boiler sizing! boiler sizing!!!!?!

[Hobo128]

Can somebody tell me the correct way to size up a boiler. Its going to be a unvented system. The house is quite big. Am i right in measuring up the radiator sizes in watts then adding up to get my boiler size?

Thanks

 

[magicno1]

Plenty of calcs online.
Buy yourself the design heating guide it's the bible for all pro heating engineers and everybody should have one my man!

 

[ShaunCorbs]

do it at a room heat loss calc and then add then unvented cylinder (normally around 15-20kw)

 

[Dwop5545]

Plenty of calcs online.
Buy yourself the design heating guide it's the bible for all pro heating engineers and everybody should have one my man!

Is this the one? Heating and Hotwater Industry Council » Domestic Heating Design Guide

 

[king of pipes]

A mears heat calculator is what i use look online it will explain more regards kop

 

[CHRISX]

Hello all,

I notice than no-one has mentioned the `Whole House Boiler sizing method` calculation which is supposed to be `the Industry Standard` for calculating the size of a new Boiler since about 2003 and was reinforced by Part L Energy Efficiency Regulations.

Also - the Energy Efficiency Exams that all Heating Installers / Heating Engineers HAD TO PASS in order to be able to continue legally install `Heat Producing Appliances` [approx. 2004 / 2005 ?] - required by CORGI for us to continue to be registered for Boilers & Gas Heaters - in my case formulated by City & Guilds - had questions about the `Whole House Boiler sizing method` - so that should have reminded Heating Engineers who took the Exams that it was supposed to become `the Industry Standard` for sizing Boilers.

I don`t know what has happened to those Mandatory `Energy Efficiency Exams` since about 2005 - 2007 - regarding Heating Engineers / Installers who have never taken them ?

To adhere to `Energy Efficiency Best Practice` regarding ensuring that a new Boiler is sized to operate at the optimum Efficiency and NOT just replace `like for like` in Boiler output or `Oversize` a new Boiler in a property that does not have one - we should have been using this Boiler sizing calculation method for about the last 14 YEARS.

I hope to have uploaded / attached a PDF copy of the Calculation method which was printed by the Energy Efficiency Trust.

Whether I agree that this is the best method to Calculate a Boiler Size / Output or not is another matter.

Chris

 

[Chalked]

That's for boiler replacement monkeys. So they just don't bang in a large, oversized boiler
It for new, or in my opinion. You should be measuring up every room and and alllowing for hot water cylinder. A whole house calculation will not allow for a 300litre unvented cylinder in a large house. Whereas, on a small semi the calculation will work .

 

[Last Plumber]

I agree with K.O.P. and Chalked.
Room by room calcs and experience.

 

[doitmyself]

I hope to have uploaded / attached a PDF copy of the Calculation method which was printed by the Energy Efficiency Trust.

The version you have uploaded is out of date; the current version is below.

It is more detailed than the original version, so there are several online "whole house" calculators based on the current version. for example: Baxi Whole House Boiler Size Calculator

 

[oz-plumber]

Can somebody tell me the correct way to size up a boiler. Its going to be a unvented system. The house is quite big. Am i right in measuring up the radiator sizes in watts then adding up to get my boiler size?

Thanks

No...your not right!

Add up the kW from the radiators, allow additional 10% for heatloss from piping.
Add onto that the kW rating required by the HWS.

Should give you a kW rating of boiler.

On top of that, depending how you set up the system will determine what size of boiler you require.

 

[doitmyself]

No...your not right!

Add up the kW from the radiators, allow additional 10% for heatloss from piping.
Add onto that the kW rating required by the HWS.

Should give you a kW rating of boiler.

On top of that, depending how you set up the system will determine what size of boiler you require.

How do you know the existing rads are correctly sized?

 

[oz-plumber]

How do you know the existing rads are correctly sized?

Welcome back doitmyself - been on a hiatus?

Without going through over technical scenarios regarding boiler sizing, I think that any well versed, experienced installer would have a feel for what radiator size is applicable for each room.

For instance - if I walked into a room 4 x 4 x 2.7 ceiling height and saw a 400 x 400 panel installed, I would know it's a cock up. If I saw a 600 x 800 panel installed I would know it's not a cock up.

So to answer your query - experience would tell me that the radiators are sized suitably or unsuitably for each room, for taking a kW rating of the boiler required.

FYI: I would never use a manufacturers online boiler sizing program to size a boiler.
I know for a fact that they would have at least a 20% margin on their calculations to cover themselves.

Any further questions?

 

[oz-plumber]

How do you know the existing rads are correctly sized?

Welcome back doitmyself - been on a hiatus?

Without going through over technical scenarios regarding boiler sizing, I think that any well versed, experienced installer would have a feel for what radiator size is applicable for each room.

For instance - if I walked into a room 4 x 4 x 2.7 ceiling height and saw a 400 x 400 panel installed, I would know it's a cock up. If I saw a 600 x 800 panel installed I would know it's not a cock up.

