Radiator & Boiler sizing

[Leoki]

I am using a maths calculation from a plumbing book by TRELOR edition no 3. I have used it to calculate the various sizes of radiators that i would need to use in a first floor flat. The calculations although time consuming take into account heat loss, u values and building materials. I have come up with the size of radiator that i need for each room and totalled up this gives me a value of 7.5kw total. The calculation then continues on showing you how you can come up with the boiler size but not when calculating for a combi boiler. A condensing combi boiler is what i need to use.

Can anyone suggest a way of calculating the boiler size now that i have the total size of radiators needed. I have been warned to be wary of on-line calculators as these can be very general.

Thankyou

 

[jpm]

I am using a maths calculation from a plumbing book by TRELOR edition no 3. I have used it to calculate the various sizes of radiators that i would need to use in a first floor flat. The calculations although time consuming take into account heat loss, u values and building materials. I have come up with the size of radiator that i need for each room and totalled up this gives me a value of 7.5kw total. The calculation then continues on showing you how you can come up with the boiler size but not when calculating for a combi boiler. A condensing combi boiler is what i need to use.

Can anyone suggest a way of calculating the boiler size now that i have the total size of radiators needed. I have been warned to be wary of on-line calculators as these can be very general.

Thankyou

combi boilers are easy to size,the more energy you put in(ie gas) the more DHW you get out.ie a 24kw combi will give you 9ltrs per min ish and a 30kw with give you 12ltrs per min ish.of course the is dependant on the amount of water you can supply at the correct pressure but you can understand the principle.35kw will give you 15ltrs ish and so on.How much DHW do you need?1 bathroom?are you putting a shower valve on?

 

[Leoki]

JPM, thanks for the response - There will be a single bathroom with bathtub and stand in shower washbasin and toilet. Not quite sure if the user wants to run the shower off the taps in a conventional way or whether they want a power shower. There is a seperate toilet with a small wash basin and then the kitchen the sink, washing machine and possibly a dishwasher.

Taking into account the total output from radiators that i need would a 24kw combi be man enough for the job (subject to gas input and water flow rate)

 

[jpm]

Taking into account the total output from radiators that i need would a 24kw combi be man enough for the job (subject to gas input and water flow rate)

yes it will be man enough,but where there is a showervalve/mixertap to shower i would explain to the cust the poor water delivery of smaller combis,a 30kw is IMHO a better option considering the lifespan of a boiler,not that much more to buy,and as it will only run at 30kw under a demand for DHW won't cost anymore to run while heating the heating circuit.

 

[doitmyself]

I would still check your calculations using the [DLMURL="http://www.energysavingtrust.org.uk/business/Business/Building-Professionals/Helpful-Tools/Whole-house-boiler-sizing-wizard"]Boiler sizing wizard[/DLMURL]. This is reliable.

It is however based on a regular boiler, i.e not a combi, so you need to deduct 2kw from the final answer to get the heating requirement then compare this with your 7.5kW.

As jpm says, combi boilers are sized according to the hot water requirement, i.e how many taps/baths/showers could be running at the same time.

You need to check the cold water mains flow rate and pressure. If the flow rate is low, there is no point getting a large combi. You need about 2.5kw for every litre/minute flow. You can check this by filling a bucket (they usually have litre marks on the inside) and timing how long it takes.

The pressure determines how many taps etc can be running simultaneously without a reduction in flow rate - you don't want to be standing under a shower when the hot water slows to a trickle because someone else has turned on the hot tap in the kitchen. If all your cold taps are fed direct from the mains, you can easily check this yourself by turning one tap on full and then turning a second one on and checking the flow rates.

 

[stevetheplumber]

i always reccomend minimum of 28kw unless its a realy small place or price is paramount ie developers