New boiler flow rate not improved

[Richard Lindsay]

Just had a new 35kw boiler installed. Everything working well except that I expected the hot water flow rate to be improved. I had a 20 year old 28kw boiler replaced. The new boiler is rated for 14 litres per min of hot water.

I measured the outside tap and get 20 litres per min at 2 bar.

The kitchen tap, which I think is flow restricting, delivers 6.5 litres per min on the hottest setting.

The shower, which I've just replaced for a manual mixer (the old shower's thermostatic cartridge was knackered and i wanted to see if the flow would improve without any restriction) is delivering 6 litres per min on the hottest setting.

Any ideas?

Thanks for your time

 

[Riley]

Pressure reducing valve somewhere? Really you need to test the pipework straightbput of the boiler to ascertain if there Is an issue with the pipework or taps

 

[John.g]

Just had a new 35kw boiler installed. Everything working well except that I expected the hot water flow rate to be improved. I had a 20 year old 28kw boiler replaced. The new boiler is rated for 14 litres per min of hot water.

I measured the outside tap and get 20 litres per min at 2 bar.

The kitchen tap, which I think is flow restricting, delivers 6.5 litres per min on the hottest setting.

The shower, which I've just replaced for a manual mixer (the old shower's thermostatic cartridge was knackered and i wanted to see if the flow would improve without any restriction) is delivering 6 litres per min on the hottest setting.

Any ideas?

Thanks for your time

The boiler should state somewhere the (dynamic?) water pressure required to give that 14 LPM.
Were you getting ~ 11 LPM from the old boiler?.

Is 2.0 bar the measured pressure at a outside tap flow of 20 LPM?.

The new manual mixer may have restrictors fitted to give that max flow rate of of 6 LPM, this seems to be a new "eco measure" around here anyway and abroad as I have stayed in lots of hotels which over the years replaced all their rain type showers with the "6 LPM" models, apart from saving the earth, it also has huge energy benefits for them.

 

[Jones82]

Don't forget that "14 litres per min" figure is at a 35 degree temperature rise. At this time of year the cold water is colder so the boiler has to slow the flow to obtain the required temperature.

Saying that 6.5 litres is low and I would be looking to the shower, possibly a restrictor in the handset or outlet if your lucky or in the mixer itself.

 

[Last Plumber]

Get the installer/person who sold it to you back to explain it.

 

[Scott_D]

What make/model of boiler?

 

[Richard Lindsay]

Cheers for the replies guys. I've tried to answer the questions below.

Pressure reducing valve somewhere? Really you need to test the pipework straightbput of the boiler to ascertain if there Is an issue with the pipework or taps

Not sure about the PRV. I'm going to test the flow rate of my dishwasher which is right beside my stopcock. It's always made a strange noise like it's not getting enough water. Could the stopcock be restricting flow?

The boiler should state somewhere the (dynamic?) water pressure required to give that 14 LPM.
Were you getting ~ 11 LPM from the old boiler?.

Is 2.0 bar the measured pressure at a outside tap flow of 20 LPM?.

The new manual mixer may have restrictors fitted to give that max flow rate of of 6 LPM, this seems to be a new "eco measure" around here anyway and abroad as I have stayed in lots of hotels which over the years replaced all their rain type showers with the "6 LPM" models, apart from saving the earth, it also has huge energy benefits for them.

Yes, the 20 LPM and 2 bar were from the outdoor tap. The outdoor tap is controlled by a stopcock in the garage, not the one in the house. Not sure of the dynamic water pressure. It's a Viessmann Vitodens 100-w 35kw. I was getting the same 6 LPM from my 20 year old WB 28CDi. Thanks for the info on the taps.

Don't forget that "14 litres per min" figure is at a 35 degree temperature rise. At this time of year the cold water is colder so the boiler has to slow the flow to obtain the required temperature.

Saying that 6.5 litres is low and I would be looking to the shower, possibly a restrictor in the handset or outlet if your lucky or in the mixer itself.

It's a pretty cheap mixer. I only changed it because the cartridge on the 15 year old Mira went. It says it should deliver 2.9Ltr/min at 0.2 bar. Can someone explain to me how I understand that? If I'm getting 2 bar of pressure, should the shower be capable of 23 LPM? Sorry for being a noob.