So to answer your query - experience would tell me that the radiators are sized suitably or unsuitably for each room, for taking a kW rating of the boiler required.

FYI: I would never use a manufacturers online boiler sizing program to size a boiler.
I know for a fact that they would have at least a 20% margin on their calculations to cover themselves.

Any further questions?

 

[doitmyself]

I haven't gone deaf so there's no need to repeat yourself.

I was on an enforced break - thanks to the previous regime of moderators.

So you don't just use the size of existing rads; you rely on your experience to tell you if the rads are the correct size, and make adjustments if required!

If you are referring to the calculator I posted the link to, I made it clear that this uses the latest version of CE54, which was produced by BRE. It is therefore not Baxi-specific (apart from recommending which of their boilers would meet the requirements) and gives exactly the same result as going though the calculations manually using CE54.

Rad-sizing calculators always 'oversize' by 10% to 20% to allow for intermittent use, i.e heating only on for an hour or so twice a day; which means the house has to heat up faster. If the house is occupied all the time, there is, theoretically, no need to make any allowance. But the next occupiers might not be there all the time, so over-sizing is probably preferable. Oversized rads will, in any case, run at a lower temperature so the boiler will be condensing more often, which saves gas.

 

[cr0ft]

I'm going to stick my 2p in. I think sizing a boiler for HW AND heating demand plus a % will mean massively oversized boilers that may well end up cycling on and off once heating is up to temperature. Take the Ideal Vogue. 7:1 modulation maximum. Say our 4 bed house has a heat loss calculations state that 15KW is required for radiators and there's an 18KW coil. These heat loss calculations are usually based on Winter temperatures so for a lot of the year the heat loss is far less than this. Also, once the house is up to temperature there's very little demand and heating requirement will be more like 3-4KW. Should we really be fitting 35KW System boilers in houses like this? I'd argue that doing so shows a poor understanding of boiler modulation and will lead to the boiler cycling on heating only - causing increased wear and tear on the boiler and inefficient running due to the boiler continuously running pre and post burn purging on the heat exchanger.

Your 18KW coil should only be added in if the homeowner uses it with heating at the same time. In practise, we should be telling homeowners to set hot water to different times to heating in my opinion and fitting boilers accordingly. This way they will modulate down to far lower KW levels which ensures cycling doesn't happen. Put it this way, a 35KW system boiler will modulate down to 5KW. I bet that will be cycling once a house is up to temperature.

By contrast, an 18KW system boiler fitted in the same house will of course be cheaper to install and far cheaper to run as it will modulate down to the levels required when the house is up to temperature (it will modulate down to 2.57KW and is very unlikely to cycle once the heating is up to temperature). End result, the smaller boiler will not cycle and is the better option. The compromise of course is hot water and heating cannot run a the same time. Not really a problem though is it?

Choose a system boiler based on the higher of coil or heating requirements and go from there to minimise the risk of cycling. This is how we size them and we've never had a complaint yet. The heat loss calculators are already worst case and already err on the side of caution.

For combi boilers, base it on hot water requirement and accept that fitting a 40KW system boiler means it will likely cycle once a house gets up to temperature.. This leads to more breakdowns than system boilers, hence why I both love and hate them (would never have one in my house but love how much money I've earned off them since qualifying!).

 

[Chalked]

I did a bungalow ( with upstairs)last year that had three towel rails on its own circuit. 8 rads on the first floor. 9kw of ufh downstairs and a cylinder with an 18kw coil.
Boiler specified by designer was 15kw system. So that's what it got.
Guess what? It works fine p, with very low bills. The trouble is a well insulated new build home has very little heat loss and no calculators take into account for this.

But.
The op was on about a large old house. This will be poorly insulated and draughty. This is where individual room calculations and experience come in. Nobody wants customers complaining that the living room , or north facing bedroom are cold. Overriding might not be green or efficient but it keeps the custards off our back.

 

[cr0ft]

Agree with you Chalked, but I would still say that adding 18KW to the heating output will produce massively oversized boilers that will unnecessarily cycle and waste energy. In the situation of an older house it's likely that peak heating output will be what determine boiler size and not the coil capacity. I'd agree that in the case of an older house, a full set of heating calculations should be undertaken unless experience tells you what's required!

 

[oz-plumber]

I haven't gone deaf so there's no need to repeat yourself.

I was on an enforced break - thanks to the previous regime of moderators.

I think they were forced into the situation, due the advice you were giving, would cause the forum legal liabilities.

So you don't just use the size of existing rads; you rely on your experience to tell you if the rads are the correct size, and make adjustments if required!

From 25 years experience, sizing and installing heating system, I do give myself the pleasure, of walking into a room and deciding whether the radiator sizes are applicable to that size of room.


If you are referring to the calculator I posted the link to, I made it clear that this uses the latest version of CE54, which was produced by BRE. It is therefore not Baxi-specific (apart from recommending which of their boilers would meet the requirements) and gives exactly the same result as going though the calculations manually using CE54.

