Which size for most efficient Boiler?

[Darems]

Hi all,
I'm having an extension put in with hydonic UFH and new radiators throughout
So we need to replace the ancient boiler & leaking vented cylinder

We've opted for Vaillant and our contractor recommended the 637 system boiler along with 170L unvented cylinder - however he has done no calculations.

The Vaillant websitegenerically recommends EcoTEC Plus 624 with uniSTOR 180L for my 4 bed Semi but that doesn't account for the extension or UFH.

so I ran some numbers and worked out the house Heat loss at 19.5KW (including air change + 20% warm up factor) with external temp at -5C
The UFH emits 3.5KW
I'm not sure whether to add this on to the 19.5KW for requirements (total 23KW) as UFH would typically start up before CH at which time it would draw much less to maintain its lower running temp.

Even at the full 23KW demand would this indicate that the EcoTEC Plus 624 (24KW) would be sufficient, and run cost-efficiently too? Which matches the Vaillant generic product finder.

Vaillant help desk were unable to quantify just how much more an oversized boiler would cost to run (e.g. running the 637 37KW boiler for 23KW demand)

Would appreciate any advice

Thanks
Darems

 

[Steelmasons]

Contractor??..if not happy then employ another "contractor".
Heat output can be range rated down to single figures.

 

[DP]

It seems to be an overkill

 

[Darems]

Heat output can be range rated down to single figures.

One person at Vaillant mentioned that to, but when I asked them another person did not advise doing that quite emphatically (not good for the boiler)

 

[Dan Robinson]

Range rating is essential with Vaillant.

Not necessary with superior boilers though.

 

[muggles]

Sounds like you need to find a different installer who actually knows what they're doing

 

[Darems]

What am I missing that a competent installer would bring?
I've got some fairly comprehensive science behind my calculations; and logically the existing 15KW coped fine, so why would we need to double the capacity for a 20% increase in demand?
In reality the boiler would only be close to full steam for the first hour? After that it's just maintaining the temperature, so real demands would be lower...

What would an installer do differently to work out the correct size?

 

[Steelmasons]

Experience..

 

[D_Hailsham]

I ran some numbers and worked out the house Heat loss at 19.5KW (including air change + 20% warm up factor) with external temp at -5C.

19.5kW is a lot for a 4-bed semi. Is it a large old house with very little insulation? What result do you get if you use the Baxi Whole House Boiler Size Calculator? (It's not biased as it uses the BRE method.)

Why do you need to allow 20% extra for warm up? If the house is occupied most of the day warm up is not really necessary; but if you are a working family, with the heating off when the house is unoccupied, 10% is the norm. This would give an 18kW requirement.

The UFH emits 3.5KW. I'm not sure whether to add this on to the 19.5KW for requirements (total 23KW) as UFH would typically start up before CH at which time it would draw much less to maintain its lower running temp.

If the 19.5kW heat loss calculation includes the new extension, there is no need to add in the 3.5kW UFH. But if the 19.5kW is for the original house, you need to add in the 3.5kW.

The installer has added the CH requirement to the cylinder coil performance (19.5kW + 22kW = 41.5kW) and then selected the nearest boiler. This is unnecessary, provided HW is timed to come on when CH demand is low, e.g overnight. HW should also be given priority.

 

[Darems]

Experience

True (and thanks for the simple answer ), but that doesn't quite explain the jump from a calculated requirement of 23KW to a recommended installation of 37KW?

19.5kW is a lot for a 4-bed semi.

Thanks for your more comprehensive reply.
Yes it's a 1930's solid bricked semi. Average size, all double glazed. Also based on external -10C (worst case scenario planning).
20% is applied on the myson online calculator.

Hadn't seen the Baxi calculator so will try that too. Thanks for the link.

My understanding is that UFH is typically on before CH, but there can be some overlap (though at a much much lower draw). But being new to this I just wanted to check what the protocol is to account for the different heating methods.

I've calculated that a 24kW boiler should be able to raise a 170L cylinder by 55C in around ½ hour (27 mins @ theoretical 100% efficiency) and the heat loss is only 1.2MJ/24h, so shouldn't need to run this at the same time as heating the house. (I know in reality it would take longer but whole cylinder would rarely be at 10C)

Thanks for explaining, it's reassured me that I can still do some basic due diligence (and maths)

 

[Razor900]

A 24kW system boiler in a proper set up will be more than enough

 

[D_Hailsham]

Yes it's a 1930's solid bricked semi. Average size, all double glazed. Also based on external -10C (worst case scenario planning). 20% is applied on the myson online calculator.

Earlier you said -5C external (has the UK ever been down to -10?). Myson make radiators; it's in their interest to sell larger rads.

My understanding is that UFH is typically on before CH, but there can be some overlap (though at a much much lower draw). But being new to this I just wanted to check what the protocol is to account for the different heating methods.

Provided the UFH is separately controlled, so it can come on earlier without the rads coming on, it doesn't matter; the heat given off by the UFH is just part of the total heat requirement.

I've calculated that a 24kW boiler should be able to raise a 170L cylinder by 55C in around ½ hour (27 mins @ theoretical 100% efficiency) and the heat loss is only 1.2MJ/24h, so shouldn't need to run this at the same time as heating the house. (I know in reality it would take longer but whole cylinder would rarely be at 10C)

.
The heat up times quoted in cylinder data is based on a 45C rise (15C to 60C). I doubt very much if cold water ever gets down to 5C.

Don't forget that the thermostats on a HW cylinder (not the electric immersion stat) has a differential of about 10C. So it would start heating the water up when the temperature had dropped to 50C - provided HW was 'on' at the programmer. This would mean a heat up time of about 5 minutes.

How did you calculate the 1.2 MJ heat loss?

 

[Steelmasons]

How did you calculate the 1.2 MJ heat loss?

Usually stated in manufacturers literature and stamped on data plate...

 

[Agile]

I presume by "contractor" you mean what we would call a builder!

If you want your heating system properly specified then you need a heating engineer and not a builder.

Its traditional to calculate boiler power for -1 C outside south of Scotland but there at a little more like -3 C.

But the trend is for higher temperatures. You have not bothered to say where you are but in London its years since we saw any proper snow!

Tony

 

[Darems]

Earlier you said -5C external

Well spotted - I've been forecasting many scenarios, but the 19KW was based on -10C (which was my father-in-laws suggestion)

heat given off by the UFH is just part of the total heat requirement.

Yes, but as it's on a different circuit from the CH I was wondering how the boiler knows.

How did you calculate the 1.2 MJ heat loss

I didn't, I trust heatraesadia's Performance and Heat Loss Figures Table

by "contractor"

I mean a Vaillant Gas Safe Registered Engineer (sorry for the "Americanism")

Yes we're in London, so much lower likelihood of reaching -10C


Can I just say, as responses go to a first post, you have been a fantastic community, and I really appreciate the great engagement from you all