Hot/cold water flow differential

[hotspot007]

Hi. I've been suffering with my shower going hot and cold (2nd floor loft ensuite) - I'm covered by boiler insurance so an engineer has looked at my boiler. It's a greenstar 30 Si. He has said it is a water flow problem form the mains. OK - I'm getting 2bar standing pressure - cold water fully open = 9 liters an min. Hot water = 6 liters a min.
My question is - is it normal to lose 2/3 liters a min flow through your plate heat exchanger, cold vs hot?
Note engineer checked and replaced flow restrictor and housing - but it is exactly the same.

 

[oldbutnotdead]

9 litres/min is a bit poor.
Dynamic pressure is useful to know (ie what pressure that 9l/min is being delivered at). Yes plate hex will cause significant pressure drop (& thus flow drop).

 

[hotspot007]

Hmmm. I rang the water board and was told that's a really really good pressure and flow ☺
Tbh I think the mains flow/pressure has always been like this since I moved in (new house 8 years ago), got the same ensuite shower (the cartridge was replaced on the shower bar). So looking at the boiler - at the flow I've always had - I think the boiler has always just ticked over on minimum to maintain a good temp without having to cycle on and off. So because I've always been near the threshold of the cycling on and off any slight pressure /flow change could cause the issue. That's in my head anyway. I questioned if losing 3 liters / min through your heat exchanger is normal and his reply was yes - and if the heat exchanger was furred up it would cause over heating and trip the boiler. My thinking is it isn't furred up enough to cause over heating issues yet, it's just because my flow is so poor that any slight variation is picked up in my shower by the boiler flame going on and off?

 

[oldbutnotdead]

Depends where you are I suppose but up here that is v poor. 30l/min out the cold tap at 2.2bar dynamic (3.1 static) in my kitchen, 24 l/min out the plate hex at 65 Deg C (hex is on a thermal store not a gas boiler).
Yes your flow to an upstairs shower (so introducing 0.4 of a bar drop through gravity) will be marginal.
Jobs for you;
A 2014 build we can assume MDPE pipe into the house.
Have you checked your stoptap at the meter and in the house to make sure they're both wide open?
What size is the cold pipe from stoptap to the combi and is it plastic or copper?
How many bends, elbows etc are there between stoptap and boiler and boiler and shower?

 

[terryplumb]

Do your hot taps give constant temperature hot water ?
Is shower a thermostatic mixer

 

[hotspot007]

Yes, mdpe into the house. I have a stop tap outside my front door with a 26mm hex head screw head just after it (thinking maybe a filter - which I plan to check). So from the drive into my lounge - the blue pipe comes up straight into my water meter and out of that into 15mm copper pipes. From there maybe 10m run to the kitchen - boiler.
As for constant temp from taps - seems so ground floor and bathroom, but not sink in top floor ensuite. Shower is thermostatic mixer - I replaced the cartridge (first thing I did) - that didn't change anything so went to a manual mixer tap - again never changed anything. That's when I called the gas engineer.

 

[hotspot007]

Also, will fitting a salamander home boost pump get my flow up to 12 litres/min
https://www.salamanderpumps.co.uk/product/homeboost/11

 

[terryplumb]

Also, will fitting a salamander home boost pump get my flow up to 12 litres/min
https://www.salamanderpumps.co.uk/product/homeboost/11

I doubt it. If you remove the shower valve ,see what the hot water temperature and flow rate are from open ended pipe.

 

[hotspot007]

Done that. It fluctuates hot and cold open ended. Same as the hot tap in the same room.

 

[terryplumb]

Are you certain that ground floor hot taps give constant temperature hot water ?
You Have lived in the property 8 years, so I assume the problem recently started ,when ?? And what changed around that time ?

 

[hotspot007]

I'm not certain in the sense that it might fluctuate in degrees that is un-noticeable to touch. Definitely the boiler does not cycle the flame on and off whilst running the ground floor tap.
Also weirdly, I have the same flow hot and cold with the shower valve open ended on the 2nd floor as I do in the kitchen.

 

[terryplumb]

You Have lived in the property 8 years, so I assume the problem recently started ,when ?? And what changed around that time ?[/QUOTE]
????

 

[hotspot007]

Yeah sorry, been in since built. Nothing has changed. The shower hot/cold started maybe 6 months ago.
There is one thing, I tried a sprinkler last summer.. And it just wasn't working - spray around 4 ft high. I hadn't tried a sprinkler since maybe moving in 8 years ago - and I was sure it worked back then, so yes I would say at some point my mains pressure may have been reduced. But it certainly not to the point where I noticed in the house by running taps or in my shower.
Do you know if that big 26mm nut head after my main isolation valve in my drive is a filter?
With regards to internals,nothing in the house has changed. Original plumbing, original taps and shower bar - other than replacing the cartridge and trying to replace shower mixer with a manual 2 tap mixer.

 

[terryplumb]

No idea what the nut is ,but doubt it's a filter.
You told us that the open ended hot pipe to shower ( with shower valve removed) gives a flow of 6 lt/ min, and the temperature runs hot / cold. Was that with the cold to shower isolated ?
That flow rate should keep the boiler running / constant temperature. It seems to me that the problem is with the boiler ,or you are not measuring flow rates correctly.

 

[hotspot007]

Yes the flow was at the shower open ended... And yes measured 6l/m. The problem arises when we then open the cold too. The engineer said this drops the pressure/flow just slightly enough for then the boiler to cycle as with the open end, the boiler is just literally ticking over to maintain the given temp output?