water pressure, tall house

[Johnking]

Can anyone help me work out how to fix my water pressure? Property is three stories, Combi boiler on lowest floor in the kitchen. The shower at the top of the house has always been a bit flakey due to lower pressure, and this has got a bit worse recently, some days failing to trigger the boiler at all, often cutting out mid shower of out does heat up.

I can't measure pressure but flow rate in basins by the boiler/showers is as follows (l/m). Vertical distance in curly brackets {approx}.

Kitchen: cold 11.2, hot 7.7 {0m}
First floor: cold 6.6, hot 5.0 {2.5m}
Second floor: cold 3.7, hot 3.3 {4m}

I had been thinking a booster on the mains, but would that work in this situation?

Many thanks!

John

 

[Madrab]

Yes it could do, the pressure reading is key here though. Obviously the height is having a significant impact on the flow therefore the pressure is obviously not the best to start with.

A mains pump can give a sustained pressure delivery especially over significant height differences but that would be primarily down to the incoming pressure and whether the pump will actually engage or not. Anything over it's base delivery of 12L/Min @ 1.5bar'ish, at the ground floor mains level, then the pump will just idle.

 

[Johnking]

Thank you. Is there any way I can measure the pressure? I have a hose pipe!

 

[Madrab]

You need a gauge

then attach it as close to the mains stop tap/where the water enters the building - an outside tap or washing machine tap. Check the static pressure - no outlets running then dynamic when more than one outlet is open - highest tap(s) would be ideal to check the maximum drop that can be expected.

 

[Johnking]

Ok, they look affordable.

Meantime, why is it the flow through the Combi is so much lower? I live in a very hard water area, might there be something going on with scale?

 

[Madrab]

I live in a very hard water area, might there be something going on with scale?

Yes, always possible in a hard water area, when possible get it serviced by a recommended Gas safe chap he should check how it's performing against what it should be and investigate if need be.

 

[Johnking]

The manual (to the extent I understand it) says it can supply DHW at 10l/m. The fact it's giving 7.7l/m out of the same tap that has a cold output of 11.2l/m suggests at least part of this is boiler-related, notwithstanding that there will be lower pressure at the top of the house. What I don't get is, it only needs 0.8bar to achieve 10l/m, and it must have 0.8bar dynamic pressure because it's getting water 8m vertically. I'm sure I've got that wrong....

DHW flow rate @ 35 °C rise 10l/min
Mains water pressure required for max. flow rate 0.8bar
Minimum water flow rate 2.7l/min
Mains water pressure required for min flow rate 0.3bar
Maximum inlet water pressure 10bar

 

[oldbutnotdead]

What temperature is the hot water coming out at/set to? The combi has to heat the incoming water from whatever (15 degrees or so?) up to whatever you have it set at instantly. A 28kw boiler would give you that 7.7ltr/min flow rate with incoming at 10 degrees and output at 65. What happens to the hot flow rate if you reduce the output temperature? There'll always be a bit of loss in the workings but if you drop the output temperature to 40 the flow rate should come up to the same as the cold (unless there's something else going on)

 

[Johnking]

What temperature is the hot water coming out at/set to? The combi has to heat the incoming water from whatever (15 degrees or so?) up to whatever you have it set at instantly. A 28kw boiler would give you that 7.7ltr/min flow rate with incoming at 10 degrees and output at 65. What happens to the hot flow rate if you reduce the output temperature? There'll always be a bit of loss in the workings but if you drop the output temperature to 40 the flow rate should come up to the same as the cold (unless there's something else going on)

Thanks, I will do some measurements. Does it make a difference if the hot flow is measured after it gets to temperature? Previously I didn't do that, so cold water was coming from the hot tap during the time it took to fill a litre jug.

 

[Johnking]

Further measurements... Maybe pressure a little higher this morning.

Cold flow ground floor next to boiler (16.5deg)12.5l/m
Hot flow set to min temp (42deg) 8.8l/m
Hot set to max (51deg) 8.2l/m

Flow rate is much lower upstairs, and thus the water gets hotter.

Boiler is 24kW ( old reliable Vaillant VUW242E)

 

[Madrab]

Your boiler is actually a 30Kw boiler - Max Input for HW.

The drop in flow higher up certainly looks like it's pressure related. The fact you are getting 12.5L/Min lower down though could compound the solution as a mains pump will not pump above 12L/min. An accumulator may be an option but again that depends on mains pressure.

 

[Johnking]

Your boiler is actually a 30Kw boiler - Max Input for HW.

The drop in flow higher up certainly looks like it's pressure related. The fact you are getting 12.5L/Min lower down though could compound the solution as a mains pump will not pump above 12L/min. An accumulator may be an option but again that depends on mains pressure.

But I only get 8l/m on the hot side, so wouldn't a booster pump kick in when the hot is running?

 

[Madrab]

Yes it would, if the mains flow drops to that level then the pump would kick in but that won't make much difference. The boiler can still only deliver a set amount @ a 35Deg rise therefore inlet water temp dependent, it would still only reach a max flow through the boiler and then that still has to travel upwards with the associated pressure and flow drop.

Your mains flow is right at the limit where a pump may not be as effective as it could be unfortunately, it sounds counter productive but it would be better if the mains flow/pressure was actually lower.

 

[Johnking]

Yes it would, if the mains flow drops to that level then the pump would kick in but that won't make much difference. The boiler can still only deliver a set amount @ a 35Deg rise therefore inlet water temp dependent, it would still only reach a max flow through the boiler and then that still has to travel upwards with the associated pressure and flow drop.

Your mains flow is right at the limit where a pump may not be as effective as it could be unfortunately, it sounds counter productive but it would be better if the mains flow/pressure was actually lower.

Oh crap! What's the answer then? Do I need a pump further up the house?

 

[Madrab]

Nope, need to check the available dynamic pressure first & get the water company to check their mains delivery levels and compare the 2.

That'll dictate next possible steps.