Sizing combination boilers

[Dannyparty92]

When you are sizing combination boilers obviously for the radiators you calculate how many BTU you need to head all the rads but I'm a little confused about the domestic hot water side of things. Do you get a flow rate reading of the cold main then the boiler you wish to buy must have that particular flow rate reading in its specs or less (as long as it has the correct BTU spec. ?

 

[Gas]

There is much more to consider than just mains water supply flow rates and pressure, you have to consider all the U-values, location, adequate gas supply to get your max HI etc refer to Building Regs Part L.

Boiler is determined by the DHW requirement, not the rads.

 

[Ray Stafford]

To be honest, I reckon half the jobs are sized on the "car on the drive" principle.

No car, or old banger: 24kw
10 yr old Ford: 28kw
5 yr old Volvo: 33kw
2 yr old beamer: 40kw

Anything better? "Have you considered an unvented hot water system?"

 

[Dannyparty92]

So how would you work out what size boiler required with a house with a flow rate of 8 litres per minute at peak demand?

 

[Ray Stafford]

So how would you work out what size boiler required with a house with a flow rate of 8 litres per minute at peak demand?

If there is only 8 litres per minutes incoming to the property, then they really need a stored water solution. Thats too low for a combi.

 

[Gas]

I wouldn't with a 8lpm flow rate

 

[Dannyparty92]

So say a dwelling with a flow rate of 11 litres per minute. From that how would I determine the boiler size needed?

 

[Ray Stafford]

Hi Dannyparty

I think you mioght be coming at this from the wrong end.

Its the hot water demand that drives the system design. Is there one bathroom, or two, or more? Do the householders prefer baths or showers? How many people live in the house? etc etc.

From that you work out whether a combi will cope or if something better is required. Assuming that a combi is specced, then you need to check that there is adequate flow and pressure into the building to make it work. (Same applies to unvented cylinders).

 

[Gas]

In addition to what Ray has said, you would need to calculate window. wall and roof U-values else you are getting no where fast.

 

[Dannyparty92]

Why would you need to work that out gas if all you are working out if the DHW demand and not the heating? Thank you Ray so basically work out the loading units for each outlets then add them up and make sure that it's under whatever the mains flow rate is? Then from there decide what boiler to go for ?

 

[Ray Stafford]

Thank you Ray so basically work out the loading units for each outlets then add them up and make sure that it's under whatever the mains flow rate is? Then from there decide what boiler to go for ?

Not quite. DHW demand is not just a function of what taps and shower heads exist. Its also connected to the number and lifestyle of the occupants, and takes into account how that might change in the future (for example if they are considering a loft conversion, or an extension).

Once you have a good picture of that, then experience, knowledge of what is on the market, and an understanding of the customers budget all feed into the solution. The incoming flow rate is just a limiting technical factor, in the same way as the presence/absence of an airing cupboard, or the layout of existing pipework might be.

All this in a perfect world of course... 🙂

 

[Howsie]

You should consider a career in the plumbing game Ray!

 

[Ray Stafford]

You should consider a career in the plumbing game Ray!

What, and mess up my lily-white hands? 🙂

 

[Simon F]

is this all a hypothetical scenario, or is this install taking place??
who is doing the work? are they not giving any pointers here??

Ray has already given good advice that a combi is not a good option

what is fitted at the moment, hope the answer is a traditional setup with boiler and hot water cylinder.
with flow rates as low as said that is how it should stay.
But it is not just flow rate, that is only part, what is the water pressure, not standing but actual working pressure? this also needs to be taken into account, anything under 2 bar, you would need to start questioning.

 

[Gas]

To again add to the above, you would need to know the dynamic flow rate, no point in having a combi and then no pressure in the pipe work if someone was to open a cold tap some where else in the building, if we are using your new 11lpm figure.

You need an RGI on site to do this stuff, other wise you are most likely going to oversize or undersize the boiler.

 

[Mike Jackson]

First thing you need to do is ascertain if the property is suitable for a combi. As a rule of thumb anything with more than one bathroom is unlikely to be suitable, if the customer insists get them to sign a bit of paper that says that they have been made aware of the possible shortcomings.

Once you've decided that a combi is suitable you need to decide what size. A 24 Kw combi will give around 9 l/min, a 40 Kw around 15. If the customer only has a shower and is happy with the restriction that they can't use multiple outlets at once then 24Kw is fine. If they like to bathe regularly you need to be looking at 28Kw minimum to give a flow rate of around 11 l/min. Anything less and your customer will be cursing you every time they run a bath and stand around for an hour waiting for it to fill. If the customer wants the ability to use multiple outlets then you need to go for a nice high output combi and look at restricting the flow to all outlets so that no one outlet robs all of the flow rate. I've got a 40Kw combi and flow restictors on all outlets and my wife can turn on the kitchen hot tap whilst I'm in the shower.

Now you've decided that a combi is suitable and you know what output is needed you need to check if the incoming cold water flow rate is sufficient. Bear in mind that the incoming main needs to supply hot and cold outlets simultaneously. If you fit a 24 kw combi and the incoming main will only supply 9 l/min the combi will be fine until someone operates a cold tap or pulls the chain when the flow rate through the combi will drop and there is a chance of people being scalded or showers dropping to an unacceptably low flow rate. As a rule of thumb when I fit a combi I like to see an incoming flow rate of twice the combi requirement. So for a 24 Kw combi I want to see an incoming cold water flow rate of 18 l/min. If I fit a 40Kw combi I want 30 l/min. This may be a little over kill but it means that I don't disappoint my customers.

 

[Dannyparty92]

Thank you mike jackson.