Flow rate vs boiler size

[sparer]

Dan,
I'm thinking that the 40kw is too much for the heating and so was trying to understand if the engineer was sizing the boiler for the showers.

http://www.idhee.org.uk/ says I need 23kw. My current boiler is 30kw and struggles to keep up for a shower in the colder months when running at full flow. Using the other shower with adjustable flow gives a better result albeit less flow.

I've read on here that the boiler can start cycling if it is too large for the heating output leading to inefficiencies.

I don't really need to know any of this and could trust the heating engineer but in interests me hence the questions.

Thanks

 

[Dan Robinson]

This is the problem with combi's.


It is less efficient to have the boiler cycling, but as with many things in life... there has to be a trade off somewhere.

As Steely mentioned, there are other boilers out there that are better at coping, but the problem will always be there to some extent.

Your house only needs 23kW for a few weeks a year at most, so even if you went small on the heating you will always be cycling anyway.

Maybe look at the IG 36/30? Not hugely better, but it is a significantly cheaper version of the same boiler (same heat exchanger as I mentioned before).


I really wouldn't bother with the 40kW. You are only gaining a couple of litres a minute.

 

[petit_pablo]

If you want to run 2 showers at the same time with a half decent flow rate, stick an unvented cylinder in rather than a mahoosive combi

 

[Steelmasons]

Dan,
I'm thinking that the 40kw is too much for the heating and so was trying to understand if the engineer was sizing the boiler for the showers.

, can't see the problem here , If you fit the larger combi for HW use then range rate the boiler for heating , a 37kw combi can be range rated for heating (6kw to 28kw) , an I missing something? , appart from the gas upgrade then what?

 

[Dan Robinson]

Indeed.

A house that genuinely needs 25kW for heating wouldn't be my first candidate for a Combi.

 

[Steelmasons]

Would I have a combi? , would I fook.

 

[Dan Robinson]

I will have to double check the boiler specs on Monday, but I am pretty sure the IG40 is only 40 on heating. I think it is still only 36 on hot water.


I wouldn't have a combi as a first choice either.

 

[sparer]

i'm starting to see a flaw in my thinking here. If a combi is not the answer for our needs than what are the alternatives?

 

[Dan Robinson]

System boiler and unvented cylinder. Bit of extra plumbing, but the flow rate will be whatever the incoming supply can cope with.

Also gives you an immersion heater backup.

Will cost a bit more tin install, but you'll be happier with the performance. Bear in mind you might need to upgrade your water main.

 

[mickyg]

I would not fit an unvented cylinder with 18l/min incoming. Bare min 20l/min and 2 bar dynamic. You could look at options of improving the main, but now your talking about stuff you really need a site survey for options..
honestly though how often will you really need to use 2 showers at precisely the same time. I had a customer fixated on this idea and got 3 or 4 quotes for a thermal store as he was determined this was the right idea. In the end I talked him into a atag a325ecx at a fraction of the cost of his proposed idea and he is happy as larry. going for 2nd yr service next week.

 

[sparer]

My current boiler is an Ariston Genus 27. It has in built hot water storage so isnt that a good halfway house? Shower does tend to run cooler when the storage is used up though.

 

[Dan Robinson]

That thing holds 60 odd litres and should be able to keep up with 2 showers unless it has a problem or you have crazy showers.


Glowworm do a decent alternative as to Viessmann - both floor standing storage combis.


They are all in effect though unvented cylinders with boilers bolted on top.

Would probably go for the glow worm over the Viessmann.

You need to know what your water main is capable of really before getting much further in the decision making.

The Atag micky mentioned is also an excellent boiler.