Mains-pressure cylinder hot water system - 15 or 22 mm piping?

I know that hot water cylinder systems which are fed by a header tank in the loft usually (*) use 22 mm piping to give a greater flow rate to compensate for the much lower pressure than the mains pressure of the corresponding cold tap.

We need to get our cylinder replaced because it has corroded and has just started leaking. We are fed up with a pathetically low flow rate and long time for the hot water to run warm, so we are considering changing to a mains-fed cylinder - obviously one which can withstand the higher pressures.

I think a lot of our hot water pipes are 15 mm (no wonder the flow rate is poor!). Given that 15 mm can give a good flow rate for cold water, is there actually a need to upgrade the hot water pipes to 22 mm, or can we manage we manage with the existing 15 mm hot water pipes?

(*) But not in our house :-(

Reply to
NY
Loading thread data ...

When I moved our cold tank from just below ceiling level to attic, I used 22mm to reach the bathroom/downstairs, but 15mm to the basins/WCs and sink. (More truthful, left in place 15mm for most of that.)

Honestly, cannot now remember whether it was 22mm all the way to the bath or not!

However, I did provide cold feed to bathroom basin from tank rather than mains to better balance the water flows. And also used tank to feed the upstairs WC which meant that we had a number of flushes even without mains water (e.g. when they replaced mains out in in the road).

Downstairs WC fed direct from mains but basin, again, from tank. That meant the most-used WC cistern filled very quickly. I added a separate 15mm feed from tank to upstairs WC to minimise the impact of cistern filling on bath/shower and basin flow and temperatures.

Remember that 22mm pipe has greater water capacity so makes it take longer for hot water to arrive.

Reply to
polygonum_on_google

If the cold is ok with 15mm then the hot will be too, at the same pressure. The downside of 22mm is the extra volume - about double - which then means it takes longer for the hot water to arrive.

Reply to
Clive Arthur

Very good point: if cold flow rate is OK with 15 mm, mains-fed hot will be too. That hadn't occurred to me ;-) What diameter hot pipe is usually used in new-build houses which have mains-fed hot water (cylinder or heat-on-demand-in-boiler)? Our last house had a heat-on-demand combi boiler and that ran hot very quickly (30 seconds max) and could keep up a fast flow rate to the bath all day long.

Two to three minutes for taps to run warm, even those which are close to the cylinder, never mind those at the other end of the house with longer pipe runs, is rather slow... And it takes a loooong time to run a bath, even once the water is running hot.

We were just about to go to bed when I noticed multiple water stains on the hall ceiling, directly below the hot water cylinder. And so began all the tasks of:

- turning off the hot water heating to the cylinder (so as not to heat an empty cylinder)

- turning off the cold water feed from the header tank to the cylinder

- running off as much water from the cylinder as possible

- finding a hose pipe to attach to the cylinder drain c*ck to run off the rest of the cylinder

- finding the carpet cleaner vacuum cleaner when the hosepipe pulled out of the window where it was draining and went all over the hall carpet

- going up in the loft to tie up the ballcock on the header tank when I realised that the hot water was still running slowly (although now cold) and that evidently the cold water feed stop tap was not fully closing

Reply to
NY

Be aware that unvented hot water systems don't necessarily run at 'mains pressure', but have a pressure reducer fitted on the rising main to limit the pressure to a few bar, and that may be lower than the 'mains pressure'.

Reply to
Chris Hogg

Yes, you?ll be absolutely fine with 15mm (assuming your mains pressure isn?t totally crap). Using too big a pipe will increase the volume of the pipework and will end up being wasteful of hot water.

Tim

Reply to
Tim+

Double the water will be wasted, but flow gets disproportionately better in a larger pipe.

With a larger pipe, and assumed flow limited by the pipe and not the taps, it should actually be quicker for hot water to arrive.

Reply to
Fredxx

There is no need, but its definitely fun to shower in a shower fed by 22mm

Reply to
The Natural Philosopher

around 2 bar plus minus

Reply to
The Natural Philosopher

A cheaper option would be just to replace the tank like with like and add a single impeller pump for the DHW. Get a Stuart Turner if you want it to last, 2 bar should be fine, as will 15 mm pipework.

Reply to
newshound

At ?normal? mains pressure, water is delivered quite fast enough through

15mm so there?s little to be gained (and energy to be lost) by oversizing.

