Heat pump to replace heat-only oil boiler?

[clockworks]

Currently have an oil system boiler that just does heating via radiators.
Hot water is done by an immersion heater cylinder and separate header tank.
Cold water is a mix of mains (kitchen tap, upstairs toilet and sink, and 2 electric showers) and tank-fed (downstairs bathroom sink and toilet).

Oil boiler is in the garage. Hot water cylinder is in the kitchen diner at the opposite end of the house, with the tank directly above it.
Water main comes in to the garage.
Originally built as a bungalow, but 2 bed dormer conversion 20 years ago, and I think some of the pipework is "under" the upstairs rooms.

ASHP is due to be done next month, pre-install visit by plumber in 2 weeks.

Plan is to replace the oil boiler, and convert to unvented hot water, both in the garage, removing cold tank and how water cylinder.

I've no idea how they are going to connect the hot and cold water though, since a lot of the pipework is inaccessible.

Worst case, is it possible to leave the water systems alone, and just use the ASHP for heating?
If so, does the ASHP still need a separate water cylinder, or is everything "built in" inside the ASHP unit?

[119]

If they are not replacing the undersized pipe work, buy lots of jumpers.

[dhutch]

You could keep the old hot water cylinder, or replace it with a modern unvented unit in the same location, but equally they can likely find a suitable location at the garage end of the house to feed the hot water into the hot pipework.

As per the other post, I would be more concerned about the size of the radiators and to and extent, the pipework feeding them.
To get a good COP heat pumps typically run a much cooler flow rate than an oil boiler, hence need larger emitters to get the heat output back up.

[clockworks]

I'd prefer it if the old tank and cylinder were removed.

I suppose the new hot tank could be plumbed into the upstairs bathroom sink feed, which is easily accessible from the garage loft.
The cold feed for the tank-fed taps and toilet will be trickier, maybe have to run around the edge of the dormer rooms in the loft to where the current cold tank sits.

I guess I'm more concerned about the pipework length to the hot tap in the kitchen (time taken for tap to run hot), and the health risks of stagnant water in any "dead legs" left in the old pipework.

We don't have a bath anymore, so hot water storage isn't an issue.


All the radiators are 15mm copper, off 22mm to the boiler, so pipework should be fine.
Heatloss calcs suggested that one or two rads might need replacing - lounge and kitchen/diner.
There are existing tails in the lounge for a second rad if necessary.
Kitchen/diner is a vertical column, which could be replaced with one double the width - just need to move a 13A socket.


I'm only switching to ASHP because it works out cheaper than sorting out the cracked oil tank (condemned at the last service) and the 25 year old boiler.