Loft conversion plumbing conundrum

[HiDBandSIL]

I am going cross-eyed trying to decide what to do about the plumbing for a new loft conversion. Please can someone tell me what the right thing to do is in this situation?:

Getting a loft conversion done going from 2 bed 1 bath to 4 bed 2 bath.
Currently have gravity fed system with 1 year old hot water cylinder and 15 year old boiler (not condensing). The hot water cylinder has a brilliant refresh rate but it (along with the boiler) are technically massively under sized for the house post loft conversion.
Not enough water pressure or flow rate for an unvented cylinder. Pipework probably lead.
Water supply pipe connection is shared with neighbours and comes in from the road over their land and under their house to get to us (they wouldn't agree to anything being done on their property e.g. upgrading that pipe).
Massive hatred of combi boilers (for their crap flow rate) and not keen on electric showers.
Shortly going to redo kitchen (home of the stopcock) at which point we'll probably replace the boiler.
Thames Water area in case that is relevant.
Water tanks in the loft need to move to make way for the loft conversion but airing cupboard housing hot water cylinder can stay.
Currently 2 little children but planning another 1 definitely and maybe 2.

I think the options are:

1. switch the cold water tank for a coffin tank (located in the eaves) and install a negative head pump. We lose very little space in the loft to this and I'm not too concerned about the noise the pump will make. My concern is that we'll end up having to replace the undersized hot water cylinder and I'd then feel as though we should be replacing it with an unvented cylinder; and
2. change to an unvented system by getting a new separate mains water connection and getting a mole to dig the tunnel under the house before starting the loft conversion, then fit an unvented cylinder in place of the new hot water cylinder in the airing cupboard. Downside - expensive and time consuming at a time when we just want to get the loft conversion done.

Having written all of that out I'm thinking option 1 because it is quicker and more economical but that is only on the basis that a gravity fed system with coffin tank and pump will be as good as an unvented system (or anyway plenty good enough for 2 simultaneous showers running back to back).

err... so maybe that's my ultimate question! Is it?

if you're doing building work, now is the best time to run a larger new pipe out to the pavement, and also run larger pipes inside the house. Ask your water company now to test your drinking water for lead content. The might be a subsidy. Get the test done before starting work. They may be slow to arrange an appointment.

What is the power of your existing boiler?

You'll be improving insulation, I expect, so heat loss of the house may not increase. You can estimate it on calculators.baxiknowhow.co.uk/boiler-sizing/

If you have any rooms that don't get warm enough, measure the radiators. They are often too small. As a very rough rule of thumb, the nominal output of a single radiator 1 metre long and 600mm high is 1kW (actual output will be lower).

It's a Baxi Solo 3 40 pf which is 9.09 -11.72 kW (31,000 - 40,000 btu/h). I'll do the calculator now. All radiators currently fine but I just found out that you need to fit the feed and expansion vessel at least 1 metre above the highest radiator which we won't manage in our loft (low ceiling) so it looks like we will in any case need to change to a system boiler.

I agree with you on the principle of "if you're getting work done you might as well" but given that our neighbours won't let us do anything on their property it will involve a new separate supply which I've been led to believe will cost a lot (particularly as we're on the wrong side of a one way road with no footpath). I will get a formal quote.

We're in a hard water area which I've heard means we're unlikely to have lead levels high enough to earn us a free upgrade. I will get it tested though.

Which gets me back to the question of whether it is worth it. I know unvented cylinders are very good but would one be much better than a coffin tank plus negative head pump? Would it be any different at all apart from the noise from the pump which I've heard isn't that bad when muffled in an airing cupboard?

Mains water pressure (garden tap) is 2.4.

Quote from Thames Water of £1,600 inc Vat for the new connection excluding traffic management.

I wrote a long reply that disappeared

Old pipes may leak when put up to pressure, so you need to renew old H&C pipes throughout, and radiator pipes if they are old or suspect. Your pressure is OK, but a new 32mm plastic pipe will greatly improve the flow (litres per minute). You will need a larger pipe to the HW cylinder in preparation for a new one. Insist that the plumbers fit full-bore stopcocks and service valves (they are more expensive and they often fit little ones thinking you won't know) to prevent constriction of flow.

If you must have a shower pump, only run it to feed the loft shower.

Piping will cause dirt and disruption, so I'd do it now.

You will need a new supply pipe if you put in an unvented cylinder now or in the future

See if you can fit long low rads in the loft to achieve the head.

Boiler looks a bit underpowered, so time the HW to heat the cylinder earlier than the CH comes on. Modern boilers usually have ample power, and modulate their flame size down according to demand.

Some open vented boilers can be sealed, I don't know about yours. The expansion vessel can go anywhere.

£1600 is that just for the connection? Will you have to pay someone for the trench and pipe?

Baxi calculator says 13.5kW excluding the loft conversion... not sure how to calculate the loft conversion bit and the Vaillant calculator comes at 40kW!

40kW is ridiculous

can you let me see the link?

Yes that's just for the connection! And the work within the boundary would include tunnelling under the house. Would also need traffic management though I've no idea what that would add.

Good point about low radiators, though it's still problematic because we've only 2m of head room and coffin tank would be put in right under the eaves. I wonder what the smallest acceptable size for a F+E tank is? It would be nice not to switch to a system boiler as we've a brand new sparkly pump and boiler controls etc.

I think this boiler would be too old to bother doing anything to it like sealing it.

I just can't think that the benefits of an unvented cylinder over a cold water tank plus negative head pump can be worth the £2.5k + that it will cost us to upgrade the main. (I'm assuming something like £600 for the tunnelling)

I'm not sure this will work: www.vaillantsystemfinder.co.uk/Result.aspx

Here (hopefully) is a snippet of the result:

And here's the input screens:

it might be the "bath" selections doing it.

Try a 120l bath and "slight reduction," if that makes a big difference, try the other bath options.

it looks like it is assuming 9" solid walls (no cavity) and probably uninsulated wooden floor.