gas rating heat input

[piperdave]

doing my acs at the moment just a general question when you put a boiler on calibration mode for example the baxi duotec combi and you have worked out your gas rate and heat input calculations , what input do you refer to in the manual to compare it to ? i know its gross but is it the CH or DHW ?

 

[GrahamM]

When you put the boiler into calibration mode, do you then turn on the hot tap to take your reading?
It is giving you max rate on the CH.

 

[piperdave]

no the hot tap was not on it was just set to calibration mode, so it looks like i should be looking at the CH max input gross am i correct ?

 

[GrahamM]

Spot on!

 

[tamz]

As the max outputs to heating and water are different, when put into calibration mode the boiler to full ie maximum power so it is the higher reading which you will compare (the water.)

 

[GrahamM]

I thought that as the calibration mode is limited to the max temperature limits of the CH then your readings would be taken based on the CH input value. Which as it goes is the same as the HW input anyway. Come to think of it, a lot of the HE boilers have the same CH vs HW input figures now.

Apologies for the miss information piperdave. tamz got few more years under his belt than myself so would go with his info.

 

[piperdave]

thankyou very much for the replies , but im confused now ..so which ever number is highest out of the CH and DHW is the one you use ?, is that correct ?

 

[tamz]

Most of the band A's includung the duotec have ch inputs lower than the HW. At full rate go with the HW input.

The duotec mi's states expect a gas rate of 2.61m3/hr (24kw boiler)

Multiply this by 10.76 to convert to kw/hr = 28.08kw.
Look up the technical data
CH input = 22.7kw
HW input = 27.4kw

Full rate is the HW figure (near enough)

 

[piperdave]

ok thankyou very much you have been a massive help!, thankyou for the quick replies

 

[GrahamM]

Most of the band A's includung the duotec have ch inputs lower than the HW. At full rate go with the HW input.

The duotec mi's states expect a gas rate of 2.61m3/hr (24kw boiler)

Multiply this by 10.76 to convert to kw/hr = 28.08kw.
Look up the technical data
CH input = 22.7kw
HW input = 27.4kw

Full rate is the HW figure (near enough)

Sorry tamz. Unless my MI listings are incorrect and better to find this out now then:
Baxi Duo Tec Combi 24 HE: CH input= 27.4kw(gross), HW input=27.4kw(gross)

and just for comparison:
Vaillant turbomax plus 824/2: CH input = 26.7(net), HW input = 26.7(net)
WB Greenstar 30CDi : CH input = 30.9, HW = 30.9

Not saying all boilers have the same CH, HW inputs but some of the usual suspect do.

 

[tamz]

Hi Graham

Here is the duotec mi's page



As you can see the CH input is less than the HW

The CDi's are the same although the 27 has both the same.



Generally, especially on bigger combis the CH input is less than the HW.
If you fire it at full rate using the commisioning button it will fire at its maximum.
CDi's put the output to water and heating when in service mode so it is a good idea to run the taps while checking it to keep it running.

One to watch for.

 

[amy]

Hi,
When I was learning this in class, we spent a couple of hours reading through MI's on several different boilers, putting them in full mode via reading through pages & pages of literature and pressing multiple buttons on some boilers......THEN, our tutor told us to just turn the tap on full rate to get exactly the same effect, this is how I have tested for months now and always get a spot on reading,,,,, I take it this is ok to do.

 

[Toddyplumb]

Of course its Ok to do . When you work on a particular make (or 2) of boiler using the inbuilt max rate function does become second nature though

 

[tamz]

So what do you write on a benchmark when filling out gas rate for CH?

 

[Toddyplumb]

I just log flow and return teperatures , and for the gas rate write " Modulating "
Obviously I do the max rate in the HW section