Pros and cons of mains pressure hot/cold water to taps?

[arg]

I'm currently starting on a major refurb/minor extension of our  80's house (upgrade insulation from really poor to Passivhaus Enerfit levels, etc, etc).  Current plumbing is traditional for the era - gas boiler feeding vented cylinder, large water tank in attic, only kitchen tap is direct from mains, all other hot/cold are from the tank.  Plan is UFH+ASHP replaces heating, 2x sunamps replace hot cylinder.  Main reason for particularly favouring sunamps here (over and above the usual trade-offs) is space to put them: the house is an odd shape with low roof height and while there's plenty of odd corners to put stuff they are all low height - existing cylinder cupboard is a triangular space in the eaves with a fairly small cylinder and no height for a bigger one,  never mind a meaningful thermal store.

 

Anyhow, the exact detalis of the heating system is a discussion for another day; I'm fairly convinced I want the sunamps and the question is how to plumb them.

 

Obviously the easy thing is to just put them in place of the existing hot cylinder - cold from the attic tank runs through the sunamp(s) and out through the existing pipes to all the taps, cold supplies unchanged, feed in and out of the Sunamp just adapts the existing pipes that go to the cylinder.  Nice, quick, easy job.....  However, I'm tempted to convert the system at least partly to use mains pressure water to the taps, for the following reasons:

 

1. As mentioned, the house is an odd shape, and in particular is split into two halves (half single-storey, half 2-storey),  with only one possible route from one half to the other for MVHR ducts (low ceiling heights and a structural beam in an awkward place means there's literally no other option).  That space is currently filled with a total of 8 pipes running from the tanks in the attic to the cylinder cupboard.  Two will disappear (boiler circulation is also vented with a header tank in the attic), but the others - 6 independent cold feeds to various taps/showers etc - will need to be relocated somehow.  At minimum, this makes me rip everything out and put it back so keeping the existing arrangement is no longer the "easy option"; also, cold mains comes from the other direction so probably makes it easier to find new routes.
2. Due to the low ceiling heights, there's not enough head for a shower on the 1st floor - existing 1st floor shower room has an electric pump to make it work, which it would be nice to eliminate.  Also, given 2x sunamps for total capacity, one option is to split them up - place one of them in that 1st floor shower room right next to the shower and also serving the kitchen immediately below, with the other one in the existing cylinder cupboard over the other side of the house feeding the bathroom and gnd floor shower room there: this would considerably shorten the hot runs and so time/water/energy wasted running the shower/tap waiting for it to warm up.
3. I've never liked having "not really drinking water" at bathroom taps, having been stored in the attic tank.  Though maybe this point is moot given I'd probably feed everything from the output of the water softener so that's not ideal for drinking either.

 

Item 1 has suddenly made this urgent, where I'd previously been leaving this stuff until the end of the build .

 

So I could now:

• Leave as-is, just find alternate routes for all/most of those pipes.
• Put the 2nd sunamp in the 1st floor shower room, and feed that with mains cold water (easy as it enters the house over that side), mains cold also used for the cold feed to shower and other taps nearby, but leave the other side of the house running off the attic tank.  This (I think) eliminates one of my problem pipes, and also frees up what was a hot-out pipe through the (also crowded) floor void that could be used in the reverse direction to replace another one.
• Scrap the attic tank altogether and run everything off mains pressure.

 

What are the pros and cons here?

 

Current system has dedicated cold feeds to all the main users (though not so much for hot), hence no issue with one tap turned on reducing pressure to others.  Is this going to become a big deal if I start using mains cold and at least some of the pipework is shared?  Water softener probably makes this worse (it is currently located right next to where the cold enters the building, so no problem for routing, but bound to add to pressure loss).

 

Are there regulatory concerns?

 

Sunamp install guide suggests I need an expansion vessel if the water meter contains a check valve (haven't yet determined if this applies to me).  Presumably only a really tiny one given the volume inside the Sunamp - do extra-small ones exist?

 

 

Note that I've written the above as if I'm doing the work myself, and I might have no option but to get do the first bit DIY since I think plumbers aren't currently working non-emergency jobs, but I'm open to being told that it needs specialist design if that is the case.

[ProDave]

When you have had mains pressure water from all taps, hot and cold, you would never ever want to go back to 19th century plumbing.

 

Beware of finding leaks in the pipework that has been fine for the last 40 years at low pressure.

[Russell griffiths]

Why would you do all that work and still keep a cold tank in the loft??

 

if you are trying to achieve that level of insulation why will you still have a small cupboard, rip the lot out, all internal walls, it will be easier to work on and you will end up with what you want instead of a compromise. 

[arg]

2 hours ago, Russell griffiths said:

Why would you do all that work and still keep a cold tank in the loft??

 

I was afraid of loss-of-pressure issues, and/or other issues I hadn't thought of that people here would pop up to tell me!

 

If running off mains pressure is that easy, why isn't everybody doing it?  Or maybe there was a great revelation in the late '90s and everybody now is?   This is really why I posted this thread, I want to understand the issues (and every house I've ever lived in has had a tank).

 

2 hours ago, Russell griffiths said:

if you are trying to achieve that level of insulation why will you still have a small cupboard, rip the lot out, all internal walls, it will be easier to work on and you will end up with what you want instead of a compromise. 

 

Well, we could knock the house down and start again (and occasionally I think that could have been a better idea, assuming we could have got planning permission).  But without doing that, there really isn't anywhere suitable.  The house was built to be as low height as possible, I am guessing for planning reasons at the time: being an almost triangular plot with the thin end at the street, if the house was built where it "should be" in line with other houses in the street, then it would have been a tiny house.  Instead, we have a gratuitously grand-looking garage built where the house should have been, and the house built further back on the plot pretending it's not there (the original planning application describes it as a "Chalet Bungalow", when it's clearly not a bungalow!).   Net effect of this is that all the upstairs rooms are built into the roof shape and any corner you partition off will have sloping ceiling and low height.

 

Anyhow, up to now I wasn't feeling particularly compromised - but maybe I didn't know what I was missing.

[Mike]

55 minutes ago, arg said:

If running off mains pressure is that easy, why isn't everybody doing it?  Or maybe there was a great revelation in the late '90s and everybody now is?

The regs were changed in about 1985, and unvented cylinders and combi boilers have certainly taken off since then, and to some extent thermal stores. No statistics, but I don't think many new builds to have vented cylinders these days.

 

Many mixer taps are also only suited to mains pressure hot water so, if you don't go that way, check before you buy.