Correct Size (Kw) Boiler ?

[BoilerMAD]

Hi

I live in a 4 bedroomed detached house which currently has a perfectly working Baxi Solo 15Kw Conventional Boiler running approx 14 rads.

Currently experiencing low pressure and sometimes lack of hot water when 2 adults and 4 kids (aged 10,8,3,1) are trying to get ready for work,school etc.

So we have decided to revamp the main bathroom (including separate shower) and en suite (with shower) to power showers along with an unvented cylinder as recommended by the bathroom designer.

British Gas have quoted us a Worcester Bosch Highflow 440CDi as the best fit and an independant Heating Engineer has recommended a Worcester Bosch 30CDi System plus a Megaflo 250l Unvented Indirect Cylinder.

Question : What is the ideal boiler size for my house and which is the better system. Can i not use my existing boiler ??

thanks

 

[Dan Robinson]

as recommended by the bathroom designer

Mistake number one.


The boiler is not your problem. The cylinder and control system is.

Can you post information on that please?

Also, have a search on the forum about fitting unvented cylinders. It is getting a little... boring... having to answer the same questions all the time.

A combi will not run two decent showers at the same time. The Highflow might for a short period. Ultimately it depends on the quality of the water supply and the installation.

 

[BoilerMAD]

I have a vented 114litre cylinder in the airing cupboard.

Not sure what you mean about control system ?

 

[Dan Robinson]

The controls that, er, control the system.... timer, thermostats, valves etc.

114l is a little on the small side. If pressure is adequate then just by a bigger cylinder.

 

[ALEC1]

If you have a 15kw boiler then you could probably get away with 12kws now...

count the radiators and multiply by 1.5kws will give you the maximum heat output of the system, no need to go much bigger unless the hot water coil is particularly big

also how water priority dedicates full output to the cylinder...so no need for a margin for the hot water

 

[dcawkwell]

A bigger tank with fast recovery coil will probably sort the problems out.

Ignore British Gas they will charge a fortune.

Although a bigger boiler or changing the system so that it does hot water priority to recover the hot water tank as fast as possible.

The boiler is a little on the small size.

 

[ALEC1]

if the radiators and coil can't diddipate the heat you gain nothing by oversizing the boiler...


how many radiators are there....

 

[ALEC1]

if the radiators and coil can't diddipate the heat you gain nothing by oversizing the boiler...


how many radiators are there....

 

[ServiceNumpty]

Why do you need power showers if you have an unvented. Has anyone given you the figures for the cold water flow rate & pressure. You risk dissapointment if these haven't checked. You could have a simple upgrade installation replacing hot water cylinder and adding extra cold water storage, maybe moving the tanks up into the loft to increase head. It really depends on budget.

 

[BoilerMAD]

We have 14 radiators in total.

We had the mains pressure tested and the plumber said it was ok for an unvented system.

We have a cold water storage and feed tank in the loft.

When we use 2 showers at the same time the pressure drops off.

If i use the calc

14 rads x 1.5kw = 21kw + 3Kw (hot water) = 24kw, is 24kw boiler more suited ?

thanks

 

[veto]

I had the same problems as you 12 months ago. Then i swapped my 24 year old Gloworm pilot lit boiler for a new Worcester Bosch Greenstar 37cdi condensing boiler. This serves 11 radiators (main rooms fitted with TRV`s),wet underfloor heating in a large conservatory and plenty of hot water for 2 showers and a bath. Nice warm house,hot water on demand 24 hours a day and my gas usage has gone down compared to the previous 12 months. Way up all the options on what are your needs before you make a choice.

 

[D_Hailsham]

If i use the calc

14 rads x 1.5kw = 21kw + 3Kw (hot water) = 24kw, is 24kw boiler more suited?

But if your 15kW boiler is heating the house OK, why would you need a 24kW one?

"Rules" such as "multiply the number of rads by 1.5kW" are fraught with danger and should not be used. After all your average rad size is only (15-2)/14 = 930W

It's much better to use a tried and tested calculator such as the Whole House Boiler Size Calculator.

 

[BoilerMAD]

But if your 15kW boiler is heating the house OK, why would you need a 24kW one?

"Rules" such as "multiply the number of rads by 1.5kW" are fraught with danger and should not be used. After all your average rad size is only (15-2)/14 = 930W

It's much better to use a tried and tested calculator such as the Whole House Boiler Size Calculator.

OK. its starting to make sense !

'Whole House Boiler Size Calculator' gave me a result of 16.37Kw.

The Megaflo recovery times are quicker with more KW from the boiler, is this a waste of time or do i need to allow any extra KW for heating up the water in a unvented cylinder quicker ?

 

[D_Hailsham]

'Whole House Boiler Size Calculator' gave me a result of 16.37Kw.

The Megaflo recovery times are quicker with more KW from the boiler, is this a waste of time or do i need to allow any extra KW for heating up the water in a unvented cylinder quicker ?

There are two options:

1. Heat the cylinder at any time, including when the central heating is on;
2. Turn off CH when heating the cylinder (usually called HW priority).

Under option 1, the cylinder can only use the excess available after the central heating has been satisfied. So the heat available for the cylinder varies. This means the reheat time will also vary.

Under option 2, all the heat is available to the cylinder, but the CH is off while the cylinder is being heated. The reheat time will be constant.

The boiler size for option 1 will have to be much greater than for option 2 to give the same reheat time.

 

[BoilerMAD]

There are two options:
1. Heat the cylinder at any time, including when the central heating is on;
2. Turn off CH when heating the cylinder (usually called HW priority).

Great Thanks !

So taking all the above into consideration, it would be sensible to :

1) Upgrade current boiler to 18-20kw System which is the most efficient boiler for my home or invest in a larger boiler and have it down rated to allow for future expansion.
2) Put in a large 210-300l Unvented Cylinder. Larger the better to avoid hot water shortage especially in the mornings.
3) Try to avoid heating the water at the same time as CH, which will increase recovery times.