New boilers - keep in tandem or replace with one?

[AndyCZ]

Hi

I am looking to replace our two Vaillant boilers (20 years old now) with some newer more efficient condensing boilers and wanted some advice on the setup.

Currently we have 2 x 28Kw boilers for our house which is split into two zones (upstairs and downstairs) and has a 300 litre unvented water cylinder.

I have done the various website calculations based on property size, glazing, insulation etc. and they come back with around 54 Kw so I think we are currently with the right sized boilers.

If we go for one boiler, we are looking at a small commercial boiler. But we could go for two boilers again and stay on the higher residential end. Is there a right way to do this or does it come down to preference? Would two boilers be slightly more efficient than a single larger boiler.

I am also looking at weather compensation controls etc. (which we do not have at present) and wonder how this would, or wouldn't, cope with two boilers linked.

Thanks for the advice!

 

[JohnD]

one boiler will sometimes break down

two boilers are unlikely to both break down at the same time

what's wrong with the old ones that means you need to replace them? How much money do you hope to save p.a. on improved efficiency?

 

[ALEC1]

are the two boilers on the same system or two independent systems?

You can cascade two smaller boilers and for weather comp you will need vaillants rather expensive commercial controller...at least two domestic boilers will be competitivel priced, an you will get a much lower modulation range rather than one bigger one...

 

[AndyCZ]

one boiler will sometimes break down

two boilers are unlikely to both break down at the same time

what's wrong with the old ones that means you need to replace them? How much money do you hope to save p.a. on improved efficiency?

We've had a few issues with them over the last twelves months. Circuit boards, diverter valves - the cost does add up in the end.

I would like to save 10-20% on efficiency per annum (annual gas bill is just over £5,000 at the moment - but we have an AGA/swimming pool boiler). Part of that would come from the improved boiler efficiency (the current ones are not condensing) but the system overall needs a good checkup. I plan to rebalance the house over the next week as we've had a number of new radiators added, taken off for decorating etc. in the last twelves months.

 

[AndyCZ]

are the two boilers on the same system or two independent systems?

You can cascade two smaller boilers and for weather comp you will need vaillants rather expensive commercial controller...at least two domestic boilers will be competitivel priced, an you will get a much lower modulation range rather than one bigger one...

The boilers are on the same system - but zoned. One boiler does upstairs + cylinder, the other does downstairs.

Each zone has a programmable thermostat but these are very poorly located just above two radiators with TRVs.

 

[ALEC1]

modern condensing boilers are prooven technology, especially if you install them with proprietary weather compensation controls around which they are designed. In germany such controls have been mandatory for about 15 years...

Integrating the pool into the system is possible, but you may find a pool boiler a better solution... they can be highly efficient..

 

[AndyCZ]

modern condensing boilers ate prooven technology, especially if you install them with propritary weather compensation controls around which they are designed. In germany such controls have been mandatory for abput 15 years...

integrating the pool into the system is possible, but you may find a pool boiler a better solution... they can be highly efficient..

The pool has it own Jandy 240k BTU boiler which was replaced a couple of years ago. Sadly with the current English summers, it rarely gets used and is very very inefficient to to use it when you rarely get above 20c here in the summer!

If I look 5-10 years down the road, I think the savings of a better, more modern system outweigh my current setup.

 

[ALEC1]

I agree, you won't have a bad experience if you change the boilers and the controls and use the system as they would on Germany!

The pool is out door then?

 

[AndyCZ]

The pool is out door then?

Yep. It would be nice to have an indoor pool given the lack of use of the outdoor one over the last few years. It takes a day or so to heat up, then the weather changes! Even if it's not heated, you still have the cost of maintaining it with the chemicals etc.

 

[ALEC1]

the solution for out door pools is probably solar...for fairly obvious reasons, or possibly air source heat pumps...I would have thought