Why is my DHW lower than stated?

[John D v2.0]

I wasn't saying a boiler is like a bucket, I was pointing out a simple analogy that if something restricts a flow rate e.g. a hole in a bucket, then it doesn't matter what rate you put in at, it will always be limited by the restriction.

I see what you mean, however I still feel the analogy is misleading. A bucket is driven by gravity, so the higher the water level, the higher the flow rate. Also with a bucket the input and output don't have to match.
With a boiler the input and output match exactly.

Just simply say it's like a tap that's turned on half way. The water in and out are the same, but if you tee off before the tap you'll get a better flow rate than teeing off after.

 

[John D v2.0]

Ha fair enough, thanks.

We're doing a rear extension this year and replacing water main at the same time wouldn't be a big deal, but is there any point?

Get a water pressure guage. Measure the pressure with everything off.
Turn on a tap at the boiler inlet position delivering your desired flow rate eg 16l per minute then measure the pressure around the boiler inlet.
Check the pressure drop, and work out whether a 25mm mdpe pipe over the distance to the road would be significantly better.
Stop taps, elbows, water meters etc all add a resistance.
The should be some information in the boiler instructions saying the pressure drop off the dhw side at the rated flow rate. You'll need the pressure drop throughout the whole system added to the required pressure for your showers to be less than the pressure at the street.

There might be a simple rule of thumb the pros can help you out with, but if not get the figures and see.
The other option is just replace it anyway if you're down the hole digging anyway.

 

[picasso]

Your boiler has a flow restrictor fitted , if you want 16 lpm at 45c take it out, if you want 13lpm at 60c leave it in, as I said before 13ltrs in 13ltrs out.

 

[Dan Robinson]

You,re correct, don't worry about Dan, i think it's fair to say he just doesn't like people on here who aren't qualified gas engineers.

Not at all.

 

[oilboffin]

I'd listen to dan if I was you I'm of the same mind there doesn't seem to be a lot of thought given to this instal in fact the way things have worked out it's been like Fred carnos circus.Bob

 

[dilalio]

No good deed goes unpunished ​

 

[ChrisZx]

Your boiler has a flow restrictor fitted , if you want 16 lpm at 45c take it out, if you want 13lpm at 60c leave it in, as I said before 13ltrs in 13ltrs out.

I had missed that in the manual so thanks for that. Unfortunately I don't think it answers anything though, I took the restrictor out and even with dhw turned off still only get 13lpm out with 20 available out the filling loop. Manual says pressure has virtually no effect on the throughput. I would be perfectly happy with 11lpm at 60C if we had the other 9lpm available as cold water as we could mix them to get the temp we wanted. But when using hhot through boiler, our total flow doesn't add up to 20. So if using 5lpm through hot, then turn on cold tap taking 5lpm, our hot output will drop to say 3lpm or something.

We spoke to ATAG and they're sending a technician out on Thursday to have a look as they don't think it sounds right, fingers crossed I guess.

As an aside, I don't think that statement about the restrictor guaranteeing water at 60C makes any sense. We can select our desired temperature to be anywhere between 40 and 65C, and we do get out water at the correct temperature, I have watched the temp readout (and no I wasn't confusing it with CH flow temps)

 

[chrisw99]

What is considered an "acceptable" hot water flow rate these days? I've recently gone to a combi for the first time - mine seems to have a limiter of 10 litres/min (see recent thread) so whatever my cold flow rate, never going to get more than 10 l/m out.

I read the average UK bathtub is 1800 litres up to the overflow, so would take 18 mins to reach that but in practice if it was that full when I got in it would overflow, 1000 litres seems about right - 10 minutes. As a bath is a rare treat for me, I'm a shower person, I can easily cope with setting it going and waiting 10 minutes.

I'm interested in what you said about mixing cold to get the temp you wanted. I set my DHW water temp at the boiler to 49C (from memory, could be 47), based on it coming out at the exact temperature needed for a bath. So I just turn on the hot, come back in 10 mins and it's perfect.

But reading what you said, I could potentially turn up the DHW higher temp, it would still come out at 10l/m, and run some cold at the same time causing a faster fill. Assuming it does still come out at 10 when the cold is also on - time to do some experimenting this weekend!

 

[dilalio]

The ratio for mixed hot and cold for a shower is about 60/40 Hot:cold. For a bath it’s slightly different as it loses heat as it’s filling and more so if you don’t get into it straight away... but you would want to set your DHW temp higher as kitchen sink should be a higher temp than a bath/shower/basin.

 

[Wildcat1961]

I read the average UK bathtub is 1800 litres up to the overflow, so would take 18 mins to reach that but in practice if it was that full when I got in it would overflow, 1000 litres seems about right - 10 minutes. As a bath is a rare treat for me, I'm a shower person, I can easily cope with setting it going and waiting 10 minutes

1800 litres?
1000 litres?
What size bath have you got?
10 litres a minute would take 3 hours to fill your mahoosive bath.

 

[mcmoby69]

Could it be as simple as, too many balofix isolators fitted in combination with those crappy thin flexible monobloc tap connectors?

 

[ChrisZx]

Technician came today, measured cold flow rate out the garden tap and said "yea something wrong with the boiler, I'll tell ATAG and we'll be in touch"

Result I guess!