Indirect unvented cylinder with 15 litre flow?

[SJRSJR]

Hello

With the addition of a couple of showers, I’m looking at getting an indirect unvented hot water cylinder in addition to the 35kW combi boiler. (When finished there’ll be a bath, 4 showers and 19 rads/23kW). The plan was to keep the bath on the combi and run the showers off the cylinder.

A local plumber has quoted for a 300l cylinder but I only remembered to ask about flow and pressure after he’d gone.

He said he’d never has any issues in the area but I said I’d check. The unadjusted pressure reducer valve on the feed shows 5bar but the flow is only 15l per minute. It’s 22mm pipe so am wondering what else can be done as I believe 20 litres is the target.

Are there any cylinders best suited to low flow? I read a Gledhill with a corrugated coil has high recovery but I’m a bit lost and don’t want to spend all that money if there’s no point. Any advice would be much appreciated. Thanks

 

[flameport]

Are there any cylinders best suited to low flow?

No, and it wouldn't be of any use even if there were.

For multiple showers or other outlets in use at the same time, the flow is shared between them, so high flow rates are needed. That applies regardless of how the water is heated - if the flow into the property is poor, then it doesn't matter whether some are from a cylinder and others from the combi or even if you fit crappy electric showers - they will all have poor flow because you can only get out what is coming in.

s 5bar but the flow is only 15l per minute. It’s 22mm pipe

That is far too low, there must be some restriction somewhere.
Or that pressure is only the static pressure and it drops off dramatically once an outlet is opened.

a corrugated coil has high recovery

That may be, but that just means it reheats quickly. Nothing to do with flow rate at all.

 

[Madrab]

He said he’d never has any issues in the area but I said I’d check.

Even though they're a local trade unfortunately there's really no way they can know if the supply is adequate or not without testing, that's not a good starting point. Get the transporter out to check the mains flow and pressure at the toby, that'll provide the correct starting point and dictates what is actually available. With that proposed system design then run least 3 full flow cold outlets whilst monitoring the pressure drop and when they're running do a flow measurement test (timed container filled over a minute) and do it during peak hours.

The key for the system is dynamic pressure - with the size of the proposed system and to maintain the ability to cope with a lot of that running at the same time, then >2bar dynamic (max demand) would be the target IMO. Obviously flow is also key but without the dynamic pressure to maintain a suitable flow then that will never be obtainable.

To give you an example - I have a 3.5 bar dynamic mains @ around 28 L/Min average (time of day dependent) on a 32mm MDPE > 22mm system pipework. With that I can run 3 good high flow thermo showers (one is a Mira Activate) at the same time, from a 250L unvented, with a little drop off in pressure. If the w/m or d/w kicks in and/or someone uses the sink or a toilet, then you do notice another dip in flow at the shower but the valves quickly react to that and recover. The lowest I've seen the mains drop to is around 2bar @ 19L/min at max demand but as the pressure is sustained, then the system can still cope.

 

[SJRSJR]

Thanks for the replies. So there’s no point in, say, a second combi boiler to feed two showers. That wouldn’t make a difference as the mains flow in is the problem?

 

[Madrab]

That wouldn’t make a difference as the mains flow in is the problem?

Exactly right , mains dynamic pressure and flow is key. As mentioned, you can only get out what goes in.