over but it’s not that much and better to have more than too little

I would be speaking with the builder as your paying the plumber for all this fair enough if he wants to know where you would like the boiler / rads but sizing and spec is down to the heating engineer, is he gas safe ?

 

[SJB060685]

As Shaun says this is the plumbers responsibility and quite frankly I wouldn't trust a word someone says until the calculations are done.
The vessel should be sized for system water volume and the temperatures the system will be running at. Based on the above I highly doubt a 7L vessel will be enough

 

[meakinsl]

1l per rad normally I allow for expansion it’s a tad over but it’s not that much and better to have more than too little

I would be speaking with the builder as your paying the plumber for all this fair enough if he wants to know where you would like the boiler / rads but sizing and spec is down to the heating engineer, is he gas safe ?

He is gas safe and to be fair seems to do a lot of installations but is allowing 1L per radiator would that mean normally all boiler installations require a separate expansion tank as even on the 28 and 32 kw models in the different range they only have 10L integral tank and just not come across it before in previous houses/friends houses, with this I also have the underfloor heating to contend with.

 

[ShaunCorbs]

He is gas safe and to be fair seems to do a lot of installations but is allowing 1L per radiator would that mean normally all boiler installations require a separate expansion tank as even on the 28 and 32 kw models in the different range they only have 10L integral tank and just not come across it before in previous houses/friends houses, with this I also have the underfloor heating to contend with.

Yes most are on the limit / under and that’s the most common fault expansion vessel overtaxed and the prv goes

 

[meakinsl]

Yes most are on the limit / under and that’s the most common fault expansion vessel overtaxed and the prv goes

Many thanks for quick reply, I was wandering if that was the case, guessing that the megaflow 28 even though an older model boiler with the 10L tank might be a better bet or convince the plumber to fit a a separate expansion vessel to the 824.

Lawrence

 

[ShaunCorbs]

It’s past that even on the mega flow I would say min you need 15 l of expansion and that’s on the minimum

 

[SJB060685]

If we had an idea of system volume I could calculate for you a suitable size. As Shaun says though oversizing a vessel will never cause a problem but an undersized vessel/vessels will.

 

[meakinsl]

If we had an idea of system volume I could calculate for you a suitable size. As Shaun says though oversizing a vessel will never cause a problem but an undersized vessel/vessels will.

Many thanks, for my understanding is the additional tank in addition to the integral tank or does the additional one bypass it ? From everything read and from replies no idea why plumber would think that 7L is enough,

Lawrence

 

[SJB060685]

You would need an additional tank as well as that within the boiler.

 

[John.g]

A 7Ltr EV with precharge/filling pressures of 1.0/1.5 bar with mean rad temps of 70C will accomodate a system volume of 75 litres with a final pressure of 2.53 bar.
A 7Ltr EV with precharge/filling pressures of 0.8/1.3 bar with mean rad temps of 70C will accomodate a system volume of 85 litres with a final pressure of 2.48 bar.

 

[meakinsl]

You would need an additional tank as well as that within the boiler.

Cool thanks, so looking like need about 15 overall so would need an additional 8L external vessel to cover things,

Many thanks,

Lawrence

 

[John.g]

If room allows why not install a 15/18 litre, if the internal one then fails, the boiler PRV will not lift and you can replace it (if ever) at your leisure.

 

[meakinsl]

If room allows why not install a 15/18 litre, if the internal one then fails, the boiler PRV will not lift and you can replace it (if ever) at your leisure.

Thanks for advice, boiler is in garage so not constrained on space so can go bigger which sounds like no downside,

Many thanks,

Lawrence

 

[SJB060685]

Cool thanks, so looking like need about 15 overall so would need an additional 8L external vessel to cover things,

Many thanks,

Lawrence

Again, without an idea of system volume we're just guessing but an educated guess suggests a 15/20 litre will be more than enough. As we've discussed too large a vessel is not going to cause system function problems and will also hold a certain amount of reserve water should the system need it, the only downsides to oversizing a vessel are additional cost being larger and space but you said above space is not an issue and there's not going to be much of a difference in cost.