Energy Efficiency Boiler Sizing etc

[NellyC]

Hi, im new to the site. I have recently did my ACS and also had to do my energy efficiency qual. Can any1 tell me if they use the boiler sizing chart. Cause i have used it and correctly calculated all the rad sizes as per energy efficiency, and it seems firstly that the boiler KW output is Low and the rad sizes seem small.

Was just wondering how every1 else works it out

cheers
Nelly

 

[gakmonkey]

go buy the CIBSE "domestic heating guide" takes all the guesswork out of all heatsizing.

 

[NellyC]

I have the proper book with all the relevant info for working out boiler size. Used the calculations for the first time the other day on a 3 up 2 down terraced house and my calc worked out at 12 KW boiler including 6 rads and adding 2000Watts for hot water. Surely this isn't right and if it is then surely the hot water flow rate will not be satisfactory.

 

[ecowarm]

I have the proper book with all the relevant info for working out boiler size. Used the calculations for the first time the other day on a 3 up 2 down terraced house and my calc worked out at 12 KW boiler including 6 rads and adding 2000Watts for hot water. Surely this isn't right and if it is then surely the hot water flow rate will not be satisfactory.

If you sizing for a combi size the boiler for hot water flow required.

Eco

Ps sized a detached 4 bed 2 bath house inc hot water allowance and it came to 11.2 kw and it all works fine with 12kw Remeha

 

[NellyC]

If you sizing for a combi size the boiler for hot water flow required.

Eco

Ps sized a detached 4 bed 2 bath house inc hot water allowance and it came to 11.2 kw and it all works fine with 12kw Remeha

Thanks for that, i can't get my head round why Most engineers fit nothing less than 18Kw and mostly 24KW and above. And so whats the point of having to take the energy efficiency qual if no 1 regulates it with what your actually fitting alongside your gas safe appliance registration?

 

[gakmonkey]

bear in mind most boilers can be range rated, so an 18kw could be reduced to 7 or 8kw and you would be leaving yourself a little scope for extension of the system and as previously mentioned all combi will be sized for dhw while heating would be reduced to your calcs.

 

[NellyC]

bear in mind most boilers can be range rated, so an 18kw could be reduced to 7 or 8kw and you would be leaving yourself a little scope for extension of the system and as previously mentioned all combi will be sized for dhw while heating would be reduced to your calcs.

Yep that seems understandable, so your saying for DHW size boiler as per flow rate requirements? add on what you need for heating. Why does all the info in regards to energy efficiency tell you to add on between 2000W and 2600W for DHW depending on which book you read!! and which engineer you talk to.

I worked out all the calcs for boiler size, worked out all the rad sizes and noted the watts of each, added 2000W for DHW and added it all together and i got just short of 12 KW.

This is in relation 2 a combi boiler

 

[dumbplumber]

i usually measure the rooms length x width x height x 6 take cylinder as 10,000 btu add them all together and you have your boiler size . by multiplying by 6 covers you for outer walls .
Its worked for me inthe passt number of years.
Hope this helps

 

[gakmonkey]

Why does all the info in regards to energy efficiency tell you to add on between 2000W and 2600W for DHW depending on which book you read!! and which engineer you talk to.

I find this nonsense yet have heard it continually so a 114 ltr cylinder requires the same amount of energy to heat it as a 300ltr, its easier to think of it as 1kw per 50ltrs, and with combi boilers you make no extra allowance for dhw mearly compare the flow rates.

 

[doitmyself]

The recommended method now is to use the "whole house" Boiler Size Calculator, which has been produced by the Building Research Establishment (BRE). It usually gives smaller sized boilers than the traditional methods (how big is the house? where is it located? What make and how many cars? ).

Radiator sizes are harder as the size of the room has to be taken into account. But it should be possible to calculate it by using the floor area of each room as a proportion of the total floor area.

If you want to ensure that the boiler runs at it's most efficient, i.e. in condensing mode all the time, radiators will have to be oversized by at least 20%. This does not affect the boiler size.

 

[NellyC]

The recommended method now is to use the "whole house" Boiler Size Calculator, which has been produced by the Building Research Establishment (BRE). It usually gives smaller sized boilers than the traditional methods (how big is the house? where is it located? What make and how many cars? ).

Radiator sizes are harder as the size of the room has to be taken into account. But it should be possible to calculate it by using the floor area of each room as a proportion of the total floor area.

