Why is my DHW lower than stated?

[John D v2.0]

But why can't it deliver more at 40C?

Because that's the amount of water that can physically get through the boiler given the available incoming dynamic pressure. Changing the boiler temperature doesn't cause any physical change in the water path through the boiler. Plate heat exchangers have a higher resistance than a same length bit of 15mm pipe

 

[ChrisZx]

Thinking again again, spec says I think 11l/min at 50C rise. So 13l/min at 55C when it's 0C air temp outside seems pretty good!

 

[ChrisZx]

Gas rated again tonight at full temp and got 35kw so seems like the boiler is going full pelt.

But running through hot taps with dhw off gave us 12l/min, whereas running through cold taps gave us 18l/min. So seems like it is lack of pressure...

Asked our installer if he measured the pressure before installation and got "No. You specified a combi". Actually he said we should go with this model combi.

This is why I never use tradesmen for anything unless in the most dire need.

Any point in upgrading water mains? It's 3/4 coming up in the kitchen, and our flow is fine so I suspect not.

 

[Dan Robinson]

Do you know the difference between pressure and flow?

Perhaps you should learn before jumping on your installer?

 

[ChrisZx]

Do you know the difference between pressure and flow?

Perhaps you should learn before jumping on your installer?

So what have I got wrong?

You don't think he should have checked the suitability of the proposed design? You think it's up to the customer to spec their own systems?

 

[Dan Robinson]

You don't think he should have checked the suitability of the proposed design

Yes. But that's not to say he didn't.

You think it's up to the customer to spec their own systems?

It's up to them to check what they're paying for and ask questions.

So what have I got wrong?

My question was a que for you to go and research.

 

[dilalio]

What system did you have before the install?

 

[ChrisZx]

Yes. But that's not to say he didn't.

He has explicitly told me he didn't.

It's up to them to check what they're paying for and ask questions.

What a thing to say. So you think a garage is perfectly at liberty to fill your oil sump up with chainsaw oil if you don't spec your own oil, or ask them what they're going to put in?

People pay professionals for their professional advice and experience. Would you go through the calculations of a structural engineer yourself to see if they are correct? We told him we were thinking of an economiser 39 so we could run two showers. He said let's go for the 35. I trusted he knew what he was talking about.

My question was a que for you to go and research

I know the difference and as far as I'm aware understand pressure. I continue to note you haven't come up with an explanation as to the cause of the poor boiler performance, so how are you judging my diagnosis to be incorrect?

I'll try again, I have a boiler connected to a cold supply that gives 20l/min. Boiler has enough gas to give 16l/min out at 40C, but won't give more than 12l/min. Even when giving 12l/min out, if I turn a cold tap on, the boiler output drops, even though total flow is below 20l/min. Even with boiler turned off, can't get more than 12l/min out of the hot taps.

So as far as I can guess, it has enough flow available and enough gas, so it must be not enough pressure to overcome loses in the boiler and still deliver enough flow.

Where is my mistake?

What system did you have before the install?

Vented with cold run off mains. Hot water supply was terrible, ran a little 12V shower booster for the shower

 

[Dan Robinson]

What a thing to say. So you think a garage is perfectly at liberty to fill your oil sump up with chainsaw oil if you don't spec your own oil, or ask them what they're going to put in?

Do you buy a car without researching its performance and suitability?

 

[chrisw99]

What is the boiler flow rate for DHW specified as?

Mine is rated 12 l/m. It doesn't matter whether 12 or 20 or 100 l/m goes in, 12 l/m will go out. This is due to the way the water flows through the fins of the heat exchanger, etc.

Look at it this way, if you have a bucket with a hole in the bottom, it doesn't matter if you are filling that bucket at 10 l/m or 100l/m, it will be coming out the hole at the same rate.

 

[John D v2.0]

Look at it this way, if you have a bucket with a hole in the bottom, it doesn't matter if you are filling that bucket at 10 l/m or 100l/m, it will be coming out the hole at the same rate.

A boiler is not like a bucket.
A bucket will fill up as it has air at the top that can be displaced. Your boiler is sealed so can't displace the air.

 

[John D v2.0]

so it must be not enough pressure to overcome loses in the boiler and still deliver enough flow.

Where is my mistake?

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.

 

[chrisw99]

A boiler is not like a bucket.
A bucket will fill up as it has air at the top that can be displaced. Your boiler is sealed so can't displace the air.

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.

 

[ChrisZx]

Do you buy a car without researching its performance and suitability?

If asked a car salesman for a car to transport my family of 5 about in and he sold me a 4 seater, think I'd be pretty miffed.

Or if I said I want one that does 0-60 in under 5 seconds and he sold me one that did it in 7, I'd be equally as cheesed off.

Do you let a structural engineer decide what size beam you require for knocking a hole in the wall, or do you tell him?

 

[ChrisZx]

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.

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?