Replacement oil boiler sizing & flushing

[Johnny7]

Hi
I am looking for some help with deciding what size of boiler to buy; all comments and suggestions most gratefully received.

I currently have a 19 year old Trianco Eurostar 70-90 (26.4kW) in an outbuilding that needs replacing.
Has to be oil.
Looking at the Grant range (local maintenenance available).
A question is should I go for a 21-26kW one or a 26-35kW model?
Current boiler has proved to be adequate and no house extensions are possible.
But the Grant ones are condensing so is a little bigger better or should it be the smaller one?

Current setup in outbuilding with boiler, 3 seperate pumped circuits (Living, bedrooms, DHW), all rads on 8 or 10mm microbore drops from manifolds in ceiling space (bungalow), not a sealed system.
Heating calcs give figures in 22 to 27kW load allowing for losses.

Recommendations for either utility range or pro range?
Differences seem to be cost and pro just a little more efficient.

Finally the system has never been flushed. All rads are steel.
Planning to flush before fitting new boiler or is it better after boiler exchange?
Any thoughts on best way to flush the drop loops to the rads (no drainage points at rads)

Thanks very much for taking the time to read this, and once again all comments most gratefully received.

 

[jtsplumbing]

Forget Power flushing if you have micro-bore , drain system, fit drain points on all drops (Radiator valves with drain off combined) remove radiators and flush out (Maybe consider replacing) add a good heavy duty cleaner (Fernox F5) and follow instructions on bottle.

 

[Chalked]

Smaller boiler, make it word hard.

 

[Reg Man]

I would not power flush micro bore, your likely to end up with more problems than when you started. Always fit the right size boiler for the job or you will not have an efficient system.

 

[kimbo]

The chances are that your old boiler was oversized. You will need to calculate what size boiler you need using the following link
Replacement Boiler Size Calculator
I would go for the pro it is basically the old multipass which in its day was the most efficient on the market and easy to service.
A condensing boiler works best when it is working hard, also you can regulate what the boiler produces that is why it is called a 15/26 etc.

 

[SimonG]

Your engineer will be able to answer all those questions for you.

 

[lame plumber]

if its in an outbuilding no need for shiny white cases etc, save some money

 

[Best]

If you need about 27 kw for your house, then a 21 - 26 kw boiler will IMO be too small in bad weather.
Remember that to get 26 kw, the 21-26 kw boiler needs set at it's limit. Frankly it is better to have the larger next size boiler & have it not at full output. The 26 - 35 kw Grant (as with most boilers this output) is at optimum efficiency as boilers go.
Just be careful to check you actually do need this high output.

 

[Reg Man]

Nice one Kimbo. Whole house sizing method every time. What ever you do, don't oversize the boiler, it will cost you dearly.

 

[WHPES]

If you have microbore, it's important that you do fit a filtration device to the system (Boilermag / Fernox TF1 / Magnaclean etc etc) to keep any dross out of the system.

Powerflushing won't work, what should be done (which I do with my customers) is each radiator removed in turn and manually swilled out in the yard using a hosepipe etc. Make sure this is detailed in the job specification and not a powerflush.

 

[Johnny7]

Hey Guys, thanks for those responses and opinions. I get the "do not power flush microbore" message strongly. I was planning on fitting a flitration device so thanks too for confirming that. I will try the recommended boiler calculator and see; I have used a few of these and they all come out 22-27kW which neatly straddles the 26kW point between the diff sizes of boiler!!Thanks once again for all this input.Johnny