What size boiler would you recommend.

[tommyleestaples]

Hi all, 

 

I have just extended my house and need to choose a new Combi Boiler. The property is 4 bedrooms and will have 16 radiators in total. There is one ensuite with a shower and then a family bathroom which has a bath with shower above. 

 

I have the option of a 32kw or 40kw. The 40kw has a higher a higher DHW floor rate. 

 

[JohnMo]

Number radiators or bedrooms is a bit meaningless less. For heating, heat demand, you need to do a heat loss calculation. But unless you have a wall missing any of the referenced boilers is fine.

 

But for hot tap water you need to look at flow rates and boiler turn down for heating. This will give a good compromise.

 

But with a 4 bed house, I would look elsewhere and you may be better with a storage combi. Similar size, better flow of DHW.

[tommyleestaples]

Thanks for your comments JohnMo.  Understand what you are saying about storage combi but looking to stick with standard combi for now.

 

In regards to heat loss calculation. I haven’t had one done but 80% of the walls are new cavity walls with 150mm rockwool insulation. The loft is insulated with 270mm rockwall. The ground floor is insulated with 100mm celotex and everywhere is double glazed. There are large bi folds to the rear 3.6m x 2.1m

 

based on that would you go with the 32kw boiler or 40kw?

[Mr Punter]

You only need to size the boiler for DHW.  The heating demand will be covered several times over.

[marshian]

21 minutes ago, Mr Punter said:

You only need to size the boiler for DHW.  The heating demand will be covered several times over.

 

Unfortunately the three boilers shown compromise space heating min output by increasingly high min modulation

 

I'd pick the smallest one on that basis

 

Actually I wouldn't I'd go for a Viessmann 200-W and have a CH min modulation of 1.9 kWh regardless of how big the HW side was

 

https://viessmanndirect.co.uk/Catalogue/Residential-Boilers/Viessmann-Combi-Boilers/Vitodens-200-W-Combi-Boilers/Vitodens-200-W-35kW-Combi-Boiler-Z020316

 

HW 17l/Min @ 60 Deg C for the 35kWh Combi

CH Modulation range 1:17

[Mr Punter]

1 hour ago, marshian said:

 

Unfortunately the three boilers shown compromise space heating min output by increasingly high min modulation

 

I'd pick the smallest one on that basis

 

Actually I wouldn't I'd go for a Viessmann 200-W and have a CH min modulation of 1.9 kWh regardless of how big the HW side was

 

https://viessmanndirect.co.uk/Catalogue/Residential-Boilers/Viessmann-Combi-Boilers/Vitodens-200-W-Combi-Boilers/Vitodens-200-W-35kW-Combi-Boiler-Z020316

 

HW 17l/Min @ 60 Deg C for the 35kWh Combi

CH Modulation range 1:17

 

I agree the Viessmann looks good but I wonder what the average actual savings are for the high modulation range given they are about £500 more expensive?

[marshian]

2 minutes ago, Mr Punter said:

 

I agree the Viessmann looks good but I wonder what the average actual savings are for the high modulation range given they are about £500 more expensive?

comes with wifi connectivity out of the box so all the parameters (boiler run time, cycles (starts) all avaliable - if the house heat loss is 8kWh at -2 I’d go for one of the OP’s selections because at 10 deg C OAT they would be a good match but if the OP’s heat loss at -2 was 4kWh I’d go for the viessmann - low and slow like a heat pump is a much better way to heat a house