When to Specify a Replacement Boiler

[DIYWell]

I've inherited a 3 bed detached house with a conventional boiler fitted in 1999. Current beast provides heating to the house and heats up a hot water tank.

Now time to think about a major refurb over the next couple of years and the order to do things. The plan will be to replace the boiler with a combi so hot water will be on demand. The cavity walls aren't insulated so that's on the to-do list. The attic was insulated in early 2000s but may need a bit of topping up; I've not been up there yet. Also a vague notion to insulated the suspended timber floor has come into my head, but may well leave again.

Question is, when a heating engineer is specifying the new boiler, does the intention to insulate play a part? Is there a possibility they would specify one model now, but once the refurb is complete, I'd find the boiler is over sized or modulates inefficiently given the bettered characteristics of the building?

 

[MeldrewsMate]

I try to get all customers who request a combi to reconsider. The 'old fashioned' hot water cylinder gives the opportunity for heating the water via solar, which isn't possible with a combi. It also provides a hot water supply substantially immune to demand based temperature fluctuations (if piped correctly), and can be heated via an immersion heater if the boiler breaks.
The weak points are that space must be given over to the hot water cylinder (most old houses will have been designed with just this space, an airing cupboard), and the risk of the loft tank freezing will be increased once the loft is better insulated (so insulate the loft tank better too).

 

[Razor900]

The 'old fashioned' hot water cylinder gives the opportunity for heating the water via solar, which isn't possible with a combi.

Erm... Are you sure about that?????????????

 

[Madrab]

Several suppliers now do a dual coil solar vented cylinder. Gledhill do one called a sunspeed 2, amongst others

The only other thing I would add though is that a combi can quite happily be installed onto a Y or S plan too, that will happily heat stored HW and the instantaneous HW output from it could be directed to one tap specifically, usually the closest kitchen HW tap.

Installed a few like that.

 

[DIYWell]

No intention of adding solar heating.

Question was about whether heating engineers consider the level of house insulation.

 

[fixitflav]

No intention of adding solar heating.

Question was about whether heating engineers consider the level of house insulation.

Why not leave it as it is if it's working OK? If you do change the boiler, I agree with Meldrewsmate that a combi may not be the best option. But if you do go down that route it will be sized to give decent HW flow,typically 30 kW, while the CH is likely to need about 1/2 that, so the state of insulation is irrelevant.
If you replace the boiler with another heat-only, just size it for the house as it is now. The CH demand won't be reduced much by improved insulation (whatever the sales pitch says) and it will still be OK. The output of the existing boiler, and how it copes in cold weather, will also give you a clue.

 

[Madrab]

When putting together a survey for a central heating system, heat loss calculations take into consideration materials, construction and insulation. That determines the radiator output required, the overall radiator output determines the boiler specification.