Are large boilers inefficient if not working flat out?

[outmyway]

Are large Kw boilers inefficient if not working flat out?

The reason I ask is that a plumber said I should avoid getting an over powerful boiler becuase it would be inefficient - Is this true?

I currently have a Potterton Kingfisher RS80 rated up to 23Kw and it really struggles in the depth of winter.

I'm also having all my rads and pipework replaced and was thinking of a Greenstar 40CDi rated up to 40Kw. If it wasnt running flat out would I loose out?

Its a 3 storey 4/5 bed house with 17 rads.

 

[doitall]

Improving the insulation would be a good plan.

You're not thinking if the CDi combi boiler are you.

Are the radiators sized for severe weather.

How long to you run the heating each day.

I agree with the plumber, too big is not always good, although they do modulate down, you certainly shouldn't need anything over 30Kw for a system boiler.

 

[outmyway]

You're not thinking if the CDi combi boiler are you. icon_eek.gif

No was looking at these
http://www.worcester-bosch.co.uk/homeowner/products/gas-boilers/greenstar-40cdi-regular

But you have got me thinking ... Does a system boiler have the pump and expansion vessel for the heating integral?
So one of these Greenstar 30CDi System boilers would be better?
http://www.worcester-bosch.co.uk/homeowner/products/gas-boilers/greenstar-30cdi-system

How long to you run the heating each day.

I want a quick heat up time, but hear modern condesing boilers are better running cooler?
Maybe my existing problems are to do with existing single pipe system and rads which will be replaced. Plumber has measured room sizes etc.

I agree with the plumber, too big is not always good, although they do modulate down, you certainly shouldn't need anything over 30Kw for a system boiler.

I see the biggest WB system boiler is 30Kw and regular boiler 40KW ... why is that?

So a 30KW boiler for a large 5 bed house should be OK?

 

[doitall]

A system boiler has all the bits built in, whereas a regular boiler you need to add the pump, system controls and expansion vessel if you go for a sealed system.

A system boiler will be more unreliable than a regular boiler as it has more cheap manufacturer specific parts to go wrong.

You're heating guy should be able to calculate the new radiators and supply a boiler to heat them.

Nothing wrong with single pipe systems, unless you want a quick heat-up and only run the boiler for a few hours a day.

 

[outmyway]

Thanks doitall.

My plumber can offer the WB 5yr warranty, so at least on a system boiler all the bits should be covered.

The existing single pipe system takes ages to heat up, ripping up the floorboards anyway to replace with ply so thought whilst I am at to go for new twin pipe system and modern rads with TRV's.

 

[Agile]

You or he should calculate the whole house heat loss before considering a boiler.

If it needs 20 kW then presumably its at least a four bed detached house?

Tony

 

[gas4you]

Its a 3 storey 4/5 bed house with 17 rads.

 

[Agile]

Well those are usually terraces with 40% less heat loss if both neighbours uses heating.

About 20% less heat loss of end terrace.

Always assuming there is good loft insulation!

Tony

 

[outmyway]

Its a detatched 3 storey house, built on a hill.
Looks two storeys from the front, but has a basement sort of area underneath at rear garden level.

Typical 1970's wimpey box construction with zero insulation. UPV double glazing.

I have a little in the loft, and have considered cavity wall insulation.

It seems to leak heat at a scary rate

 

[compheat]

I cant see why a system heat only boiler is more unreliable than a open vented heat only boiler.
Most are fitted with grunfoss pumps, and expansion vessels are usually Zimet.
The Intergas HRE boiler as been mentioned on other threads can be used on different types of systems, open vent or sealed system, the expansion vessel can be sited internally or externally if required and can be used as a combi.

 

[D_Hailsham]

Its a detatched 3 storey house, built on a hill.
Looks two storeys from the front, but has a basement sort of area underneath at rear garden level.

Typical 1970's wimpey box construction with zero insulation. UPV double glazing.

Use the Boiler Size Calculator to find out what size boiler you need.

I have a little in the loft, and have considered cavity wall insulation.

Get that done first. It will mean a smaller boiler and lower gas bills.

Run the calculator with your current insulation then with full loft and cavity insulation and see the difference.

There are many schemes to supply free or low cost insulation. Use Energy Savings Trust Grant Search to find them.

If there is someone over 70 living in the house you can get loft and cavity insulation for free.

 

[outmyway]

Thanks D_Hailsham for those links.

Interesting about too large a boiler cycling all the time.
Thats answered my orginal thread question.

Surprised I can get cavity and loft insulation for £200.

I had another house cavity insulated and always swore I would never again - they made no effort to match the mortar to fill the holes. My external walls looked they had chicken pocks!

I'm also wary of filling the cavity in case of damp issues, but gues the modern cotton wool looking balls they pump in dont absorb moisture and still let the wall breath?

 

[D_Hailsham]

Surprised I can get cavity and loft insulation for £200.

The previous government was responsible for encouraging the cheap deals.

But don't worry where the money comes from, get it done asap.

I'm also wary of filling the cavity in case of damp issues, but gues the modern cotton wool looking balls they pump in dont absorb moisture and still let the wall breath?

According to The National Insulation Association, "These materials are all resistant to water penetration and will not transmit water across the cavity or from below d.p.c. level by capillary action." So any damp problems must be due to another cause.