40kw Navien NCB Combi install with v.poor DHW? Is this adequate?

[ecswtrav]

Correct you have a restriction on the cold main somewhere need to follow this pipe all the way back to the main

The Istor never suffered from this issue so could this issue possibly be installation related? Seems strange how all of a sudden the flow rate has dropped after installation of a new boiler. The Istor even had a prv limiting the pressure....this doesn't and have anything limiting on the supply line now. Just for reference too, I also popped outside and tried the cold tap again....approx 21/22lpm getting to that still.

 

[ecswtrav]

Don't want to be awkard but its strange that a open ended pipe via that flexible hose is only giving you 9LPM when you can get up to 12.5LPM from two HW taps, one of which is upstairs, and the water must also pass through the boiler heat exchanger.

Yeah I agree....the only time I've managed to get a higher flow rate reading on the boiler screen is when I've opened a few hot outlets. A single outlet stays around the 9-10 mark.

 

[John.g]

Is the blue isolating valve just attached to the cold supply to the boiler and is that "1.5 bar" PG just monitoring the boiler system pressure?

Also check for a PRV installed somewhere on the cold feed to the boiler, it may be faulty because in order for the flow rate to increase from 9LPM to 12.6LPM would require a pressure increase from 2.2 bar to 4.3 bar for any fixed restriction/obstruction in the cold water feed.

 

[ShaunCorbs]

Don't want to be awkard but its strange that a open ended pipe via that flexible hose is only giving you 9LPM when you can get up to 12.5LPM from two HW taps, one of which is upstairs, and the water must also pass through the boiler heat exchanger.

but that would of had 22mm pipework and not 15mm so there’s a joint somewhere maybe an iso valve etc

 

[John.g]

Maybe my thinking is wrong but a test was carried out on the cold feed supply only to the boiler by breaking a pipe (hose) so would expect the maximum possible flow rate from this as the cold feed is now open at one end, the same supply is then feeding the boiler, the hot water pipe or pipes to the hot taps and gives 40% more flow than flowing out the open end of the supply pipe.
The only other explanation is that the hot taps have mixers.

 

[ecswtrav]

Well, no contact from the fitter as yet although an electrician was here fitting some opentherm controls as part of the deal. Checked again at the outside tap and confirm it was still 22lpm. I did try the washing machine inlet hose and that didn't quite match the 22lpm and was around 17.5 but I think by looking it was really restrictive with a very tiny hole at the end. Either way it exceeded 9lpm.
Just a few feet upstream from the stopcock are the two isolation valves....one on the cold supply and the other on the boiler supply....so as suggested maybe the boiler supply one is faulty? I believe these were full bore when fitted. From looking they're both open it would seem fully.
The prv has been removed that was used to limit the istor boiler feed to 2 bar so there is nothing else on this feed to the boiler....just a left hand turn which then leads straight to the boiler.
The fitters have pieced back some plasterboard behind the boiler so I wonder if there is something restrictive they've fitted beyond this hidden out of sight?
John.g you are correct - the bathroom tap is a mixer tap.
I will keep this updated anyway until we get this resolved as I appreciate everyone's help.

 

[John.g]

Sometime, you might try this.
What flowrate does the boiler show (1) if you ran the shower only, with the boiler temp at minimum of 35C and (2) with boiler temp at max temp where you will also have mixing.

 

[ecswtrav]

Sometime, you might try this.
What flowrate does the boiler show (1) if you ran the shower only, with the boiler temp at minimum of 35C and (2) with boiler temp at max temp where you will also have mixing.

I just gave that a try....ran the shower in the ensuite in the loft as that is the furthest away of the two. There was minimal difference for some reason and both temps hovered around the 5.8lpm mark on the boiler display....the lower temp exceeding it intermittently. The shower itself has a rain head and is perfectly usable at those flow rates but just not what we were used to.

 

[John.g]

A rain shower should give a torrent of water.
If the shower is maintaining ~ 45C showering temp (thermostaic mixer?) then it should have flowed ~ 9LPM with boiler at max, still very poor.

 

[John.g]

You could also measure the actual shower flow rate with the shower temp control to minimum and on full flow, if temp setting is low enough ie a "cold" shower then no water will flow through the boiler, if this works then measure the actual shower flow rate with the shower at say 50C (or max) and the boiler at its minimum temp setting, this should then force all the water flow through the boiler and compare both flow rates.

 

[ecswtrav]

You could also measure the actual shower flow rate with the shower temp control to minimum and on full flow, if temp setting is low enough ie a "cold" shower then no water will flow through the boiler, if this works then measure the actual shower flow rate with the shower at say 50C (or max) and the boiler at its minimum temp setting, this should then force all the water flow through the boiler and compare both flow rates.

Thanks for that. Little bit of progress...Navien are coming to see the boiler with the installer...not sure when yet but hopefully very soon so will be interesting to see what they say and whether they consider any testing like the above. Will definitely report back....

 

[ecswtrav]

A quick update as things are moving in the right direction at last and I'm grateful for the help here. Fitter came out and measured the flow rate at the boiler to be 20lpm. Navien visited earlier and the chap tried several new flow restrictor devices so as pointed out earlier in the discussion the flow restrictors within the boiler seem to be the culprit. It's now flowing at around 13-14lpm occasionally peaking briefly at 15lpm which is more on par with the spec for the 37kw boiler. The chap is going to investigate with Navien whether there is an alternative flow restrictor with a more open bore. I got the impression the thing he was showing me just couldn't be pulled out. Certainly progress though so thank you to those who have been extremely helpful. I'll update this if we manage to squeeze a few more lpm out of the boiler....I'm hoping so.

 

[ShaunCorbs]

Or maybe remove the flow restrictor and fit an external one on the hot out of the boiler etc ?

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[John.g]

Post #22 shows a link to restrictors which may be one of the ones installed in your boiler, they are orange, yellow and blue with respective flowrates of 25.5, 22.7 & 18.0 LPM at a minimum dynamic pressure of 2 bar, the outside tap pressure might give some idea with full usage.

 

[John.g]

Why do boilers need restrictors at all when the flowrate can be controlled at the end users?.

 

[ecswtrav]

Ran a few showers earlier and opened up a third outlet....the boiler was registering 18-18.5lpm so something has freed things up. It seems the more outlets open the higher the flow rate it registers Previously doing this I think got 10-11lpm maximum regardless of how many open.

A single outlet won't register 18lpm but the bath tap does 15lpm which is far better than when first installed and not far off what the istor kicked out. Assuming this is how a combi works so I think problem solved. Thank you to everyone for your help.