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

[ecswtrav]

Hi all,
Just had a Navien NCB 40kw combi installed by an approved fitter. The DHW rates of the boiler seemed promising on paper - 16.4lpm at a 35 temp rise and 22.9lpm at a 25degree rise. More than adequate for what we need. The only issue now its installed is that I'm not getting any more than 9 or 10lpm using the boilers display. I knew it would be a compromise fitting a combi as I had an Ideal Istor previously (cylinder & system boiler all in one), but the reduction in DHW is quite noticeable.

I've a new 25mm mains - fitted years ago mind, the Istors 22mm pipe feed now goes to the Navien (although reduced to a 15mm connection). I have 25lpm at the outside tap off the kitchen wall and 3bar pressure (not sure if this is static or dynamic but it was a gauge screwed to the outside tap???). Gas pipe is 22mm probably 20 feet to boiler). There is a tee off at the stop tap where it enters the property leading off to the cold supplies for the house - the other leading to the Navien. The hot water supply pipes to a bathroom and ensuite and kitchen from the boiler are 22mm (again from the old Istor) which tee off in 15mm to the fittings, the cold water pipes seem to be in 15mm to the various outlets. I've a bathroom and an ensuite. Boiler is in the basement, the bathroom 2 floors up and ensuite in the loft conversion.

The fitter assures me he's seen much higher readings on 40kw Naviens. He just doesn't know why it only reads between 9-10lpm.

I just wondered whether the quoted figures related to the flow rate when hot water is mixed with cold? I guess then it probably would be a much higher flow rate which is why the showers seem ok. Apparently the incoming temperature is 9 degrees, so setting the boiler hot water temp to 45degrees should give approx 16lpm...but it's a mile off based on the display.

Just wondered if I was missing something as my fitter couldn't shine any light on it and said he would speak to Navien? I never expected it to match the Istor with multiple outlets on at the same time, but I did expect a slightly better flow rate with one outlet...particularly to my bath tub which we used to fill in no time, which now takes noticeably quite a bit longer to fill.

Is the reality that what I'm getting currently is actually normal for a 40kw combi? Thanks for any comments.

 

[Scott_D]

Any Pictures of the install?
Is there a filling loop at the boiler?

 

[ecswtrav]

Hi thanks for replying. Here is a pic of the underside of the boiler. Assume there is a filling loop in the photo although I'm no expert.

The boiler was commissioned with a gas pressure reading of 21mbar...I wondered whether it was undergassed so couldnt do its job? Between 17 and 25 mbar is recommended so I assume this ok. Although I don't remember being asked to turn on multiple outlets to get the boiler up to its max operation.

Puzzles me why it won't perform anywhere near its specs.

Thanks

 

[ShaunCorbs]

I would suspect low cold mains flow into the boiler if it’s only reading 9-10lpm

 

[Scott_D]

If you have 25lpm at an outside tap
You should have similar at other cold taps eg bath cold?
Hot is probably restricted through the boiler but as it’s a 40kw I would imagine a 15-16 LPM Restrictor.

I know nothing about nevian but it may have a filter that needs cleaning.
You can test at the filling loop pressure and flow to give an idea of what the cold supply just before the boiler is.

Any other valves been shut or touched?
I would ask the installer to return and have another look at it.

 

[ecswtrav]

I would suspect low cold mains flow into the boiler if it’s only reading 9-10lpm

I've got approx. 22lpm at the outside tap which makes me think surely the boiler must be receiving that? or does it need to be receiving more than that to achieve more than its 9-10lpm heated up?

 

[ShaunCorbs]

It might not be due to Pipework valves or other restrictions

or could be a restriction on the hot somewhere

 

[ecswtrav]

If you have 25lpm at an outside tap
You should have similar at other cold taps eg bath cold?
Hot is probably restricted through the boiler but as it’s a 40kw I would imagine a 15-16 LPM Restrictor.

I know nothing about nevian but it may have a filter that needs cleaning.
You can test at the filling loop pressure and flow to give an idea of what the cold supply just before the boiler is.

Any other valves been shut or touched?
I would ask the installer to return and have another look at it.

Is this something I could do or should I get the installer back? I've mentioned the issue and I've since seen he even recorded on his commissioning sheet 9LPM so he was well aware. His answer was that he'd phone Navien and ask. There are a few isolation valves next to the stopcock - one for the cold and one direct to the boiler but these are fully open and full bore as far as I know.

 

[ecswtrav]

It might not be due to Pipework valves or other restrictions

or could be a restriction on the hot somewhere

Do you mean a restriction within the boiler? I'm only assuming that as its the boiler that tells me what lpm its kicking out - the hot tap in kitchen for example seems to match this rate too.

 

[ShaunCorbs]

Don’t think so would need testing

best option would to put the hot into a bucket From the valve and see what you get coming out of the boiler if it’s 13-14lpm you know your problem is on the outlet / hot pipe work

but if your only getting 9lpm you know it’s probably cold pipework

 

[ecswtrav]

Don’t think so would need testing

best option would to put the hot into a bucket From the valve and see what you get coming out of the boiler if it’s 13-14lpm you know your problem is on the outlet / hot pipe work

but if your only getting 9lpm you know it’s probably cold pipework

Great - apologies though - which valve are you referring to as I'm not overly familiar with all of this? So supposedly if I release the hot water from this valve I should hope the lpm reading on the digital display would increase and show a true reflection of what the boiler is capable of? Looking at the commissioning sheet the temperature of the cold is 9degrees? Thanks

 

[ShaunCorbs]

Tbh the installer should be doing this else there was no point in fitting a 40kw when you could of used a 28kw etc

 

[ecswtrav]

Tbh the installer should be doing this else there was no point in fitting a 40kw when you could of used a 28kw etc

That's exactly my issue. He told me 40kw was ideal. Had I have know I'd get 9 or 10lpm I would never have gone this route or saved a few hundred quid and got a smaller boiler. Just to be certain, when/if he returns what should I be suggesting he do exactly (without telling him how do his job!?)

 

[John.g]

If you open multiple hot water taps can you measure this and compare it with the boiler display?, a 22kw boiler at 9 LPM will give you a deltaT of 35C, they don't make combis much smaller than this.

 

[ecswtrav]

If you open multiple hot water taps can you measure this and compare it with the boiler display?, a 22kw boiler at 9 LPM will give you a deltaT of 35C, they don't make combis much smaller than this.

Just opened two hot taps - one in a sink next to the boiler and one in a sink upstairs in the kitchen (boiler in basement). kitchen was 5.4lpm and basement tap 7.2lpm - the display on the boiler was just passing 12lpm which was progress in some ways. the water temp is 47 degrees so a 37 degree rise if the 10 degree water temp the fitter recorded on his commissioning sheet is correct.