Get the installer/person who sold it to you back to explain it.

They were pretty useless if I'm honest. Tried to install a different boiler at first and then couldn't wait to leave at the end of the day. The reason I went with them is that they were the Viessmann accredited installer for my area and therefore I get the 10-year guarantee.

What make/model of boiler?

Viessmann Vitodens 100-w 35kw

 

[John.g]

2.9 LPM @ 0.2bar should flow 9.2 LPM @ 2 bar. you are getting 6 LPM so that would equate to only 0.86 bar so it would seem that you have a water pressure problem somewhere.
To get 14 LPM from that shower head would require 4.7 bar.
(Flow is prop to sq.root of head) & (head is prop to the square of the flow)

 

[Aire]

If you had poor pressure before your boiler install then a new boiler will not fix that.
First thing to check would be your stop tap, is it fully open? Do have 1 old stop tap that is partially closed with a new stop tap fitted above it?
Your getting 20ltm in your garage so your mains is up to it so something in your house is causing a restriction.

 

[John.g]

Viesmann's should be able to inform what is the sustained or dynamic water pressure required to give the rated 14 LPM, I don't see it mentioned in the users manual, do any manufacturers state it in their manuals I wonder.

Have been "reliably informed" that 1 bar differential head is more than adequate to give that 14 LPM so 2 bar main would require a shower head that will give a flow rate of 14 LPM at 1 bar or the existing shower head will require a mains head of 5.7 bar to give that flow rate. ( 1 bar drop through heat exchanger & 4.7 bar through the shower)

 

[Rob Foster]

please keep us informed of what you find out and do , its
an increasing line of enquiry on
ukpf and we like feed back to,help us learn..regards
Rob Foster aka centralheatking

 

[Richard Lindsay]

please keep us informed of what you find out and do , its
an increasing line of enquiry on
ukpf and we like feed back to,help us learn..regards
Rob Foster aka centralheatking

Will do. I'm going to start with measuring the flow from this dishwasher then having a look at the stopcock. How common is it for them not to open fully? I'm going to measure the pressure and flow from the outdoor tap again as well, just in case I've dreamt these numbers

If that doesn't turn up anything then I'm going to have to go under the floor

I live in a little bungalow with 2 sinks, 1 toilet and 1 shower. There shouldn't be that much pipe to check, surely?

 

[Rob Foster]

A 35 kw boiler is way too much
for the gaff you describe
centralheatking

 

[Richard Lindsay]

A 35 kw boiler is way too much
for the gaff you describe
centralheatking

Will be converting the loft in the next few years, master bedroom and new toilet with bath and shower. Will be adding a bath downstairs too

 

[Welder]

Will be converting the loft in the next few years, master bedroom and new toilet with bath and shower. Will be adding a bath downstairs too

Still FAR too big. Viessmans actually advise having heat calcs carried out, fitting a small boiler with a UVC. For that prop you'd be looking more like 12-15KW.

See here to hear it for yourself.

 

[John.g]

OP needs 35W HW output @ 14 LPM but a bit of a mismatch then for CH but the Viessmann has modulation ratio of 6:1 I think which means ~ 6 kw to CH at full turn down, still a bit high but not the end of the world with a bit of cycling, if he could be happy with say 9/10 LPM HW then a 24/28 kw unit would do with a turn down to ~ 4kw for min CH.
I see its already installed but may work out OK.

 

[Richard Lindsay]

Still FAR too big. Viessmans actually advise having heat calcs carried out, fitting a small boiler with a UVC. For that prop you'd be looking more like 12-15KW.

See here to hear it for yourself.

Cheers for the video, interesting watch. It would have been nice if our Veissmann accredited installer could have advised this. With regards the UVC, we just don't/won't have the room once the loft in converted

OP needs 35W HW output @ 14 LPM but a bit of a mismatch then for CH but the Viessmann has modulation ratio of 6:1 I think which means ~ 6 kw to CH at full turn down, still a bit high but not the end of the world with a bit of cycling, if he could be happy with say 9/10 LPM HW then a 24/28 kw unit would do with a turn down to ~ 4kw for min CH.
I see its already installed but may work out OK.

