Unvented Cylinder vs. Combi

We had a gravity feed loft tank and 2 shower pumps and went to a vented hot water cylinder with 160L when we installed a hot water solar panel system. We have mains pressure and just a thermostatic mixer for both bathroom and ensuite shower. This was done a few years ago and we "only" have a copper hot water cylinder which "only" takes 3 bar - steel cylinders go much higher without pressure reducing. We kept our heat only boiler. This is in a 6 bed house with 2 bathrooms and 16 radiators.
 
bernardgreen said:
Opening a cold tap will reduce the cold water pressure at the cold water inlet to the boiler. If this reduction in pressure causes the flow rate to drop below the minimum level for the boiler ( it cannot modulate low enough to match the low flow rate ) then the boiler has to shut dow
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As said.not major!...lose h/w for less than a minute.
 
whall3y said:
What utter rubbish. Yes if you don't have a thermostatic mixer the shower will get hotter, but it doesn't stop the toilet being flushed. And you can run more than one tap OK.
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If someone is in the shower and runs another tap or flushes the toilet the water pressure drops and you either get burnt or sudden shock of cold. Same happens if running a hot tap.

It might not happen or at least to the same extent with a thermostatic shower but the flow of water drops significantly and in a busy household it will be very annoying.

I've had a combi boiler in the past and my parents, my brother's, friends, and they all been the same.

In the op's case a combi boiler is not the correct solution.
 
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@Ash D unfortunately you have opened a real can of worms with this subject.

Each type of advanced hot and cold water system has its own merits, interpretations and implementations and you will always get a whole load of mixed advice and advocates for each type when you visit a public forum, some good and some not so good and some downright mental.

The key for you is what is achievable for your circumstances, taking into consideration what is currently available (mains supply, current system, etc), what the site layout is like, ease of installation, costs and in my view the most important - your personal requirements.

That I'm afraid really needs someone who is non partisan, who has broad certified experience with advanced hot and cold water systems and can be onsite and perform a detailed survey. They can then advise on the merits and downfalls of each system applicable to your situation (invariably that can be more than one type or a mixture of types).

I appreciate it maybe more difficult to get a professional at the moment but this is a complex piece of work that you want to get right from the start and not get taken in a million different confusing directions. I really would wait until someone recommended can visit, if at all possible.
 
I know this is an old topic, but incase (like me) anyone is struggling with the same issue, and to give something back (and say thanks again) this is what I have done:

Since installing the new bathrooms, with new thermostatic mixer showers, the water flow from the shower heads (from the old gravity fed system), was at best, just a trickle.

Asking the neighbours, it seems everyone installed a different solution, including pumps and Combis with mixed results.

The answer, following various surveys, for us, was to go with a Unvented Cylinder, based on our cold water flow to the house being 14ltrs / min and 2.95bar.

We did not upgrade the cold mains supply from 15mm to 22mm, and agreed with the installer that we might consider this later if we still had issues.

The cylinder installed is a Telford Tempest 250L. The shower mixers are all Bristan, with a minimum pressure requirement of 0.2bar (which we thought might work, as the old gravity tank was 2m above the shower head), and have a maximum flow of 33 ltr /min.

I am aware that I’ll need to get it serviced each year (£80) and the 15mm cold supply is less than Telford recommend, but I’ve heard the noise of pumps and we were not sure that we could live with that.

I was conflicted about the Combi, it would have been a cheaper option, but was worried about the compact nature and limits in a larger house of water and heating supply.

Anyway, incase anyone is struggling with the decision, the pressure from the shower is really good. I’ve not been able to measure it, but it’s same pressure for cold and hot now, and although the flow seems acceptable.

Although not a consideration, the hot water flow from all the taps has also improved massively to around 13ltrs / min.

Even when a toilet is flushed or tap opened, we’ve not noticed a drop in pressure of flow, but haven’t used it in anger.

I hope that helps someone else as so many on here helped me!
 
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