Oh well....good for you....you have your way - we have ours

Rad-sizing calculators always 'oversize' by 10% to 20% to allow for intermittent use, i.e heating only on for an hour or so twice a day; which means the house has to heat up faster. If the house is occupied all the time, there is, theoretically, no need to make any allowance. But the next occupiers might not be there all the time, so over-sizing is probably preferable. Oversized rads will, in any case, run at a lower temperature so the boiler will be condensing more often, which saves gas.

So what are you trying to say?

There was a reason for things in the past - end of gripe

 

[oz-plumber]

Half my post disappeared.
Maybe a good thing.

 

[CHRISX]

The version you have uploaded is out of date; the current version is below.

It is more detailed than the original version, so there are several online "whole house" calculators based on the current version. for example: Baxi Whole House Boiler Size Calculator

Hello and Thank You doitmyself,

Thanks for uploading the updated Energy Efficiency Trust `Whole House Boiler Size Calculator`.

I have had the version that I uploaded on my Computer for years and it did not occur to me to check online for a more up to date version.

A comment to All - I did write in my previous message:

`Whether I agree that this is the best method to Calculate a Boiler Size / Output or not is another matter`.

END OF QUOTE

I would calculate the Heat requirements of every Room [and find out the Heat Output of the existing Radiators] and I would add an allowance for Heating the Cylinder - but in this case even with a large Hot Water Cylinder I would not be adding 18kw.

I know the so called `Ideal`/ Theory regarding allowing for the Hot Water Cylinder to Heat up / Recover in 40 minutes to an Hour - but in practice that would definitely mean that a Boiler size calculated to that parameter would be vastly oversized for most of its operating times - as Member croft and others have already stated.

I would want any Boiler that I installed to be able to Heat the Rooms / Property based on MY Calculations irrespective of the Heat output of the existing Radiators - which I would have calculated to see whether they were correct - and I would allow enough additional output for a large Cylinder to Heat Up in about 2 - 3 Hours - unless the Householder definitely required a faster Heat Up.

However I have done quite a few Heating & Hot Water Installations where there was a 450 Litre Hot Water Cylinder and some of them had to have an individual Boiler to Heat the Cylinder because the Heating system requirements were such that I could not install one Boiler to Heat both the Radiators and the 450 Litre Cylinder without having to create a `Boiler House` and even then the Primary pipework would have been prohibitive regarding size, length and route.

Those jobs have been in large Houses so there have been no problems with finding locations to install a dedicated Boiler for the Cylinder and another for the Heating.

As We know with a dedicated Boiler just Heating a large Cylinder - it can then be sized to Heat Up the Cylinder in a much shorter time as all of the output will be going into the Hot Water.

Chris

 

[oz-plumber]

Bit strange Chris,

From your OP to you second post, you seemed to have acquired and awful lot of knowledge.

Then to you last post - an inordinate amount of knowledge & experience.

Come clean

 

[oz-plumber]

Bit strange Chris,

From your OP to you second post, you seemed to have acquired and awful lot of knowledge.

Then to you last post - an inordinate amount of knowledge & experience.

Come clean - Fess up

 

[CHRISX]

Hello oz-plumber,

I don`t know what You mean - could you explain further.

I don`t really want to be giving my `Life Story` on here but as You seem to be questioning something here is some information on my `Experience`:

Regarding my `acquired knowledge` - I have been in the Plumbing, Heating & Gas Trades for over 50 Years including my Apprenticeship.

In my 30`s I worked as a Site Manager and Site Supervisor for some very large Heating & Plumbing Companies - running some very large Contracts with up to 70 Tradesmen as my Team - but I have spent many years of my working life Self Employed / running a small reasonably successful Heating, Plumbing & Gas Installation Business in partnership with a lifelong Friend who is also a Heating Engineer, Plumber & Gas Engineer.

I have been a Registered Gas Engineer for 44 Years - with CORGI from 1973 to 2009 - both on the voluntary CORGI register and then on the Mandatory CORGI Register from its inception in 1991 to when Gas Safe took on the UK Gas Engineer Register in 2009 - and then registered with Gas Safe from 2009.

During those 50 Years of working I think that I have gained a fair bit of knowledge and experience in the Heating, Plumbing and Gas trades.

However - when I see your reply hopefully explaining what you meant in your previous message I will be able to respond better.

Chris

 

[oz-plumber]

Can somebody tell me the correct way to size up a boiler. Its going to be a unvented system. The house is quite big. Am i right in measuring up the radiator sizes in watts then adding up to get my boiler size?

Thanks

Did I miss something - or did you change your name?

For some reason, I thought you were Hobo who posted the original post.
And your knowledge had on the subject had increased at the speed of light.

Maybe I should take more care in the future of who is posting what.

 

[CHRISX]

Hello oz-plumber,

Thanks for your reply.

I did wonder what had prompted You to post your previous message.

Thanks for explaining what had happened.

Chris