Water pressure and boiler type

sjb92

Hi, I'd appreciate some advice from the experts in this forum. 

Our house is a 1930s detached house. We are close to the top of a hill - about 15m below the height of the local supply reservoir. Our builder has found that we still have the original, narrow, pipe running from the water main (via a water meter). The house is well insulated and we do not have anything like underfloor heating - just (modern) panel radiators in each room.

When we moved in we replaced the c 25 year old boiler with a Greenstar 30SI combi boiler, situated in the kitchen with the bathroom above. We have always had an issue with flow rate - e.g. the shower pretty much stops (rather than just the cold water stopping) if someone flushes the loo.

We currently have one bathroom but are in the process of having two en suite shower rooms installed, planning for the time where we'll have four adults in the house, and also for those times in the year when we have extended family staying. So we'd hope to be able to comfortably cope with two showers running at once (and a tap running, or the washing machine, etc). 

We assume this means replacing the combi boiler. And we know it makes sense to do it when the new pipework is put in for the new bathrooms even though the boiler is still OK (although it's had one small breakdown and is out of warranty. But I have a few questions which I would appreciate help with.

• we'd prefer a system boiler to avoid having a cold water tank, etc. But what's the minimum water pressure/inlet flow rate for a system boiler? (Our neighbours had their supply pipe upgraded a couple of years ago so we plan to check with them to see what is achievable - it wouldn't be too difficult to upgrade ours.)
• does the cyclinder need to be adjacent to the boiler? We no longer have the airing cupboard that the old tank was in. We're thinking of putting it about 11m from the boiler (the other side of the house from the boiler) - but someone has suggested to avoid heat loss between the boiler and the cylinder they should be next to each other. As the boiler is the only gas-fired thing we have, moving the boiler to alongside the ideal tank position would mean moving the gas supply.
• one of our other neighbours mentioned they had two hot water tanks, but had no idea if it was a system or vented set up. Does having two tanks point one way or another? 

Thanks in advance for your help.

nofoollikeold

Before you do anything you need to make sure you can get the flow rate to achieve two showers at once, if you are going either the combi boiler route or the unvented hot water route.  Look for a minimum static pressure of 2.0 bar and a minimum flow rate of 20 litres / minute.  You should also have a dynamic (a mains cold tap running) pressure of 1.5 bar.  If you can't achieve those figures with what you have, or what you are certain WILL be achieved with a new mains pipe, you need to look at other options, such as an accumulator or stored hot water. 
If you go for a system boiler, you will need a cold water storage cistern and a hot water cylinder.  Are you perhaps confusing a system boiler with an unvented hot water system?
The boiler and hot water cylinder do not need to be next to each other.  

southcoastrgi

nofoollikeold said:

If you go for a system boiler, you will need a cold water storage cistern 

🤔.................

nofoollikeold

Whoops, quite right southcoastrgi - a system boiler only needs a cold water storage cistern if used with a vented hot water cylinder.  With an unvented, it doesn't.

sjb92

nofoollikeold said:

Whoops, quite right southcoastrgi - a system boiler only needs a cold water storage cistern if used with a vented hot water cylinder.  With an unvented, it doesn't.

Hi both - thanks for your replies. Yes, I think I was assuming a system boiler with an unvented cylinder.

More broadly - that's really useful information, thank you. No one had mentioned an accumulator to us but having quickly looked at what they are, it might be ideal for us as it would save having to move the boiler (and thus the gas pipes) - or indeed having to get a new boiler just yet. And avoids having a hot water cylinder for a peak hot water demand that isn't going to often be there. Are there any particular advantages/disadvantages about them that you would think I should know about? We are in a hard water area if that's a factor.

sjb92

Thanks again for your advice. As an update - I am alking to the plumber this week. So far all contact has been via our builder because of social distancing so it's been a bit slow. But the plumber isn't keen on the accumulator. He is suggesting we keep the combi boiler for the heating and kitchen tap (and I guess for the downstairs shower that is used about once per year) and that he installs an unvented cylinder to handle the bathrooms with a motorised valve. (I assume the reference to the motor means it wouldn't be indirectly heated?) I can see the attraction of this - not least as we have solar PV so we could heat the water during the day with our own generated electricity - but still worry about pressure/flow rate being insufficient - unless we have a huge tank in which case we'd be keeping a load of water warm all the time which feels wasteful. 

I'd really appreciate a second opinion. Thanks in advance.