(Part?) replacement central heating installation

[Rick Bowlby]

I have read as many FAQs, previous threads etc as i can find, and learnt quite a lot, but would still appreciate advice if anyone can spare it.

I live in a 100 year old semi-d house, with existing conventional floor-mounted boiler (approx 18 years old), open vented central heating with a mixture of radiator types, hot water from cylinder, and a pumped shower. I intend to get rid of most of it, and install a new condensing combi boiler in the cellar, convert the central heating to a sealed system (keeping about half the radiators, replacing / relocating the rest), and convert the hot water to mains pressure (get rid of the shower pump and cylinder etc). There's one bath, one shower and one kitchen sink, with only one likely to be in use at any given time.

The work will be done by an installer (so this isn't really d-i-y) but I'm planning to specify a Worcester Greenstar 35CDi combi boiler, Honeywell CM67 programmable digistat (probably the wireless one, so that I can experiment with locations), Myson (round-top) rads to match the retained ones already installed, ? TRVs. I've calculated the required radiator sizes using an old copy of Myson Heatloss Manager (Myson don't seem to have this on their website any more, which is a shame).

I would be grateful for any reassurance that this all sounds reasonably sensible, but in particular:

1) With condensing boilers is it still important to get the 'right' size or might there be benefit in over-sizing a little? 2) Any problems using a Honeywell digistat with a worcester boiler? There seem to be other similar products (eg Danfoss) - are there significant differences? Surpisingly the manufacturer's websites don't seem to give clear details. What we need is 7-day control, with the ability to over-ride for periods, both for days (holiday) and hours (during the day), without having to re-programme. 3) Controls generally... I've seen a number of threads about the wisdom or otherwise of a traditional room thermostat in the hall, no TRV on the radiator etc. (I think) I understand the requirement to achieve 'boiler interlock' by one means or other. However I wondered if anyone has knowledge or experience of more 'hi-tech' solutions, eg Honeywell CM-Zone? Also, any preferences for TRV manufacturer?

Sorry about the length of the post. Any comments would be much appreciated.

[Ed Sirett]

1) Yes, see BoilerChoice FAQ (this is quite new so you may have missed it). 2) Well an on connection is an on connection regardless of the name on the unit. None of them are proportional controls AFAIK. A Horstmann Centaur Stat 7 will do what you want, other units too. 3) As far as I am concerned a thermostat (programmable or not) is the only plausible way to achieve the required interlock. Leaving the TRV on a radiator which is near to the thermostat will give an _opportunity_ to make the thermostat worse than useless. However leaving a TRV on the radiator nearby will allow to fine tune the balance between the thermostat and the overall house heating.

I have no experience, as yet, of the high tech stuff.

[Rick Bowlby]

Which is 'yes'?! Actually I had read the FAQ, which was helpful - "Modern boilers can adjust their output power over a range automatically, therefore over-sizing is less of a sin that it used to be, nevertheless getting this right will save some gas". However I do want to ensure that there is a decent HW flow rate. Heatloss manager reckons I need a 25kW boiler for heating, so I was thinking of going up to 30-35 to be on the safe side with HW.

[Ed Sirett]

Seriously. That's a gross over estimate of the HW requirements when combined with a cylinder/thermal store. You'd only need that sort of HW if you have a stack of people (12+) people living at your place.

28-30kW (load of models around that size) would be quite enough.

However you should allow 40 litres of stored HW/person.

[Rick Bowlby]

But I want to get a combi boiler and mains pressure HW, does that make any difference?

[Ed Sirett]

Hugely. Just get the biggest combi you can afford to install. Thewre seem to be a cluster of premium models around the 33-37kW mark as opposed to the 24k or 28kW more basic models.

There are some combis which have substantial stores of HW built in these units may suit your requirements better.

[Rick Bowlby]

Thanks, that's useful.

Are there any sites (eg retailers) where you can compare boiler features?

[Ed Sirett]

Just put a boiler name into google. Ignore the manufacturer. Find any of the numerous on line retail sites most of which list many manufacturers. You can then compare features and prices. The relative prices may well hold true in general.

[Tony Bryer]

Our free QSEDBUK program will let you select boilers with particular output, fixing position etc. There are other factors which are harder to categorise: for example in our church the Keston Celsius is particularly suitable because of the fluing options and being able to remote wire the panel run/fail lights back to a visible position.