Condensing bolier flow and return temps - effect on rad sizing?

[Rory]

Had a quote for complete new CH system including Worcester Greenstar 28i Junior.

Not sure I totally understand this, hence asking here: I was pretty surprised at how small the rads spec'd were as I thought condensing boilers needed as low a return temp as possible? The installer (WB accredited) is telling me that once the boiler gets going there will be hardly any difference between flow and return temp, and the rads are rated assuming flow temp of 75C.

Am I mis-understanding or does this seems far too high? Indeed the boiler's user guide suggests running the CH at between 1 & 2 on the dial - around 50C - and it also says "the radiators may need upgrading", which I assumes means "replaced with bigger ones!".

 

[diamondgas]

Hi Rory - Do you have concerns about the installer? If not then trust his professional decisions; if you do then get other quotes IMHO

 

[LPC]

Had a quote for complete new CH system including Worcester Greenstar 28i Junior.

Not sure I totally understand this, hence asking here: I was pretty surprised at how small the rads spec'd were as I thought condensing boilers needed as low a return temp as possible? The installer (WB accredited) is telling me that once the boiler gets going there will be hardly any difference between flow and return temp, and the rads are rated assuming flow temp of 75C.

Am I mis-understanding or does this seems far too high? Indeed the boiler's user guide suggests running the CH at between 1 & 2 on the dial - around 50C - and it also says "the radiators may need upgrading", which I assumes means "replaced with bigger ones!".

My opinions:

28i Juniors aren't very good. I'd recommend going for a Worcester CDi (if you want Worcester) unless it is a flat/rental property. Personally in that price range a Vaillant Pro 28 installed by a Vaillant Advance installer (5 year warranty instead of 2) is a much better buy.

Unless you live in a very large property I wouldn't worry to much about F+R temps. You can size the radiators yourself by googling 'radiator sizing' and measuring your rooms.

Worcester Accredited means absolutely precisely nothing. Just means he installs a fair few boilers. It is no indication of quality, and in most cases you are paying for that extra warranty indirectly. Just so you know.

 

[newcastle phill]

Condensing boilers operate at maximum efficiency with a low return temp.
Most radiator manufacturers quote a delta which is based on temps for non condensing boilers and need calculating but with some this is changing.
Typically a lower delta will in effect increases the size of the panel.

Ill see if I can find a link that explains in more detail a little about the general idea

 

[newcastle phill]

http://blog.tradeplumbing.co.uk/the-mystery-of-delta-t-revealed/

Sort of a bit here.
But to be frank I think delta 50 is even too high.
Delta 40 is getting there more where condensing boilers need to be.
And some heating engineers even calculate more towards a delta t of 30-35 ish.

Simply put if the return temp is too high the boiler wont condense fully and the efficiencies will not be realised

 

[Rory]

Thanks for the responses.

[FONT=&quot]Do I have concerns about the installer? Hmmm….he’s very local, recommended etc. I’m confident he can do the job, I’m just not sure he “got” the more technical aspects. I did get other quotes but central heating installers seem to have very strong and fixed opinions![/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]Re the boiler – I understand the 28i Junior was updated last year. The installer says they fit a lot of them and will be guaranteed for 8yrs (although if it fails even at that point I’ll be a bit dismayed). Other installers said they wouldn’t touch WB with a barge-pole. One said he only fits Glow-worm – not a name that comes up much in discussions these days – take it or leave it.[/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]Thanks for the link about Delta-T. I used to be an industrial electrician, although I’m well out of trade now, but I’m used to having to calculate stuff like this – you can’t just throw a long bit of cable in and hope for the best! My reading is that you really need to substantially oversize rads to be able to run at lower flow temps.[/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]One of the issues with the current installation (which might be single pipe) is the return gets very hot, very rapidly, so the boiler shuts off for 3 mins and everything goes cold. Rinse and repeat. This means the house gets warm in series of very brief steps. I explained that to the installer and I really want to make sure that doesn’t happen in the new system.[/FONT]

 

[Ermintrude]

We use 35! And yes it should be less than 63c !

 

[Rory]

We use 35! And yes it should be less than 63c !

So for Delta-T 35 that would be a flow of say 60C and return 50C?

And sorry, but what should be less than 63C?

 

[doitmyself]

Had a quote for complete new CH system including Worcester Greenstar 28i Junior.

Not sure I totally understand this, hence asking here: I was pretty surprised at how small the rads spec'd were

How old is the house, how large and how well insulated?

Did the installer do a detailed heat loss calculation? Has he provided this info? If not ask for it.

I thought condensing boilers needed as low a return temp as possible? The installer (WB accredited) is telling me that once the boiler gets going there will be hardly any difference between flow and return temp, and the rads are rated assuming flow temp of 75C.

1. The return needs to be below 55C for any condensing to occur.
2. The lower the return, the more condensing occurs.
3. If a flow of 75C has been assumed and a return of 55, the rads will be producing about 85% of the output given in the catalogue, so the have to be "oversized" by 17%, say 20% for safety.

 

[wattsie1987]

The boiler should modulate and adjust pump speed according to the heat demand and keep itself condensing. Trust your installer. Your property will be loosing more heat than you will save through flue gases.

 

[Ermintrude]

The boiler should modulate and adjust pump speed according to the heat demand and keep itself condensing. Trust your installer. Your property will be loosing more heat than you will save through flue gases.

But the idea is to recover latent heat as it will save money. If my can had a hole in the fuel tank I wouldn't drive fast just because the losses from the leak exceeded what I could save by driving sensibly? What an odd argument?

 

[doitmyself]

The boiler should modulate and adjust pump speed according to the heat demand and keep itself condensing.

In most cases the boiler will modulate to keep the flow temperature constant at whatever value the installer/user has set. Most systems with modulating boilers do not have variable speed pumps.

 

[Rory]

Thanks again all.

The house is a decent size early 1970’s 3 bed end-terrace. It’s double glazed. Gable wall is cavity insulated. Has a lobby rather than a hall so the front door basically opens straight into the living room. However, like many houses of this age and older, they weren’t designed to be sealed boxes so it’s kept well ventilated.

The installer did measure the rooms and used a rad size calculator but came up with that seem to me to be very small sized rads, especially in the bedrooms. At the moment I’ve convinced him to use larger ones more based on appearance than anything else.
The Junior doesn’t have a modulating pump but other WB models do. I did wonder about this – like all aspects of CH systems, there seem to be very polarised opinions on whether modulating pumps are a good thing or not!

 

[Ermintrude]

I'd say 30% of new installs do. ( ststems and Heat only)

 

[wattsie1987]

Maybe its because I only fit the high end boilers. Some will learn the system size and modulate accordingly thus keeping it at optimum dew point for longer. Oversizing rads is fine but will cause intense heat up periods. Balancing the system will help a lot with better condensing. I would personally go for the best boiler and controls you can afford.