Boiler for large 5 bed house

johnacky · 14 Oct 2010

Can anybody give me any advice on a boiler for a large 5 bedroom house, with two bathrooms, and cloakroom toilet please.
Space is limited, so looking at a combi boiler, such as the Worcester Highflow 550 cdi, or 40kw Baxi Platinum.

Think it will have to be a combi, as space is very limited and its looking to be fitted under the staircase.

Many thanks

mickyg · 14 Oct 2010

measure your mains water flow rate from the kitchen or garden tap or both first. This will determine what maximum power boiler you can get away with. Pretty much all combis will be able to satisfy your heating demand so its really about sizing it for hot water.

Use a bucket and a stopwatch to measure litres per minute flow rate.

johnacky · 14 Oct 2010

Just checked water flow rate the main which has been reduced to 15 mm at the rear of the property is providing 16 l/m. If the lead water pipe was increased to say, 22mm would this give much of an improvement in flowrate.
many thanks

Agile · 14 Oct 2010

You have only measured the open pipe flow rate. the dynamic flow rate is often about 65% of the open pipe.

You need about 20 li/min dynamic flow.

Upgrading the supply from the street will help a lot but all depending on what you have now.

Tony

mickyg · 14 Oct 2010

it would be worth upgrading the supply,but you'd best get some professional onsite advice for that. With 16l/min any storage style combi would be a waste of money. A 35-38 kw combi will provide 14l/min which is about as good as you can hope for with your current flow rate, even then I would be warning the customer you may not get the max flow from it.
an ATAG A325ec would be my choice with the current flow.

aelitaman · 14 Oct 2010

First off I am not a plumber or an expert so I can only give you my view of living with a large combi in a large house.

I have a worcester bosch highflow 400 (smaller than the one you mentioned)

Mine is meant to have a max flow rate of 18l/min I measured mine and I get 12 l/min in the summer and 9 l/min in the winter (colder incoming water I assume).

This is just enough to run two taps at the same time.

For a shower I have a victorian style jobby with the shower rose above my head. This is 8 inches in diameter, so not small. No way can you describe it as a power shower experience, but it is not weak there is a lot of hot water coming out and somebody turning a tap on does not affect it.

Now running bath - obviously depends how big it is but it takes 20mins to run my bath, wich may not be acceptable to you.

Also if you do buy a highflow model they work by having a local hot water store in the combi that is kept to a high temp, usually 75C so that when demand comes they mix this water with the water that is being heated to get the temp you have set on the boiler so you get the highflow rates for a period of time. When I had my boiler installed the programmer did not control the time the hot water was on so this store was being kept at 75C 24/7. I now only have the hot water turned on at the boiler when I need hot water not 24/7 and this has reduced my gas consumption by 3.5m3 a week or about 2000Kwh per year. So make sure the boiler you buy can have the hot water timed.

Hope this helps.

boilerdoktor · 14 Oct 2010

aelitaman said:
First off I am not a plumber or an expert so I can only give you my view of living with a large combi in a large house.

I have a worcester bosch highflow 400 (smaller than the one you mentioned)

Mine is meant to have a max flow rate of 18l/min I measured mine and I get 12 l/min in the summer and 9 l/min in the winter (colder incoming water I assume).

This is just enough to run two taps at the same time.

For a shower I have a victorian style jobby with the shower rose above my head. This is 8 inches in diameter, so not small. No way can you describe it as a power shower experience, but it is not weak there is a lot of hot water coming out and somebody turning a tap on does not affect it.

Now running bath - obviously depends how big it is but it takes 20mins to run my bath, wich may not be acceptable to you.

Also if you do buy a highflow model they work by having a local hot water store in the combi that is kept to a high temp, usually 75C so that when demand comes they mix this water with the water that is being heated to get the temp you have set on the boiler so you get the highflow rates for a period of time. When I had my boiler installed the programmer did not control the time the hot water was on so this store was being kept at 75C 24/7. I now only have the hot water turned on at the boiler when I need hot water not 24/7 and this has reduced my gas consumption by 3.5m3 a week or about 2000Kwh per year. So make sure the boiler you buy can have the hot water timed.

Hope this helps.

hope you don't mind aelitamen but i should like to address a few points!

firstly did you realize that the 400 was only designed for 3 bed type houses with 2 bathrooms and as such is only a 24kw boiler on central heating! amazing i know and not many know this!

how can you get 12 l in summer and 9 in winter? why does your water pressure coming INTO the prop drop so much in winter??

I think you may have poor pressure! the boiler will deliver 18lpm but obviously you need the water pressure getting to the machine in the first place!

75degrees?? the stat on the right hand side dictates the temp of the "stored" hot water and ranges (i think) from 55degrees to 75 degrees depending on the setting on the boiler.

Worcester provide dual heating/hw controls for all their boilers (400/440/550 etc) if you weren't provided with one...blame the installer!!

Hope this helps!

In answer to original op...dont buy a 42 kw boiler etc UNLESS you have the flow, you are wasting money! Yes improving the mains in will make your choice easier! Change the main to alkathene and THEN and THEN ONLY make your choice of boiler! And the 440 and 550 are supremely built machines, engineered perfection ( vast improvement on the 400) I have fitted many to extremely happy customers! 5 year warranty available as well!

aelitaman · 14 Oct 2010

Hi Boilerdokktor no problem.

First off I shoulds say I am very happy with my boiler but I will also admit that I did not know its limitations when I bought it buthat was 6 years ago now and technology has moved on a lot.

To answer your questions.

No I did not know it was designed for 2 bedroom house. Like the OP I had space constraints I wanted a combi. I wanted to buy made in the UK (my family is generations of English manufacturers) and at the time this was the largest combi WB made.

~I may well have poor pressure, I was just trying to illustrate to the OP that what the spec sheets sya you may not get but it may be acceptable.

No that stat on the right hand side sets the temp that the hot water exits the boiler not the temp of the heatstore that the boiler uses to get the highflow.

I do not blame the installer for not providing a programmer that times the hot water on a combi boiler because I did not ask them to provide one, because at the time I just read the brochure that said hot water on demand with out understanding how the boiler provided this hot water. The installer did what I asked him to do.

mickyg · 14 Oct 2010

boilerdoktor said:
firstly did you realize that the 400 was only designed for 3 bed type houses with 2 bathrooms and as such is only a 24kw boiler on central heating! amazing i know and not many know this!

24 kw is more than enough for most 5 bed houses. average 3 bed house only needs around 14 kw. unless your in northern scotland or something

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Boiler for large 5 bed house

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