Combi with stored water tank? To solve water flow issues.

[David]

Background - since we replaced our hot water tank in the airing cupboard with a combi boiler (also in the airing cupboard) about 10 years ago we have always had poor water flow. This doesn't seem to be the boiler as the cold water flow is also poor. So far it hasn't been enough to really inconvenience us because we shower, but trying to fill the bath is a long term task.

We are now contemplating replacing the bath with a shower and would like more flow rate.

Investigations are pending to try and find out where the problem lies. Allegedly neighbours don't have a similar problem although someone across town has said that water pressure and flow has dropped over the last couple of years, so perhaps this is Anglian Water managing leaks and finding the original problem might not fix the issue.

So is it feasible to fit a new hot water cylinder in the loft with associated cold water tank to allow a shower pump to be fitted? Using the combi as a system boiler.

At the moment I'm musing on if the cold water storage tank needs to be higher than the top of the hot water storage tank or if perhaps a pump could be used to push water into the hot water tank, as well as pushing it out to the shower.

Cheers

Dave R

[David]

<snip>

<snip>

Just found

which offers a horizontal hot water cylinder. Assuming it is indirect and the heat exchanger uses pumped hot water this could be an interesting option. Placing of the thermostat would also be interesting.

Cheers

Dave R

[NY]

But I'd always go for a mains-fed cylinder rather than a tank-fed one, as long as mains pressure isn't feeble. Tank-fed never seems to give a usable flow rate because there's only about 3 feet head of water between the tank in the loft and the shower head close to the ceiling on the floor below.

I prefer combi because you get unlimited hot water, rather than the dreaded problem of the water running cold if a second person wants a bath, so you have to turn on the immersion heater and run the boiler (both in parallel to heat the cylinder as fast as possible).

Even a mixer tap, on the fully-cold setting, should give a flow rate similar to what you'd get with a plain tap.

You say "since we replaced our hot water tank in the airing cupboard with a combi boiler (also in the airing cupboard) about 10 years ago we have always had poor water flow" which seems to imply that at one time you had good water flow. I'd get that looked at. If the fitting of the combi changed *cold* water flow to a cold tap (or mixer on cold setting) then something was installed wrongly.

[alan_m]

Isolation/service values not full bore? Flexi pipes to taps not full bore or too much of a restricted bore?

[The Natural Philosopher]

I believe this may well be the case. I feel too that water pressure is not what it was in the Anglian region.

Whoa there ...you are coming up with solutions without first having diagnosed the problem.

I can assure you that whatever the water *pressure* is it will be far far more than a cold water tank in the loft can provide, or that tank would never be filled!

What you have is a flow rate problem, and you have jumped to the conclusions it is in fact a pressure problem.

It is far more likely that your incoming mains - if the problem applies to the kitchen cold water tap - is restricted

It may well be that the installers of the pathetic obscenity known as a 'combi boiler' have deliberately turned down water flow at the mains to get a decent temperature rise. Having sold you or a previous occupant an inadequate system.

Now the first thing to do is measure the pressure. You should have a couple of bar at least coming off the mains. You can hire pressure gauges to do this. It the pressure is indeed below that than you have cause for complaint to Anglian water etc.

Then you need to measure the peak flow rate through probably the kitchen cold tap. This is generally the nearest to the incoming mains. Basically time how long it takes to fill a 5 litre watering can or plastic container.

Anglian water have deginitely been managing pressure to reduce wastage, they are proud of it.

You should get about 5 litres per minute out of your tap. If you are not then you need to check that there has not been a flow restrictor fitted somewhere at some time. Or the mains stopcock is not fully open.

Once you have an adequate flow rate, the last thing you want to do is fit a atmospheric pressure loft tank and an electric pump.

By all means ditch the combis instant hot water feature and use it as a system boiler to heat a tank for better hot water flow, but make that tank a mains pressure one. And you can fit that anywhere convenient. It runs off mains pressure, not gravity.