Tim

Reply to
Tim+
<snip>

I'm looking to either fit an instant water heater by the downstairs hand basin or replace the 22mm pipework with 15 for that exact reason.

For the first run for a while it take about 30 seconds for the hot to come through from the multipoint water heater.

The pipework was originally specified for a conventional cylinder based HW system.

Cheers, T i m

Reply to
T i m
<snip>

Isn't it just a 'waste of water' and 'a waste of energy' (not just a waste of hot water)?

Eg, when you run the tap it takes a lon(ger) time before the hot comes though (wasting water) and then you use the hot as required, but leaving a greater quantity of water between the heat source and tap to go cold before the next usage (wasting energy)?

A mate has recently fitted a Mira instant hot water hand washer and he says that's up to temp in about 6 seconds.

If you were using it to run a basin I should imagine that you could start with the plug in and by the time you have enough water to fill the basin it should be a useable temperature (no water or energy wasted).

Cheers, T i m

Reply to
T i m

The other consideration is what works best when you are in the shower and someone turns on another hot water tap in the house.

Reply to
Joshua Snow

If you do go down the route of a pressurised hot tank make sure that you get the cold main pressure tested. Having just done exactly that (without the pressure test) we find some modern shower mixers are overpowered by the cold and back-feed into the hot over-pressurising (or would have done but for the pressure relief valve) the tank.

In summary in our case tank designed to operate at 3 bar (with a limit of 10). shower mixer valve limited to 5 bar.

Cold supply at 7.5bar fed back through the mixer and caused the tank pressure relief valve to blow off.

Easily remedied by putting a pressure regulating valve (set to 3.5 bar) in the rising main.

Plentiful hot water supplied through 15mm pipes.

Reply to
Chris B

Sounds like a non-standard installation as usually the cold supply for showers etcetera is taken off the down-regulated cold feed to the tank ensuring that the pressure is balanced. That is at least ?good practice? and I?m pretty sure it was in the installation instructions with our tank.

Tim

Reply to
Tim+

My water supply (the mains is about 10 bar) has a 4 bar regulator at the water board stopcock. I don't know how unusual that is.

Reply to
Roger Hayter

What does a pressure-regulating valve look like. The only thing I can see in the rising main is the T piece to the water softener that feeds one tap of the kitchen sink (it's a lift-for-flow/rotate-for-temp valve for unsoftened, with a separate integrated tape for softened), with the other branch of the T going off to the rest of the house (washbasins, toilets, header tank, filling loop of boiler pressure vessel). Otherwise I can't see anything..

Anyway, we've got the cylinder guy coming on Monday to give us a quote, and I imagine he'll make any checks that he needs. There are more pipes going into the loft that I would expect: There's 15 mm cold main to ballcock, 22 mm expansion pipe T-ed from hot water outlet from top of hot water cylinder into tank, 22 mm feed from tank to bottom of hot water cylinder. But there are a 15 mm and a 22 mm plastic pipe as well as the three copper pipes I've mentioned.

Reply to
NY

If you can establish that you have adequate flow rate and pressure into the house, then an unvented system can work well. Typically the controls on the cylinder will include a pressure reduction valve to limit the DHW pressure to around 3 to 3.5 bar. 22mm piping might be a bit quieter, and will support more concurrent users of the DHW (if the mains supply can keep up)

Many of the pros, cons and practicalities are discussed here:

formatting link

Reply to
John Rumm

Two different beasts. With a combi boiler, typically 15mm everywhere since the boiler itself is normally the limiting factor on how quickly DHW can be delivered. (unless going for slightly more exotic "storage" combis).

Unvented cylinders typically expect 22mm piping for the main feeds and outputs.

For certain values of fast. A powerful combi (i.e. >= 30kW) can provide DHW at 15 - 18lpm typically depending on the incoming water temperature. An unvented cylinder could supply 50 lpm or more if the mains can keep up.

I would say 10 - 20 secs top for hot water to a tap is reasonable with a unvented cylinder.

If you have long pipe runs, and delay is an issue, then you can use a secondary circulation loop, to provide near instant hot water all round. (many unvented cylinders include tappings for a circulation loop return)

With the cold feed connected and the heating turned off, you can run off any stored hot water. However it will always remain full - you can't drain it at all using the taps in the house.

Yup, that is usually the only way to get water out of the cylinder.

Reaching into the tank in the loft and putting a bung into the outlet to the cylinder will stop that being re-filled, but still leave the other cistern fed outlets working.

Reply to
John Rumm

HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.