by ericmark »
Sat Oct 10, 2020 4:52 pm
My house has 5 bedrooms, three floors, 2 kitchens, two bathrooms, plus shower room i.e. a big house, boiler 24 kW oil.
No boiler can supply three showers at the same time, only way to fill a bath fast is to have a water store.
The main reason any house has a 28 kW boiler is to fill the bath fast, 18 kW would likely keep house warm.
So 14 radiators if each was 2 kW that's 28 kW total it can use, have a look at each radiator and compare it with ones for sale is say screwfix and work out a reasonable size. A 600 x 1200 mm double radiator is rated 2146 watt at Delta T50°C what the Delta T50°C means is whole radiators is 50°C hotter than room, since we want cool water returning or the boiler will close down the water would need to leave boiler at 80°C and in real terms that is unlikely, so that radiator is likely to give less than 2 kW in real terms.
With a modulating boiler the output reduces as the return water heats up, so you don't notice that the boiler is not at full output, because mine does not modulate you notice it turning off/on, and when the house asks for the temperature to change from 16°C to 20°C if the radiators were sized for the boiler it should run for at least 2 hours, but in fact lucky if it runs for one hour before it starts to cycle.
So in the main what you look for is not so much maximum output but minimum output. If the boiler will only close down to 18 kW from 24 kW common for oil, then it will still cycle, if it can close down to 6 kW then far less likely to cycle and so you have a lower hysteresis in the room temperatures.
Think about it, the TRV controls the temperature in each room, it is an analogue device which slowly reduces or increases flow to maintain the temperature, as soon as boiler turns off, the radiator cools and the TRV opens, so when boiler turns on again the TRV starts to close, but not fast enough so room temperature over shoots, but if boiler turns down, then TRV does not open too wide so you get an even temperature.
I would say radiators capacity about 1.5 times boiler size as you never want all rooms heating together, I set the program for each of my TRV's according to room use, so bedroom does not start heating until 10 pm, and living room is not heated after 12 pm, the kitchen is heated first as I approach home, followed by dinning room, then living room, i.e. in the order the rooms will be used.
With gas I would want OpenTherm and if boiler not OpenTherm it would be removed from list of contenders, the way to lower bills is control, so your looking for a boiler that can be controlled well, but also one which can be maintained.
I was reading a report plumbers had written about Which Mag and it was rather damming, they pointed out some boilers with very easy maintenance were bottom of the list, it seems Which has sold out to highest advertiser and is not longer to be trusted.
Sorry to say but Mags have done this for years, they would compare Skoda cars to Lada cars push rod engine to over head cam engine and claim the Skoda push rod engine could get more MPG to the Lada OHC engine, in their dreams, neither was a good car, but the Mags gave wrong info even in the 90's.
What you want is cheap to maintain and cheap to run, and if the heating engineer knows the boiler well he can service it faster and cheaper than one he does not know, so ask the guy fitting the boiler what make, in last place the guy did not deal with Bosch, they sell parts it seems to their own engineers cheaper than to general engineers so general engineer could not compete with prices, so did not bother with Bosch boilers. Be it right or wrong does not matter, but fact was if an Ideal boiler broke down he would normally get around and fix it in 8 hours normally less, but Bosch would take a couple of days as he did not hold parts.
Here with oil boilers Bosch seems to be common, so local guy can fix a Bosch fast. So in North Wales I would not want Bosch and Mid Wales Bosch no problem.
Same with cars, if nearest Jaguar dealer is 50 miles away, even if a really nice car, maybe not the best option.
But main point is three bathrooms does not seem to line up with combi boiler, at least if used, combi is great in a small house but as the size of house goes up, then stored hot water is more the order of the day.
Having said that I could likely use a combi if I had gas, as hardly ever run a bath, and two out of three showers are electric.
