Planning New Boiler an Rads

[looneyfitter]

running the rads say 60/40 is effectively reducing the heat ouput of the rads, so fitting a slightly smaller boiler will have the same effect only it will work harder but more likely to be in condense mode.

What does AFAIK mean bud\?

 

[vulcancontinental]

When I first read the undersize the boiler theory and saw mickyg’s reply I smiled then thought about the lower return temp and saw what you’re aiming for.

The problem with the theory is of course when you need the heating most at times of peak demand the boiler cannot supply the output required. Granted the periods of max heat are infrequent but are you installing to keep the client’s boiler in condensing mode as much as possible or to keep the client warm when he/she needs it most?

I can hear the conversation with a client “yes, I realise you’re cold this January but I remind you your boiler is working more efficiently in September, October, March and April”.

WC is to reduce the operating flow and return temps as much as possible to suit prevailing conditions without leaving the client short when they need max output.

Interesting.

BTW: 30KW heating is the equivalent of 30 K1:600:1000 Stelrad radiators! How many houses require that? Most three bed semis are around 10-12KW heating which means the vast majority of combis are grossly oversized for heating so the point is moot. I know there are a great many combi fans here. I mention that because nearly all the outputs given in earlier posts as examples are 24, 28, 30 and 40KW – common combi outputs.

On a light-hearted note the most efficient way to run the system for the customer is boiler off – it costs them nothing then.

The most efficient way to run the boiler is flat out continually with the output matching the demand – no downtime losing heat through the flue etc.

WC tries to hit a happy medium.

 

[mehran]

all this under/over size business is missing the point really.

what you want is a boiler that can modulate down as low as possible level but have it's max output greater/equal to the house heatloss.

for example you you have a house with 16kw heatloss then a boiler that can modulate from 6-32kw is better then one that can modulate form 10-16kw.

the 10-16kw boiler maybe correctly sized but that does not make it as good as the one that can modulate down lower then it....

 

[Diggory]

Ooooer - seems i have set a hare running.
All interesting stuff but may take me a while to get head around [as they say].
Just to pick up a few points back up the thread.
No - not fitting a boiler myself.
Yes - would clean with DS40 as have used both DS40 and superfloc [i think was called] on system in past.
Milder cleaner didnt really work so moved on to DS40. This did quieten things down for couple of years.
Yes - will power flush also [good idea].
& really would like not to change pipes - more the disruption than the cost.
Thanks D

 

[looneyfitter]

When I first read the undersize the boiler theory and saw mickyg’s reply I smiled then thought about the lower return temp and saw what you’re aiming for.

The problem with the theory is of course when you need the heating most at times of peak demand the boiler cannot supply the output required. Granted the periods of max heat are infrequent but are you installing to keep the client’s boiler in condensing mode as much as possible or to keep the client warm when he/she needs it most?

I can hear the conversation with a client “yes, I realise you’re cold this January but I remind you your boiler is working more efficiently in September, October, March and April”.

WC is to reduce the operating flow and return temps as much as possible to suit prevailing conditions without leaving the client short when they need max output.

Interesting.

BTW: 30KW heating is the equivalent of 30 K1:600:1000 Stelrad radiators! How many houses require that? Most three bed semis are around 10-12KW heating which means the vast majority of combis are grossly oversized for heating so the point is moot. I know there are a great many combi fans here. I mention that because nearly all the outputs given in earlier posts as examples are 24, 28, 30 and 40KW – common combi outputs.

On a light-hearted note the most efficient way to run the system for the customer is boiler off – it costs them nothing then.

The most efficient way to run the boiler is flat out continually with the output matching the demand – no downtime losing heat through the flue etc.

WC tries to hit a happy medium.

Hard to e plain my theory (30kw was just an eg)........ Basically if the rads are are oversi ed then under si e the boiler whick will inturn be working flat out, achieve the desired heatout put for the dwelling and be in condensing mode....

 

[mehran]

I think I understand what you are saying but you dont need to undersize the boiler just size it correctly.

for example if the house had smaller rads it would need to run the flow/return temp at 80/60 for example to get 30kw output...but oversized rads could be run at 60/40 or even 50/30 to get the same output from the rads.

the lower the return temp the better the efficiency of a condensing boiler.

I think your theory may come from b rated atmospheric boilers were they get there best efficiency when they are running flat out...were as pre-mix can maintain its efficiency through out its modulation range

 

[looneyfitter]

I think I understand what you are saying but you dont need to undersize the boiler just size it correctly.

for example if the house had smaller rads it would need to run the flow/return temp at 80/60 for example to get 30kw output...but oversized rads could be run at 60/40 or even 50/30 to get the same output from the rads.

the lower the return temp the better the efficiency of a condensing boiler.

I think your theory may come from b rated atmospheric boilers were they get there best efficiency when they are running flat out...were as pre-mix can maintain its efficiency through out its modulation range

 

[looneyfitter]

My theory maybe wrong but it has always been my understanding that it is a good idea to have over sized rads at a lower temp for longer.

Of course WC is the best option, but that isnt what the OP has asked for.

I am more than happy to retract my theory's if im completely wrong, but it makes sense to me.

Most installations are fixed controls and not variable temp

Just read through a few of my 1st posts, as wondering why im looking wrong. dont know what i was thinking. Long hard days and not reading peoples posts correctly & not explaing my thoughts correctly maybe?

 

[Norcon]

Perhaps theres arguments for both oversizing and undersizing.
Bloke I know (controls engineer) oversizes by about 40 percent on underfloor heating systems.
His argument is that when the controller goes into "boost" on start up with 350 odd sq/m of screed underfloor to heat up , the mixing valve gets what it needs in a very short space of time and the home owner has a toasty house in a very short space of time.