System boiler requirement for separate hot water return and low loss header

craig1234 · 12 Mar 2020

I am renovating a house (3 bedroom Victorian semi) and am looking to have my existing boiler replaced. The house has a shared water supply and will have two bathrooms so I think I will be better going for a system rather than a combi boiler (the water pressure isn't great either). I will be converting the loft so am thinking of creating a separate heating zone for this (hot water cylinder would also go in the loft).
I will run the pipework myself and have noticed that some system boilers have a separate return for the hot water whereas others share a return for both heating and hot water. My question is, to be safe, would I be wise to run in a separate hot water return (22mm?) even though my final choice of boiler might not need it or can all boilers actually share a single return?
Also, for a relatively small system (11 radiators), would it be good practise to install a low loss header?

muggles · 13 Mar 2020

If your cylinder is going in the loft, where are you putting the tank that supplies it?

A cylinder's return pipe should always be the last connection before the boiler, so yes run it all the way back

No, you don't need a LLH - they're for big systems

craig1234 · 13 Mar 2020

Thanks Muggles. I should have said that it would be an unvented cyclinder so no need for a water supply tank. I'm in two minds as to exactly where to locate the cylinder as I am going to build a utility room downstairs (this will be quite close to the boiler which will be at high level in an adjacent toilet) so it may be better off there I think. Excuse my niavity but would there be any noticeable loss in water pressure (I know the cylinder will be pressurised) if I locate the cylinder at ground level (incoming mains water pressure is not great as I previously mentioned)? As I write this I am actually thinking of having the new boiler installed within the utility room as well!

Gasguru · 13 Mar 2020

It makes the job a whole lot easier if the cylinder and boiler are together and installed in a heated space...once you're dealing with lofts and garages it's a lot more complicated than you think to protect them against extreme weather (and the majority or properties have totally inadequate frost protection).

craig1234 · 13 Mar 2020

Thanks Gasguru - I will be converting the loft so frost protection wouldn't be a problem but the more I think about it the more I think it would be best to have both the cylinder and boiler installed downstairs in the utility room that I will build. I still have the concern regarding water pressure (especially to the loft conversion) but even with a lowish mains water pressure (seems low to me although I haven't had it checked), with a non-vented cylinder/system boiler is this something I don't need to worry about?

muggles · 13 Mar 2020

Your desire to have an unvented cylinder is at odds with your statement that your mains performance is poor. They aren't magic - you can only get out what you put in, so get your mains tested before you commit. If you're getting 17 litres per minute or less at 1.0 bar, you may as well have a combi because it's going to give you exactly the same hot water performance without taking up all that extra space.

A chap called Bernard will no doubt be along shortly to tell you I'm wrong, and that combi boilers are the work of the devil, but he can safely be ignored - he's not a heating engineer

craig1234 · 13 Mar 2020

Thanks Muggles - with hindsight I should have got the pressure tested before but anyway I have made an arrangement with South East Water this afternoon. The reason for a system rather than combi boiler was the fact that the property will have two bathrooms and I read that a system boiler may therefore be best in these circumstances - is that not always the case? Back to my pressure issue again (hopefully will disappear once the pressure is tested) but I also understood that I could add a shower pump to a "system boiler system" if I needed to boost pressure.

bernardgreen · 13 Mar 2020

""A chap called Bernard will no doubt be along shortly to tell you I'm wrong, and that combi boilers are the work of the devil, but he can safely be ignored - he's not a heating engineer""

You mis quote me, ""combi boilers are the work of the devil,""

I have not said that, they are good when used in the situation they were created for,

The devils connected with combi boilers are the installers who install combi boilers when they are in-appropriate for their customer's property and life style. Those installers are not heating engineers.

muggles · 13 Mar 2020

bernardgreen said:
""A chap called Bernard will no doubt be along shortly to tell you I'm wrong, and that combi boilers are the work of the devil, but he can safely be ignored - he's not a heating engineer""

You mis quote me, ""combi boilers are the work of the devil,""

I have not said that, they are good when used in the situation they were created for,

The devils connected with combi boilers are the installers who install combi boilers when they are in-appropriate for their customer's property and life style. Those installers are not heating engineers.

And like whack-a-mole, up he pops! I think I'll just include a screenshot at this point...

muggles · 13 Mar 2020

craig1234 said:
Thanks Muggles - with hindsight I should have got the pressure tested before but anyway I have made an arrangement with South East Water this afternoon. The reason for a system rather than combi boiler was the fact that the property will have two bathrooms and I read that a system boiler may therefore be best in these circumstances - is that not always the case? Back to my pressure issue again (hopefully will disappear once the pressure is tested) but I also understood that I could add a shower pump to a "system boiler system" if I needed to boost pressure.

You can't pump directly off the mains at more than 12l/min. If you want to boost more than that then you need some form of stored cold water - either an accumulator, or a cold tank in the loft. Wanting to use two showers at the same time is all well and good, but once again a system boiler and cylinder setup is not magic - it won't deliver any better performance than a combi if your incoming mains isn't up to the job

craig1234 · 13 Mar 2020

Ok thanks Muggles - South East Water will visit on Tuesday so I will see where I stand with the water pressure then.

ianmcd · 13 Mar 2020

craig1234 said:
Ok thanks Muggles - South East Water will visit on Tuesday so I will see where I stand with the water pressure then.

water dynamic flow rate is a lot more important than pressure

hard-work · 14 Mar 2020

As your water flow is poor on a shared mains, the best solution is a cold water accumulator with a high flow combi. Combis are a world away from what they were 15 years ago. Then you will have decent water flows, with only cold water storage.

oldbutnotdead · 15 Mar 2020

Look at sorting your cold water supply first- since you're on a shared supply then flow won't be great to start with and you'll get fluctuations when neighbours run the bath.

Once you've done that, see what pressure/flow are like and then make your system decisions.

Madrab · 15 Mar 2020

My advice would be don't make any decisions regarding what you can/should put in without knowing what you currently have regarding flow and pressure and what could be achievable by renewing the main. The water company will tell you what you have at the mains but it's really important to know dynamically (more than 1 outlet open at the same time) what may be achievable as @ianmcd mentions.

Anyone that suggests otherwise is just guessing.

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System boiler requirement for separate hot water return and low loss header

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