[Robin]

While generally true we lived with a good deal less than that for years until Thames replaced the leaky Victorian pipes. We only have a /right/ to seven metres static head.

[The Natural Philosopher]

A water softener is a well known source of slow flow rates. Especually if it is fitted with 15mm pipes or is clogged., Try bypassing it temporarily

All these issues can be solved with a floor level mains pressure tank, but you have to fix the flow rate issue first. Your 3.5 bar mains sounds about right, but what flow are you getting from the UNSOFTENED tap.

[The Natural Philosopher]

Well if you skimped on hot water tank side of course that will happen, but a comnbi IME means you cant run tow baths anyway.

The water softener is a definite possibility. Ex's in laws despite my advice left theirs in soften mode when the water people injected a shit load of silt into the mains. It stopped the flow completely. I switched it to by bypass and told them to get it seen to, They never did Fucking ArtStudents to a man.,

[John Rumm]

It is about more than static pressure - to get a good experience from an unvented (i.e. mains fed) cylinder, you need a decent flow rate as well

- at a minimum 20 lpm. The cold main will need to be able to meet the total demand for hot and cold water at any given time. So if the OPs problem is lack of flow rate, an unvented cylinder will offer poorer performance than a gravity fed one.

That depends on the layout, the height difference, and the way the pipework is done. With 22mm pipes and brassware designed for gravity fed operation, things like bath filling performance can be very good with a traditional system.

However showers are a special case were you are deliberately introducing a constriction at the shower head. A large rain drop style head can be good with gravity fed water. However if you expect a powerful jet, then you will need a pump.

That is really a failure of design (not installing a suitably sized cylinder) and also a lack of decent controls.

With a well specced unvented cylinder with a fast recovery time, and controls that bring the boiler into action quickly enough then you can get "unlimited" hot water from a cylinder since the boiler can reheat water as it is being consumed.

The flaw in this argument however is that to allow this to work you would need to reduce the demanded flow rate from the cylinder to combi level performance - obviously something you may not want to do, having grown used to the much better performance of the unvented cylinder.

Well yes, the trick is working out what :-)

[David]

Please read my full reply to JR (everyone) where I think I covered most (if not all) the points being raised.

Specifically, if I have enough flow for a mains pressure hot water storage tank, why do I need to swap out the combi?

"cold bath tap" is included in the category "all inside taps are mixer taps".

The outside cold isn't close enough to the rising main to be a reliable indicator, as it is inboard of the pressure limiting valve (which is my first suspect).

The boiler is a Worcester Bosch 38 CDi which is rated at 19.1 l/min for a

30C rise. Noting that in our previous property we had a much smaller combi boiler yet managed a much larger flow of hot water.

Noting that static pressure and volume of flow are related but not interchangeable.

The most likely issue is a constriction somewhere in the pipework.

Please note that this thread started out as a "what if" question exploring the options if there wasn't enough mains flow after best efforts to find a reason inside the house.

As already stated, with a cold water storage tank there was no evidence to suggest a limited cold water flow. It never occurred to us that there might be having never seen this before in previous properties.

Cheers

Dave R

[David]

Thanks John. The point about the mains pressure cylinder and flow rate is one I have been trying to make.

Also noting I have mentioned an unvented cylinder combined with a shower pump as giving very good results for two simultaneous showers in a previous property.

I currently strongly suspect the pressure limiting valve installed to protect the water softener but lack the energy to fully investigate, and also my list of more urgent things is quite long.

At the moment we are breaking out tasks that someone else can be paid to do so we can make more general progress.

Cheers

Dave R

[The Natural Philosopher]

You dont.A combi will drive a hot water tank as easily as it will drive a central heating system

What pressure limiting valve., You didn't mention the water softener, and now there is a 'pressure limiting valve' That is very likely the problem. When my EX was in the house alone the water softener failed and scale destroyed the pressure limiting valve to the mains pressure hot water tank,. so when I finally got the house back, I had to replace that (and the water softener) Symptoms were extremely poor hot water flow (it was only in the hot water circuit)

So the problem is upstream of the buffer tank. The two things you failed to mention in the original post - the water softener and the pressure regulator - are both very likely candidates.