Thanks for the info.

So, I checked the tap on the garage again and I was correct, 20 LPM at 2.25 bar. The flow is great.

We actually have another outdoor tap at the kitchen that is controlled by the main stopcock. I get exactly the same 20 litres at 2.25 bar at this tap, so it's not the main stopcock that is the restriction.

Guess I'm going under the floor or can someone recommend a plumber in Central Scotland?

Cheers

 

[John.g]

You may not be able to get any higher mains pressure where you live but its probably just the shower/shower head is the restriction, you might just try the HW flow rate from the shower with the shower head removed, depending on what you find, a new shower head might give you the required flow rate at 2.25 bar or else you should be able to source a shower+shower head that will give you the required flow rate at 1 bar differential head but you will have to stipulate that when sourcing one.

 

[Richard Lindsay]

You may not be able to get any higher mains pressure where you live but its probably just the shower/shower head is the restriction, you might just try the HW flow rate from the shower with the shower head removed, depending on what you find, a new shower head might give you the required flow rate at 2.25 bar or else you should be able to source a shower+shower head that will give you the required flow rate at 1 bar differential head but you will have to stipulate that when sourcing one.

Just checked the shower without the head on and it delivers 10 LPM on the hottest setting. Absolutely awesome, thanks a lot.

Where would i get a shower head that supports those kind of rates?

 

[John.g]

Nowhere! because a shower head has to have some restriction to give "jets of water" so you will have to settle for a compromise, maybe say 9/10 LPM. You should try and source a gravity type shower head that will give you the biggest flow with the least resistance, something like 6 LPM @ 0.2 bar which will give you 9 LPM @ 0.5 bar which is the ~ differential pressure available after flowing through the boiler and shower.

I just did a test there with shower head (off my Mira Sport 9 kw electric shower) on gravity flow and I got 7 LPM @ 0.25 bar which is almost 10 LPM @ 0.5 bar so I would think that you should realistically be able to achieve that with the proper head.

 

[Richard Lindsay]

Nowhere! because a shower head has to have some restriction to give "jets of water" so you will have to settle for a compromise, maybe say 9/10 LPM. You should try and source a gravity type shower head that will give you the biggest flow with the least resistance, something like 6 LPM @ 0.2 bar which will give you 9 LPM @ 0.5 bar which is the ~ differential pressure available after flowing through the boiler and shower.

I just did a test there with shower head (off my Mira Sport 9 kw electric shower) on gravity flow and I got 7 LPM @ 0.25 bar which is almost 10 LPM @ 0.5 bar so I would think that you should realistically be able to achieve that with the proper head.

Great. Should I also look at a wider bore shower hose?

 

[John.g]

Yes, a good idea, everything helps.

 

[John.g]

Actually a very good idea...
If you can get a 12mm ID hose it will almost compensate for the head loss through the shower head even if a 10mm is already fitted, should give at least 2/3 LPM more than on the 10mm hose.
https://www.amazon.co.uk/d/Showers-Shower-Parts/12mm-luxury-large-shower-1-5M/B006WLNXQ4

 

[Rob Foster]

A shower power booster from Flowflex might be good they dont tale up much space
Rob Foster aka centralheatking

 

[Richard Lindsay]

Actually a very good idea...
If you can get a 12mm ID hose it will almost compensate for the head loss through the shower head even if a 10mm is already fitted, should give at least 2/3 LPM more than on the 10mm hose.
https://www.amazon.co.uk/d/Showers-Shower-Parts/12mm-luxury-large-shower-1-5M/B006WLNXQ4

A shower power booster from Flowflex might be good they dont tale up much space
Rob Foster aka centralheatking

Thanks a lot for the suggestions.

So, I removed the little restrictor from inside the shower hose and what a difference. I had an old Mira Excel and the shower hose has an internal diameter of 11mm. Would it still be worth it going up to the 12mm?

I also noticed that one of the washers is missing and water is running down the outside of the hose. For some reason I cant find a replacement. The washer at the other end has an external diameter of 18.5mm and an internal diameter of 11mm. Every shower washer I find is smaller internally.

Cheers