'Range Rating' a combi

[bster]

I have just got back from the C&G Energy Efficiency course and was told that combis should be adjusted to the correct output for the rads it serves, this apparently does not effect DHW output.

The question is do all combi's allow adjustment down to the correct output for the system it serves and how do you know what burner pressure equals what kw output?

 

[Agile]

As far as I am aware ALL combis have an adjustment on the max CH power. Details are given in the MI.

The problem is that 90% of installers are so dumb that they dont think its necessary and dont do it. At least one manufacturer does not include it in the MI because all the dumb installers cock it up. If you sound intelligent they tell you how!

If you dont then its quite likely to deposit limescale in the primary HE from the initial fill.

I much prefer giving you advice than seeing you trying to give advice to others!

Tony Glazier

 

[bster]

Cheers Agile, maybe i'll stop ''advising' for a while.

I'd like to say that i have learnt so much from your replies along with 'The Scruff','Billy the Kid etc (mostly on plumbingpages) and that for anyone just getting into plumbing i suggest spending an hour an evening going through the threads as its not as tedious as reading a book (better to use them for reference).

 

[chrishutt]

suggest spending an hour an evening going through the threads as its not as tedious as reading a book

Gee, thanks .

 

[bster]

meaning?

 

[Agile]

i suggest spending an hour an evening going through the threads as its not as tedious as reading a book .

I think that I would find reading a book considerable less tedious!

Tony

 

[bster]

maybe you spend to much time on the net!

 

[bster]

Agile - back to original post why do so many installers bmanage to cock it up and what causes the lime scale build up on initial fill- is it the fact that the boilers return temp is to high or what?

cheers again

 

[Agile]

The limescale occurs when a 30 kw boiler comes on full power with only 5 kW of rads in a small flat.

For a few seconds the 30kW heats the water in contact with the heat ex and deposits the lime scale. This happens because there are a few seconds before the sensors warm up and turn the burner down and even then there is latent heat in the hot fins.

In the extreme it can sound like a jumbo jet!

If the boiler is set to 5 kW output then it jently heats up the water without boiling it.

As to why most installers cock it up then I dont know! Many seem to think that when you fit a boiler you dont look at the MI and you dont make any measurements or fill in the Benchmark. Last month I saw a band A combi fitted in 6½ hours! Getting them to adjust a gas valve they use stilsons and mangle it up.

Modern boilers are precision devices! Would you repair a computer, TV or video camera with stilsons?

Tony

 

[bster]

i know that our company does not adjust the the CH output and i'll be 'advising' on our next install! - did you say that the output figures and related gas pressures our stated in the MI's?

How are you so knowledgable - you should be a college tutor, more money than a plumber probably! or are you a boiler\system designer of sorts?

 

[Agile]

I fix boilers because I enjoy the chalenge of finding faults. To do that I have to understand how they work so that when they dont I know what to look for. My electronics background is very important.

Training in the colleges is not very well paid and is distressing because of the lack of organization and communication internally. The private training firms are much better and it must be good to get people through the ACS.

What make/model does your firm instal ? I will look up the MI and point you to the CH setting section.

Tony

 

[bster]

I'm with you on the disorganisation of the colleges- it frankly takes the p**s but i wont bother you with my endless stories of college incompetance.

We mostly fit Worcester Greenstar, Baxi 105HE and Glowworm CXi, As you can see they seem to think combis are the only boilers available i really do beleive that we should be installing unvented DHW fed by a sytsem\combi boiler in some situations but nobody listens to me because i am an apprentice that knows nothin (they might be right).

By the way whats the difference between the Gloworm SXi (system) and HXi ranges - i understand what system boilers do but but the SXi is called a heating only boiler but goes on to mention DHW as well, im confused.

Think i've got it - the HXi has no pressure gauge and exp vessel - i think.

 

[bster]

sorry for the multiple posts

You say that - lime is deposited when burner ignites on max output with a smaller rated CH system..but if its only for a small period doesnt this happen even if the CH requirements meet max boiler CH output. Or is it the CH thermistors that are (in theory) causing the damage as they dont but will kick in before max output is acheived because a 'softer' start up from the burners gives time for the thermisters to warm up to operating temp-acheivable by reducing output by 'range rating'. How is limescale not deposited on a 30kw rad system fed by a 30kw boiler?

does that make any sense because i think i've just confused myself?

 

[Balenza]

bster wrote

You say that - lime is deposited when burner ignites on max output with a smaller rated CH system..but if its only for a small period doesnt this happen even if the CH requirements meet max boiler CH output.

Good question bster and a perfectly valid one. I am left wondering why your query was not answered.
Can anyone answer this ?.
Also , what purpose does the CH pump fulfill whilst the burner is igniting on full tilt either on the 5kw rad system or the 30kw rad system. ?

 

[gas4you]

Back to op. No, not all combis allow physical range rating, most rely on modulation to control this. Obviously there are some that do, including Vaillants I fit, but others such as Ideal do not.

It is slightly easier when fitting a system boiler or open vent boiler as you can install the one with the closest output to what the property needs, but I always range rate all of mine