DHW flow rate query

[beefster]

My fitter has queried my boiler choice and would like views...

i had gone for the Worcester 30 CDI.

Design value for heating requirements is 21kw.
Only DHW feed from this boiler will be to a shower room containing a thermostatically controlled shower and a basin.

He thinks the 28i junior would fit the bill plus physically smaller in the kitchen. Says i could save a few bob?
I am tempted only real differance i can see that would effect me is... 11.1 litre flow rate instead of 13+ with the 30CDI.
24KW heating o/p should be ok.
No control on DHW temp - do i really need it?

It makes sense to me but... would appreciate an expert opinion? is the 30 CDI worth the extra money, would i re coup it with energy savings additional to the 30CDI, modulating pump etc. Seem's more to go wromg really....

Any thoughts most welcome.

 

[Agile]

Your figures are not correct or comparable.

The power difference is 28/30 thats a difference if about 6 percent. Not much!

More interesting is if you really need 21 kW of heating! Is it a 5-6 bedroom detached house?

Tony

 

[Agile]

Your figures are not correct or comparable.

The power difference is 28/30 thats a difference if about 6 percent. Not much!

More interesting is if you really need 21 kW of heating! Is it a 5-6 bedroom detached house?

Tony

 

[beefster]

Thanks Tony...
used
http://www.idhee.org.uk/calculator.html

The junior 28i has a 24KW heating o/p according to the bumf I have so slightly more than the 6% differance.

"Central heating output: 24kW
Domestic hot water flow rate: 11.4 litres per minute at 35 degree temperature rise (provided adequate mains pressure is available)"



Its a 4 bed link detached converted bungalow, so virtually detached.... I will double check my figures as they seem wrong from what you infer... There is a lot of glass so that may be the prob. But it looks like the 24kw heating O/P will be more than adequate. As you didnt mention the DHW flow rates I assume you think the reduction from 13 to 11.3 would not impact on the shower room?? Previous boiler offered 13L as does the 30CDI.


Would seem my fitter is spot on and saving me some cash. Found a good one Appreciate the second opinion though.

 

[ianniann]

24kW is very high for a 4 bed detached anything. Re-check the calculation, or consider whether you need some insulation.

In any case, you can't size a combi to your heating needs (unless they are crazy high) because most people would like 30kW to heat their water but less than 10kW to heat the house.

Indeed the 28i provides heating output in the range 7.2kW-24kW. The 30CDi provides heating from 7.7kW-30kW. That is not the important difference though. Either are plenty big enough to heat your home (if not you really need some insulation!) and the hot water outputs are quite close. The difference is in quality and controls. The CD range is much better build quality with a much larger heat exchanger and more flexible controls. You will instantly see the difference side by side with the CDi being substantially bigger and much heavier. So heavy in fact that it can't practically be fitted by one person. If you want to save a couple of hundred on the initial installation, or you are really tight for wall space, then the 24i will perform similarly to the 30CDi, but otherwise I'd choose the CDi every time. And an installer with a mate

One important control difference, which might save you a fair amount of money in the long run, is separate hot water and heating temperature controls on the CDi. Another feature that Worcester likes to flag in their literatire is that the CDi range "condenses in hot water mode". In reality there is no fundamental difference between the two, but the larger heat exchanger and a fairly low cap on hot water output temperature (about 65C maximum) means the CDi will on average condense a little better than the junior. Possibly the most important difference that most people will never notice is the modulating pump on the CDi range, which saves electricity and reduces noise, especially when TRVs start to shut down some radiators. Seems trivial but 20W-30W less can be significant if you're operating your heating for many hours a day at a fairly low temperature as is ideal for condensing boilers.

 

[Happyplumber]

The most important thing is the junior is Basic budget model made solely for the UK market and not sold anywhere else in europe because they would never be able to get away with it!

You pay for what you get and the heat engine inside the CDI model is far superior in comparison thus slightly more expensive

Your choice is the correct one and your installer should know better !


Just a quick edit ,its not possible to time it better,have read and enjoy your "recommended junior boiler"



//www.diynot.com/forums/viewtopic.php?t=189915

 

[MikeyGas]

Have you not thought about going for the 27CDi instead?

 

[beefster]

Thanks chaps, think the extra for the CDi will be worth it but the 30 is oversized for what i want.

Calculations where indeed wrong.... must get the decimal place in the right position.... doh! Easily spotted when your doing it all the time, I suppose you lads can look at a house and say thats going to be approx 12KW or whatever.

Given all of that i am prone to move to the smaller CDi. I did say to the guy I was on the border line with his estimate so maybe he was offering me a way out... to be fair to him.