Boiler / HW Cylinder choices

[HarryW]

nickd01 said:

It's approx 2500 sq ft, 3 showers, 2 baths and 6 people.
From what I've read (Always dangerous!) - a property similar to ours needs as little as 12kW to heat but it's the DHW that requires more 'power' to warm up. So I'm looking at around an 18-24 kW boiler but this is based on a hunch rather than a calculation.
It looks like the heat-only boilers are less common, and the system boilers with the expansion vessel in seem to be more available as well as more advanced. I can't see why I'd chose heat-only, as the one with the expansion vessel in is the same size so surely it'd save me some space in the cellar where the hot water tank goes?

With vaillant boilers they are designed for priority hot water systems and you must use their controls to fully utilise it, simple aftermarket 230v switched controls like nest and Hive will just treat the boiler as a single heat source for both and not utilise the boilers features properly. Vaillant controls use 24v logic over an eBus to control.
That is vaillant deal with hot water and central heating demands separately. For instance the 620 system boiler can provide between 2.8-21.2kW in condensing heating mode and 2.5-19.8kW in non condensing heating mode. I’d suggest you’d only be interested in the condensing mode though. The same boiler can provide 23.6kW when in hot water mode. All from a notionally rated 20kW system boiler.

See what Vaillant say and make sure the guy is a master tech (qualified) for boilers, controls and cylinders, it wouldn’t be the first one who was only boiler qualified. You need someone that can design and spec a complete system. Don’t be surprised they recommend the Vaillant unistor cylinder too.

I have a year 2000 4 bed 1 bath 2x shower house, my heat losses are under 8kW and I have 13kW of radiators. I range rate my heating circuit output to 14kW, my 210ltr unvented cylinder for hot water has a 20.5kW rated coil I range rate my hot water output to 21kW. That’s all from a 18kW boiler that can produce up to 19.5kW heating and 25.2kW hot water when using vaillant controls. If using after market then it would be 19.5kW for both, ergo slower HW heating times and inefficient use across the board.

Edited by HarryW on Wednesday 8th May 11:50