Two condensing boilers instead of one system boiler? Ridiculous idea or not?

[DonnaP123]

So we currently have a decent condensing boiler which we picked to have extra ‘welly’ in case we ever added a second bathroom. Well we’ve gone a bit further than that and built a large L-shaped wrap around ICF extension. The current boiler can easily service 2 bathrooms that will be above it, without moving it or adding much extra pipe work.

 

However, our en-suite, the kitchen and the manifold for the UFH are at the side of the extension, about as far away as you can get from the existing pipe work.

 

we are wondering if it wouldn’t be simpler to add a second condensing boiler in the new kitchen for the en-suite and UFH. 
 

This would take up way less space than a system boiler, and would be relatively easy to get the gas to it and much simpler pipe work. I think the cost would even out with the labour bill being lower.

 

We will have a highly insulated and airtight house when done, so the demand for space heating should be low. We’ve ruled out an air source heat pump for now (and realise we are possibly ruling one out forever if we don’t put the pipe work in for a single central point now when everything is easy to get to).

 

Just after views as to whether this is a ridiculous idea or not?

[Mr Punter]

All gas boilers are condensing.  I think you mean combi boilers?

 

You don't need lots of hot water for a kitchen.  Dishwasher heats cold water.  You could just get an under sink heater for any washing up or hand washing.

[JohnMo]

7 hours ago, DonnaP123 said:

we are wondering if it wouldn’t be simpler to add a second condensing boiler in the new kitchen for the en-suite and UFH. 

It may sound easy, but that UHF will not want many kW of energy to keep it up to temperature, you are likely to short cycle the boiler to death, without a big buffer.

 

But to be more clear with your terminology 

 

All boilers sold since the 1990s have been condensing.

 

A system boiler has a hot water cylinder, DHW is provided by the cylinder.

A combi boiler provides DHW on demand from the boiler and has no cylinder.

[DonnaP123]

3 hours ago, Mr Punter said:

All gas boilers are condensing.  I think you mean combi boilers?

 

You don't need lots of hot water for a kitchen.  Dishwasher heats cold water.  You could just get an under sink heater for any washing up or hand washing.

Totally right - I meant 2 combi boilers!

[DonnaP123]

2 hours ago, JohnMo said:

It may sound easy, but that UHF will not want many kW of energy to keep it up to temperature, you are likely to short cycle the boiler to death, without a big buffer.

 

But to be more clear with your terminology 

 

All boilers sold since the 1990s have been condensing.

 

A system boiler has a hot water cylinder, DHW is provided by the cylinder.

A combi boiler provides DHW on demand from the boiler and has no cylinder.

Totally right - I meant 2 combi boilers! But UFH is okay to run from a combo boiler isn’t it? I’m not sure what you mean by needing a buffer?

[JohnMo]

16 minutes ago, DonnaP123 said:

Totally right - I meant 2 combi boilers! But UFH is okay to run from a combo boiler isn’t it? I’m not sure what you mean by needing a buffer?

The basics are

A boiler has a rated kW output that's it's max output. It also has a minimum output. The difference between the two is the modulation range. My Atag combi boiler would do 34kW max and 6kW minimum. When the output was below 6kW it would switch on and off to manage the heat output.

 

Short cycling is when a boiler starts and stop to quickly. What causes a short cycle.

1. Not enough water volume to carry the heat away from the boiler efficiently. You would add a buffer to meet this requirement.

2. Not enough flow rate.

Both 1 and 2 are required.

 

To support 6kW output without short cycling you need a flow rate of around 16L/min and over 50 Litres of water actively engaged in the circulation circuit.

 

Not sure if the size of your extension if it's zoned etc, or the boiler you propose to use, but unless it's huge you are unlikely to meet the required min flow or volume for a combi boiler. Unless it's a Viessmann model (£1500 to 2000 approx) that can turndown to around 1.8kW.

 

 

[SimonD]

16 hours ago, DonnaP123 said:

Just after views as to whether this is a ridiculous idea or not?

 

This is a ridiculous idea, don't even go down that route. It's never as simple as people think to 'just' run a new gas pipe and then re-plumb the system for the 2 boilers.

[JohnMo]

28 minutes ago, SimonD said:

 

This is a ridiculous idea, don't even go down that route. It's never as simple as people think to 'just' run a new gas pipe and then re-plumb the system for the 2 boilers.

I need to be more blunt and to the point. But at least he now knows he has a condensing combi boiler. He just has to figure out what the condensing bit means.

[Mr Punter]

14 minutes ago, JohnMo said:

But at least he now knows he has a condensing combi boiler

 

With a user name of DonnaP123 the OP is probably female (or whatever you are supposed to say now).

[JohnMo]

1 minute ago, Mr Punter said:

 

With a user name of DonnaP123 the OP is probably female (or whatever you are supposed to say now).

Why should that make a difference, worked with really great engineers of all genders, also worked with some really dumb ones mostly blokes.

[Mr Punter]

16 hours ago, JohnMo said:

Why should that make a difference, worked with really great engineers of all genders, also worked with some really dumb ones mostly blokes.

 

Because you referred to them as "he" three times.

[joe90]

In my little cottage (with a combi boiler next to the sink) I wash up using a kettle fir hot water (if I don’t use the dishwasher) the combi is 28Kw and the kettle 3Kw and the kettle boils quicker than the combi can supply hot water to wash up. 🤷‍♂️

[SimonD]

1 hour ago, Mr Punter said:

 

Because you referred to them as "he" three times.

 

Do we need a pronoun signature, I wonder? If we did, mine would definitely be 'it' 😉 I'll go back to my cave so as not to cause further offense.

[SimonD]

1 hour ago, joe90 said:

In my little cottage (with a combi boiler next to the sink) I wash up using a kettle fir hot water (if I don’t use the dishwasher) the combi is 28Kw and the kettle 3Kw and the kettle boils quicker than the combi can supply hot water to wash up. 🤷‍♂️

 

You highlight the biggest problem with combi boilers - and long runs from the combi. I always ask my customers who are considering a combi whether their use of water involves frequent small draw offs and in those cases, it's always more efficient to have a cylinder, or some kind of small hot water storage. Some combis with preheat help, but not by much.