Sizing a combi boiler

[SadieLovatt]

Hi,

We have a small 3 bed house (by small, I'm talking 17' x 23' x two floors, no dining room, no ensuites, 1 of the bedrooms is barely big enough to fit a bed in), and using conventional wisdom (ie 'system' boiler calculations) this equates to 13½Kw of heating plus 2½Kw of water heating or a minimum of a 16Kw boiler. Thing is, these are system boiler calculattions and I need to relate this to a combi boiler not a system boiler.

As we rarely have baths (we have showers, and its an electric shower at that), the hot water is not fed into the washing machine (ie its fed from cold only) and the only other DHW outlets we have are two hot water taps (kitchen and bathroom) then is it fair to say that a Vaillant EcoTec 824 would be more than enough (24Kw for DHW, 19-20.2Kw for CH)?

Sadie.

 

[HarrogateGas]

Yeah spot on.

 

[D_Hailsham]

We have a small 3 bed house (by small, I'm talking 17' x 23' x two floors, no dining room, no ensuites, 1 of the bedrooms is barely big enough to fit a bed in), and using conventional wisdom (ie 'system' boiler calculations) this equates to 13½Kw of heating plus 2½Kw of water heating or a minimum of a 16Kw boiler.

I doubt if you would even need 13½Kw for heating - unless, of course, your house leaks heat like a sieve!. Use the Whole House Calculator on the Sedbuk website to find out how much you do need need for heating.

As you only use hot water for taps and the occasional bath, you may find that you can install a smaller combi than 24kW. Don't forget that a combi can only do one thing at a time; it is either heating the house or heating the water - it can't do both simultaneously. So the size of the boiler is determined by the Hot Water requirement.

 

[SadieLovatt]

13½Kw was worked out from the Energy Saving Trust 'whole house' calculator, but going through the figures I think a few of it's screens are out or misleading... which is a little disturbing.

Using the SEDBUK whole house calculator, the calculations come to 8.5Kw heat loss plus 2Kw water heating, total 10.6Kw. (D_Hailsham, I assume this is closer to what you were thinking).

If DW flow rate is the critical factor here then I really don't think that its worth paying the extra for the next boiler up (the 831) if all it will do is fill a bath up 2 minutes quicker (80 litres @ ~12l/min vs ~9l/min).

Sadie.

 

[gas4you]

Don't worry about sizing the boiler for your heating needs.

You only need size a combi for your hot water needs. The boiler will modulate down for the ch, or in the case of the Vaillant be range rated during commissioning.

 

[SadieLovatt]

...or in the case of the Vaillant be range rated during commissioning.

Hang on a sec, what does "range rated" mean and once installed, how would I be able to tell how this was set?

Sadie.

 

[gas4you]

Your installation engineer will set the onboard computer to only run at maximum out put for the size of your system, in your case approx 9Kw.

 

[D_Hailsham]

13½Kw was worked out from the Energy Saving Trust 'whole house' calculator, but going through the figures I think a few of it's screens are out or misleading... which is a little disturbing.

Using the SEDBUK whole house calculator, the calculations come to 8.5Kw heat loss plus 2Kw water heating, total 10.6Kw. (D_Hailsham, I assume this is closer to what you were thinking).

Yes, that more like what I was expecting. I have had a look at the EST calculator and it's certainly not very easy to use. I must check out what I need using both calculators and compare the results.

 

[SadieLovatt]

Your installation engineer will set the onboard computer to only run at maximum out put for the size of your system, in your case approx 9Kw.

Ok, excuse my ignorance here but firstly, what is the benefit of this (let's face it, although heating systems are designed around specific temperatures - eg. 21° in living areas - I, as a user might want my house to be a lot warmer and therefore use more Kw). Secondly, would there be anyway as a layman that I could see what this is set to?

Sadie.

 

[mickyg]

Your installation engineer will set the onboard computer to only run at maximum out put for the size of your system, in your case approx 9Kw.

Ok, excuse my ignorance here but firstly, what is the benefit of this (let's face it, although heating systems are designed around specific temperatures - eg. 21° in living areas - I, as a user might want my house to be a lot warmer and therefore use more Kw). Secondly, would there be anyway as a layman that I could see what this is set to?

