System Boiler Size

[Jupiter01]

Hi,

I'm planning for a 250 Litre unvented system (megaflow most likely) and a system boiler (Worcester most likely) to go with this. I'm struggling to obtain hard stats on how I should size my boiler and was wondering if you can help.
I will have 14 radiators in total. It's a 4 bedroom house and some of these radiators are on the larger side. I could provide the BTU's if that is required.

I'm interested in your thoughts and also any comments you have on the brands I have selected.
Thanks in advance.

 

[Agile]

You need to calculate the heat loss of the house. Then add 2 kW for the hot water.

As a further check calculate the heat output of all your rads together.

Heat is measured in kW nowadays!

"four bedroom house" tells me very little!

But if a detached built about 1980 probably about 18 kW boiler would be required.

Tony

 

[Agile]

You need a gas reg engineer to fit the boiler and someone with the G3 qualification to fit the unvented.

Normally they would be able to do the calculations for boiler size and cylinder size.

But of course if they are the cheapest of the cheap they may not be familiar with the niceties of heating installation.

Tony

 

[D_Hailsham]

Use the Baxi Whole House Boiler Size Calculator. It's not brand specific.

The number and size of rads are only relevant if they are not large enough. As most houses have larger rads than necessary, this should not cause a problem.

Although HW cylinders now expect around 20KW so they heat up quickly, there is no real need to add this to the central heating requirement, provided the HW cylinders are reheated at the appropriate time, e.g. overnight or in the middle of the day - not first thing in the morning when the central heating is working hard to bring the house back up to temperature.

 

[Agile]

It is normal to time the cylinder reheat to start about 20-30 min before the morning heating period so that its fully heated for the morning by the time heating is needed.

And a later reheat time from 1700 hrs ideally continued all evening so that the hot water useage is topped up.

Tony

 

[Dan Robinson]

 

[Agile]

What does that symbol mean Dan?

Is it that Worcester boilers are shit ?

 

[Dan Robinson]

2kw this
Whole house calcs that
20 minutes here.
Worcester wherever....


It's like the bad school of system design.

 

[Steelmasons]

2kw's hot water requirement????

Guess it might be better to install a direct cylinder running off a 3kw immersion...

 

[D_Hailsham]

2kw's hot water requirement????

Guess it might be better to install a direct cylinder running off a 3kw immersion...

You are making the mistake of assuming the boiler is always running flat out for the CH, so there is nothing left for the HW. This will only be true when the house is being heated up from cold. Once that has happened the boiler is idle most of the time and only comes on when the room stat has dropped a degree or so. It doesn't then need the full output of the boiler to raise the temperature one degree; it only needs about 5% of its output. This leaves plenty spare to reheat the cylinder.

Setting the timer so the cylinder reheats overnight and in the middle of the day obviates the need to allow any extra for heating the cylinder.

 

[Steelmasons]

Let's assume coil rating 24kw.
Building heat loss 8kw.
......what size boiler would you propose?????

Hey DH .......you explain your above post to my wife as we have a family of 6 all wanting showers/baths between the times of 6-9pm on a cold winter night.......she also likes to dry clothes on the radiators .....get in the real world.

PS..I don't make mistakes when it involves system design.

 

[Agile]

Not many UK four bed houses would be only 8 kW.

But he has not told us how many live there or why he chose a 250 li cylinder.

Tony

 

[Steelmasons]

Not many UK four bed houses would be only 8 kW.

But he has not told us how many live there or why he chose a 250 li cylinder.

Tony

Then don't assume 2kw for HW rating when you clearly have no idea regarding occupancy/usage.

 

[Agile]

I am not assuming but following the EST best practice instruction. Didn't you go on the energy efficiency course too?

Tony

 

[Steelmasons]

Didn't you go on the energy efficiency course too?

Tony

Yes....and what a load of shyte it was too......
So 2kw for hot water rating then Tony?....oh dear.

..but then you had no "proper" training as you came into this industry late in life.....city and guilds? apprenticeship?

No I didn't think so...