If you want to ensure that the boiler runs at it's most efficient, i.e. in condensing mode all the time, radiators will have to be oversized by at least 20%. This does not affect the boiler size.

surely rad sizes will affect the boiler size because the bigger the rad the more BTU and Wattage is being used. I have the whole house boiler sizing worksheet and once worked out boiler size. I worked out all the rad sizes and added the wattage of them all to the final result.

I find this nonsense yet have heard it continually so a 114 ltr cylinder requires the same amount of energy to heat it as a 300ltr, its easier to think of it as 1kw per 50ltrs, and with combi boilers you make no extra allowance for dhw mearly compare the flow rates.

I know what your saying, so does this mean i should just forget trying to follow all this energy jargon boiler sizing chart. I only did the energy efficiency course 2 week ago and i just wanted to make sure i was following good practice as being fairly new to the trade. It doesn't even mention anything about flow rates in the info pack etc. Seems a waste of £100 to me!!

 

[prs1]

with a larger radiator the boiler can operate at a lower temp to give the same heat output.

The flow rates come into play when determining your customers requirments if its a family house and the kitchen and bathroom are in use at the same time 24kw wont cut it, but there is no point banging a 37kw boiler in when incoming cold mains is only 12 litres/pm if they havent got the cold flow then a combi is not for them better looking at unvented & system boiler but if they have good cold up size the boiler for dhw requirement and range rate!
Its all about whats best for the customer get this wrong and they wont be happy when shower stops because someones using the kitchen.

 

[NellyC]

with a larger radiator the boiler can operate at a lower temp to give the same heat output.

The flow rates come into play when determining your customers requirments if its a family house and the kitchen and bathroom are in use at the same time 24kw wont cut it, but there is no point banging a 37kw boiler in when incoming cold mains is only 12 litres/pm if they havent got the cold flow then a combi is not for them better looking at unvented & system boiler but if they have good cold up size the boiler for dhw requirement and range rate!
Its all about whats best for the customer get this wrong and they wont be happy when shower stops because someones using the kitchen.

That makes sense, thanks for that. When you say range rate what exactly is meant by this term? is it taking into consideration amount needed for all rad usage?

 

[doitmyself]

surely rad sizes will affect the boiler size because the bigger the rad the more BTU and Wattage is being used. I have the whole house boiler sizing worksheet and once worked out boiler size. I worked out all the rad sizes and added the wattage of them all to the final result.

A radiator is not like an electric fires, which gives out a constant amount of heat. The output of a radiator depends on three temperatures: flow, return and room.

To ensure that you can compare the rads from different manufacturers, there is a standard method of measuring rad outputs, which is BS EN 442. This specifies a flow temp of 75C, return temp of 65C and room temperature of 20C. Without going into the maths used to calculate the output if different temperatures are used - it's complicated - the output of a rad using 75C/55C/20C (flow, return, room) will be 85% of the manufacturers output, i.e a 1kW will only give out 850W. So if you need 1kW you will have to install a 1,176 (1/.85) or 1200W radiator.

Similarly if you run the boiler at 65/55/20 the rad output will reduce to 74% of the manufacturer's figure and you would have to install a 1,350W rad to produce 1kW.

In both cases the rad is only producing 1kW, so the boiler only need to produce 1kW.

 

[prs1]

Range rating is what a gas engineer would do to a new boiler upon commisioning, basicaly he will set the burner pressure to the desired level required for the system.
A 20kw boiler when r ratable acually means it actually 10 to 20 kw output so its set accordingly

 

[NellyC]

A radiator is not like an electric fires, which gives out a constant amount of heat. The output of a radiator depends on three temperatures: flow, return and room.

To ensure that you can compare the rads from different manufacturers, there is a standard method of measuring rad outputs, which is BS EN 442. This specifies a flow temp of 75C, return temp of 65C and room temperature of 20C. Without going into the maths used to calculate the output if different temperatures are used - it's complicated - the output of a rad using 75C/55C/20C (flow, return, room) will be 85% of the manufacturers output, i.e a 1kW will only give out 850W. So if you need 1kW you will have to install a 1,176 (1/.85) or 1200W radiator.

Similarly if you run the boiler at 65/55/20 the rad output will reduce to 74% of the manufacturer's figure and you would have to install a 1,350W rad to produce 1kW.

In both cases the rad is only producing 1kW, so the boiler only need to produce 1kW.

Yep i understand that. I think ive got confused as i use a BTU calculator which when entering all the criteria, e.g. room size, window material, above and below rooms etc. It basically tells me the radiator i need and i took this as Gospel!
cheers.

