Glad I read this thread, we've been considering getting a combi so that we can get rid of the tank and have space for a bath in our upstairs bathroom shower room. I shall have to look into the low flow abilities of the boiler currently top of the list.
The distribution box is immediately below the bathroom
Does your shower drain downstairs or horizontally between the floors? If the former you've got the ideal setup for Drain Water Heat Recovery. It's a year since I looked into them but back then there were three types:
1) Vertical pipe in pipe, the only type legal in the UK
2) Horizontal, becoming a thing in Trumpistan
3) Horizontal compact for fitting under a shower tray, some European manufacturers.
See EcoDrain for example.
Exploring the Eco-drain idea, and if I've got my maths right:
* Energy required to heat water = 4200J/kg/C
* 1kg water = 1 litre. Let's say the average shower is 40 litres.
* let's assume water enters the shower at 50C and leaves at 40C. Energy remaining in water entering drain is:
40 x 40 x 4200 / 3,600,000 = 1.86kWh.
* Let's now assume 46% of that heat is recoverable, therefore 0.86 kWh of heat is recovered. If you have an electric shower and your electricity costs 12p/kWh then the Eco-drain saves you 10.3p/shower. However, if you are using gas it's saving you perhaps a quarter of that.
Showers can have a flow rate of 3 - 30 litres per minute with the average shower apparently 8-12 minutes (far less for us baldies, especially if you turn off flow while you soap up). A shower therefore ranges from 24 to 360 litres! Assuming an aerating shower head limited to 9 litres/minute that gives a volume range of 72 to 108 litres. Both of which seem high (our household of four uses a modest total of 75 litres/person/day including 2 showers and 1 bath on the average day).
On the face of it the Eco-drain seems to make sense IF you have an electric shower. However, is it really 46.6% efficient? The last bit of heat it recovers in each shower pre-heats incoming water which isn't used for example. It also has lots of small bore pipes which won't much like hard water! I think I'm also optimistic in assuming water enters the drain at 40C as a good deal of heat will be lost running down the shower enclosure, falling through the air etc. Interesting though.