why the different boiler spec and which is right?

dirtmother

We're intending to get a replacement combi boiler and thus far have two quotations: one from Worcester Bosch's local recommended installer, and one from British Gas.

The first specified a 38... British Gas chap said that was way more than we needed and would be more expensive to run and has specified 28 (not that quotation works out cheaper)

We currently have a circa 15 yo Worcester Bosch 24. Very temperamental hot water supply, central heating fine. And horrendous (over £200/month DD) gas payments despite comparison sites, insulation, double glazing, thermostats on radiators, general thermostat set to 18C. Running 14 radiators (if you include heated towel rails), one mains shower, one electric (and we do have baths from time to time).

WB site says 34... although initially agreed with 38 but if I've understood correctly that would be for 2 mains showers.

macman

The BG guy is clueless. A higher rated boiler won't cost any more to run, assuming that the same level of heat and hot water is consumed. It'll burn gas faster, but it'll heat the house and hot water faster (subject to the rest of the system being in balance with it).
BG normally come in around 40% more expensive than an independent RGI anyway (as your quotes demonstrate), so why pay the same for a lower rated boiler?
Any combi will run your CH side with capacity to spare, what's important is the DHW flow rate. If that's inadequate at 24kW, then 28kW is not going to improve it very much. Even a powerful combi isn't designed to supply two baths or showers simultaneously, even supposing that your water pressure is sufficient to so so.
There is a WB Greenstar 34CDI available, but I doubt that the overall cost will be much less than the 38 (since the install costs the same for a cheap or an expensive boiler).
Do you actually want a combi? Is your pressure and flow rate sufficient to run a higher powered combi? Did both installers check both the mains pressure and flow rate?
Which brand of boiler are BG proposing?

dirtmother

Yes, combi is what fits our circumstances. We have good water pressure and flow rates of boilers were discussed at some length. We have one mains shower and that's in the bathroom so wouldn't want to fill both of those at once but it isn't unusual for there to be two demands for hot water in the house at once. Current boiler copes reasonably well with that, if it is in the mood to provide hot water at all that i - and indeed the way to persuade it to heat the shower is often to go and turn a hot tap on elsewhere...

verydeeppockets

As above. A 38kw boiler will cope with two showers, and won't use any more gas than a 28 for space heating (rads).
A local recommendation for installer is key - that excludes british gas as their prices are usually way off. A small to medium sized local firm is usually best value vs best service and likely to respond if there's problems.
If a recommended installer is also WB approved you'll get either 5 or 7 years warranty as well.