Range rating is what a gas engineer would do to a new boiler upon commisioning, basicaly he will set the burner pressure to the desired level required for the system.
A 20kw boiler when r ratable acually means it actually 10 to 20 kw output so its set accordingly

most new boilers are zero governor though, so how can you adjust burner pressure. do you mean air gas ratio?

 

[doitmyself]

I think ive got confused as i use a BTU calculator.

Does your instructor come to work on a horse and use log tables?

BTUs are old hat. Calculation is so much easier with Watts.

 

[NellyC]

Does your instructor come to work on a horse and use log tables?

BTUs are old hat. Calculation is so much easier with Watts.

Sorry! thats just what the calculator is called it gives you both readings and i also work out using Watts.
And i don't have an instructor i have my own business and am fairly new to all the calcs as i've never had to measure up and survey for gas installations as some1 is usually doing it for me

 

[Plucky]

A radiator is not like an electric fires, which gives out a constant amount of heat. The output of a radiator depends on three temperatures: flow, return and room.

To ensure that you can compare the rads from different manufacturers, there is a standard method of measuring rad outputs, which is BS EN 442. This specifies a flow temp of 75C, return temp of 65C and room temperature of 20C. Without going into the maths used to calculate the output if different temperatures are used - it's complicated - the output of a rad using 75C/55C/20C (flow, return, room) will be 85% of the manufacturers output, i.e a 1kW will only give out 850W. So if you need 1kW you will have to install a 1,176 (1/.85) or 1200W radiator.

Similarly if you run the boiler at 65/55/20 the rad output will reduce to 74% of the manufacturer's figure and you would have to install a 1,350W rad to produce 1kW.

In both cases the rad is only producing 1kW, so the boiler only need to produce 1kW.

Good post

Can we over complicate things a little as I would like to know how you calculate sizing of rads for different flow temps etc.

Been fitting a few heat pumps onto systems with rads and we have had to change rads for larger ones but all the sizing is done at the design stage by someone else. I am keen to learn to do this for myself.

For example we fitted a 14kw Mitsibushi Ecodan onto a system, all the rads came to around 25kw. Couldnt quite get my head around it at first

 

[Bernie2]

I suppose the underlying reasons of looking at dhw flow rates rather than central heating requirements, is simply that the dhw is heated instantly from cold so requires the most heat input. Usually its far in excess of what the central heating needs are.

The boiler then modulates or turns itself down according to what the central heating requires.

So suite the dhw flow rate to the house size and usually there is bags left for the heating.

Problem with condenser combis of course is getting shut of the heat out on the circuit so that the return temperature drops enough for it to condense.

Its all good fun!!

 

[doitmyself]

Can we over complicate things a little as I would like to know how you calculate sizing of rads for different flow temps etc.

You asked for it!

It's probably easiest if I give it as the Excel Formula. It assumes that the manufacturers rad output is measured using 75/65/20.

(49.83*LN((D2-G2)/(E2-G2))/(D2-E2))^1.33

D2, E2 and G2 are cells which contain the following values:

D2 = Flow temp
E2 = Return temp
G2 = Room temp

LN means take the log to base e of the bit in brackets.

48.83 is a constant which depends on the starting point (75/65/20)

1.33 is a factor which varies slightly with type of rad (single/double/convector) but not enough to worry about.

Using the formula with temps of 75/55/21 produces a value of 1.208. So you need to multiply the calculated radiator size by this to get the size you need to buy if you want to run at those temperatures. In other words, if you need 1kW of heat you will have to install a 1200W rad.

If you have a 1kW rad already installed and want to workout its actual output at 75/55/21, then just divide 1kW by 1.208 and you get 828W.

 

[NellyC]

You asked for it!

It's probably easiest if I give it as the Excel Formula. It assumes that the manufacturers rad output is measured using 75/65/20.

(49.83*LN((D2-G2)/(E2-G2))/(D2-E2))^1.33

D2, E2 and G2 are cells which contain the following values:

D2 = Flow temp
E2 = Return temp
G2 = Room temp

LN means take the log to base e of the bit in brackets.

48.83 is a constant which depends on the starting point (75/65/20)

1.33 is a factor which varies slightly with type of rad (single/double/convector) but not enough to worry about.

Using the formula with temps of 75/55/21 produces a value of 1.208. So you need to multiply the calculated radiator size by this to get the size you need to buy if you want to run at those temperatures. In other words, if you need 1kW of heat you will have to install a 1200W rad.

If you have a 1kW rad already installed and want to workout its actual output at 75/55/21, then just divide 1kW by 1.208 and you get 828W.

Easy as that!! think i'll go back to joinery