And, As I said, the solution depends on what the problem actually is.

Otherwise you are just hacking your way in an expensive 'parts cannon' style approach.

How many properties with water softeners and pressure regulators have you had?

[David]

The unsoftened tap goes through a water filter so isn't a reliable guide.

I'm pretty sure that I switched the system to bypass the softener but that didn't make any difference.

That is something that I will re-check.

As noted elsewhere, this is a "what if we don't have a good flow rate" thread looking past the initial investigation and speculating on options should we need to move to stored hot water and pumped feed.

Oh, it is a Monarch water softener.

Cheers

Dave R

[The Natural Philosopher]

Then get a plumber to replace it and/or the ware softener. BY LAW you need one tap that isn't softened - generally the kitchen cold water tap. You should check flow rate there.

[David]

Can I repeat for emphasis that this isn't a thread asking for diagnosis. Otherwise I would have included much more detail.

This thread was specifically asking about options should there turn out to be an unfixable slow flow rate.

"Investigations are pending to try and find out where the problem lies." is from the OP.

Noting also that if the problem is a slow flow rate then the only solution seems to be to have stored cold water and some kind of pump to get the flow rate required.

I believe that this would require stored hot water as well, as I have memories of a thread exploring using pumped cold water through a combi which was considered sub-optimal.

I am therefore exploring option around having both a vented hot water cylinder and a cold water tank in the loft, and the possibility of avoiding having to build a tall and very strong platform to get the cold water tank above the large hot water tank.

Cheers

Dave R

[The Natural Philosopher]

The point is that without a diagnosis, your solutions may not work.

Whole house water pump. Contact Anglian water. A flow rate unable to support a single shower is below statutory limits and they MUST fix it. Ergo, unless all your neighbours have the same issues, you have a house specific issue. Which is by definition something you can (get) fix(ed) . So fix it. It will be cheaper than a new tank.

ANd better

[David]

I have one tap that isn't softened. Please check what I wrote.

[David]

On Sun, 05 Mar 2023 15:02:27 +0000, The Natural Philosopher wrote: <snip>

Which bit of separate investigation did you fail to understand?

[NY]

I was meaning two baths *in succession*. The cylinder in my last house was probably about 4 feet tall and about 2 feet 6 inches diameter (including built-in lagging) so about 500 litres. I was surprised that I sometimes ran it "cold" when it was that large. But how much does incoming cold water mix with outgoing hot water: it's not as if there is a perfect "barrier" between the two as the tank is emptying of hot and filling with cold.

[NY]

Our present house had a tank in the loft, a fairly small cylinder on the first floor and a bath on the ground floor. So a head of about 15 feet (middle of one floor, height of another floor, most of next floor). The hot water out of any hot tap flowed at a pathetic rate. Hardly surprising: the pillocks who'd installed it (for the previous owners of the house) had used

15 mm pipe for the hot instead of 22 mm as you are recommended with gravity (rather than mains) fed hot water.

When that cylinder developed a leak, and the heating engineer said that the heat exchanger in the boiler was close to perforating, we had a combi boiler fitted instead. Hot water flows a lot faster. It takes a while for the water to run warm because it is routed via the site of the old cylinder (*) which is a longer run than it needs to be, but once it's warm it just runs and runs. We've tended to have showers since the price of gas has shot through the roof, but if gas prices come down again, we may sometimes splash out (pun intended!) on baths every now and then.

Our cold water flow rate was around 17 litres/minute, if I remember correctly. I *think* the hot water rate through the boiler is not much less, so the boiler isn't restricting the flow rate much.

(*) To reuse as much of the original piping as possible, to avoid having to chase any new pipes into the plaster of the walls.