Why is my DHW lower than stated?

[ChrisZx]

Curious if anyone's got any ideas on this. I'll be speaking to installer as well, but still. Just had an Atag Economiser 35 fitted, supposed to get 16.1l/min DHW out at 35deg rise. I'll bullet point as there's a lot to take in!

• Cold input not an issue, 20l/min directly before boiler
• With output set to it's lowest of 40deg, get 13l/min directly after boiler (well after about 15cm of 15mm pipe and an isolation valve). It's cold today, 0deg air temp tonight, but would the input be much less than 5deg? Not sure that would account for a 20% drop anyway

So lack of gas I was thinking. But:

• With hot connected back up to taps etc, set the water to it's lowest output of 40deg, all taps on, get 12-13l/min
• But with hot set to 60deg (it couldn't actually get it over 53/55deg but hovered around 50-55deg), still getting 12-13l/min out

Err what?!? How can that be possible? I believe the water store is only 0.6l, so that shouldn't affect it much. Surely with enough input and seemingly no real restriction on the output of note, shouldn't the DHW rate fall with increased temp? The fact that it doesn't sort of indicates that there isn't a lack of gas to me....

I've run this test a few times now since Tuesday, it was about 10deg last week, and I've always got 12-13l/min out no matter what I do or when I do it.

Just trying to think it through before I talk to the installer in a few days time when he comes back to finish a few things off. Any ideas? I'm stumped!

 

[Dan Robinson]

Run a bath.

Gas Rate boiler.

Post results with measured flow rate.

 

[ChrisZx]

Ah never heard of that, thanks. Used 0.077m3 in 2mins, 12.5l/min, 40C output. That a long enough test?

 

[picasso]

if your getting 12-13 ltrs out you must have 12-13 ltrs going in or have I missed your point ?

 

[Dan Robinson]

It's not taking in enough gas. Bet the gas pipe is to small.

 

[Nozzle]

.

 

[ChrisZx]

if your getting 12-13 ltrs out you must have 12-13 ltrs going in or have I missed your point ?

I measured the output of the filling loop which is the same connection as the dhw input and got 20l/min so I assumed that was alright. Guess I could turn dhw off and see what comes out the hot output to see what the loses through the boiler are

It's not taking in enough gas. Bet the gas pipe is to small.

Thanks, thats what I thought, but then why doesn't my flow rate go down when I turn the temp up? Pipe run in only 3 or 3.5 meters, mostly in 15mm though

 

[ChrisZx]

.

That's gas!

Edit: Ha beat me to it

 

[Nozzle]

I twigged that when it was too late to delete. Doh!

Nozzle

 

[Dan Robinson]

why doesn't my flow rate go down when I turn the temp up

Why would it?

 

[John D v2.0]

Ok so this isn't like an electric shower where the temp control adjusts the flow and the power stays constant. The flow is varied purely by adjusting the tap.

55c at 13l/m sounds about right given the specified 35c rise, unless your water is coming in at 25c you wouldn't get 60c out at 16l/m. So probably not your gas pipe causing any issue there.

The internals of the boiler have some resistance, so you won't get the same open pipe flow rate as before the boiler. What is your (residual/dynamic) pressure before the boiler with the outlet running at 13l? What is the rated pressure drop off the dhw side at 13l/m?

PS I'm not a gas engineer

 

[muggles]

Ah never heard of that, thanks. Used 0.077m3 in 2mins, 12.5l/min, 40C output. That a long enough test?

That's about 25kW gross input. Are you certain that it's a 35 and not a 27 that's been fitted?

 

[John D v2.0]

Ah never heard of that, thanks. Used 0.077m3 in 2mins, 12.5l/min, 40C output. That a long enough test?

You need to set to max dhw temp and wide open taptap bef you gas rate it

 

[ChrisZx]

Why would it?

Because it takes a certain amount of energy to raise the temperature of a body of water. If it can raise 13l/min to 55C, it should be able to heat more water to 40C for a given input temp and gas input.

Guess the way to tell would be to gas rate at 40C and 55C and compare

Ok so this isn't like an electric shower where the temp control adjusts the flow and the power stays constant. The flow is varied purely by adjusting the tap.

55c at 13l/m sounds about right given the specified 35c rise, unless your water is coming in at 25c you wouldn't get 60c out at 16l/m. So probably not your gas pipe causing any issue there.

The internals of the boiler have some resistance, so you won't get the same open pipe flow rate as before the boiler. What is your (residual/dynamic) pressure before the boiler with the outlet running at 13l? What is the rated pressure drop off the dhw side at 13l/m?

PS I'm not a gas engineer

Yes I agree I would think 13l/min at 55C is to be expected. But why can't it deliver more at 40C?

I don't know the pressures I'm afraid, but as I said I measured the output almost directly from the boiler output, so virtually no resistance after the boiler. I think I'll measure the output through the hot with dhw turned off, should tell me if the boiler itself is restricting it.

That's about 25kW gross input. Are you certain that it's a 35 and not a 27 that's been fitted?

Worth a double check!

You need to set to max dhw temp and wide open taptap bef you gas rate it

Yes I only realised the point of the test after I did it, makes sense now if you want to measure max usage

 

[ChrisZx]

Now I think again after the above, if 55C at 13l/min is to be expected then it should be using I think 40kw of gas, so will gas rate again tonight to check that.

If that checks out then at that increase we are getting the max out of the boiler. It should be more efficient at a lower temp increase I think though, and a lower output wouldn't burn our hands so would still be good to get to the bottom of it