Sadie.

if you want a hotter room you turn the room stat up

 

[gas4you]

If you have radiators that give 10Kw output in total, having a boiler that is running at 20 Kw will not give you any more heat to your rooms.

Where do you expect the other 10 Kw to go to?

By range rating the boiler down to 10Kw it will help keep it in condensing mode longer and prevent undue damage to the boiler HE due to the excess heat produced having nowhere to go.

Vaillants are susceptible to this unneeded heat production.

 

[SadieLovatt]

"Where do you expect the other 10 Kw to go to? "

I did say excuse my ignorance.

 

[D_Hailsham]

13½Kw was worked out from the Energy Saving Trust 'whole house' calculator, but going through the figures I think a few of it's screens are out or misleading... which is a little disturbing.

Using the SEDBUK whole house calculator, the calculations come to 8.5Kw heat loss plus 2Kw water heating, total 10.6Kw. (D_Hailsham, I assume this is closer to what you were thinking).

Yes, that more like what I was expecting. I have had a look at the EST calculator and it's certainly not very easy to use. I must check out what I need using both calculators and compare the results.

The two calculators are using exactly the same factors so you should get exactly the same result from both. As you suggest you must have misinterpreted the info required on the EST version.

You asked about having the temperature higher, say 22°C; that's easy! In theory you need a boiler and radiators which is larger by a factor of 23/22, i.e. 5%. This is because the heat loss is proportional to the difference between the external and internal temperatures and the "standard" temperatures used are -1°C and 21°C.

But boilers are sized on a worst-case basis - external temp of -1°C - and this occurs fairly rarely (even less if global warming is true). So you will easily be able to reach temperatures of 25°C or more using a boiler sized using "standard" temperatures.

As for radiators, they are usually oversized by 10-15% to make heating up from cold quicker; so they already have spare capacity to allow a higher room temperature.

 

[SadieLovatt]

The two calculators are using exactly the same factors so you should get exactly the same result from both. As you suggest you must have misinterpreted the info required on the EST version.

Excuse me but if you look at the EST calculations for external wall area they are: -

NoOfExternalWalls x (Length + Width) x RoomHeight x NoOfFloors

Now I don't need to be a heating engineer to understand basic maths here but "(Length + Width) x Room Height x NoOfFloors" = the area of two external walls not one. Using this method, a detached house would result in equivalent of 8 external walls (an octagonal house?). Do the same calculation on the SEDBUK calculator which asks how many external walls you have on width/length seperately and you get the correct area of its 4 external walls.

Also, if you look at the "Total Fabric Heat Loss" page it says: -

WindowHeatLoss x WallHeatLoss x RoofHeatLoss x FloorHeatLoss x LocationFactor

Despite the calculations actually being: -

(WindowHeatLoss + WallHeatLoss + RoofHeatLoss + FloorHeatLoss) x LocationFactor

If I work out my Total Fabric Heat Loss by what it states (rather than what it calculates) then my Heat Loss would be 336,000Kw !!!

Like I said, the EST pages are inaccurate and misleading.

Otherwise, thank you for educating me.

Sadie.

 

[D_Hailsham]

The only calculator I can find on the EST website is CE54 Whole House Boiler Sizing. Is this the one you are using?

Excuse me but if you look at the EST calculations for external wall area they are: -

NoOfExternalWalls x (Length + Width) x RoomHeight x NoOfFloors

According to CE54 you have to calculate:

((width X no of ext walls) + (length + ext walls)) X room height X no of floors.

The only confusion here could be the "no of ext walls", but I would have thought it was obvious that it meant the "no of external width walls" or the "no of ext length walls", as appropriate, not the total number of external walls.

Also, if you look at the "Total Fabric Heat Loss" page it says: -

WindowHeatLoss x WallHeatLoss x RoofHeatLoss x FloorHeatLoss x LocationFactor

Despite the calculations actually being: -

(WindowHeatLoss + WallHeatLoss + RoofHeatLoss + FloorHeatLoss) x LocationFactor

Sorry but CE54 does say:
(WindowHeatLoss + WallHeatLoss + RoofHeatLoss + FloorHeatLoss) x LocationFactor

which is correct.