Mole a new water main or fit combi and fit accumulator?

11 May 2012
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United Kingdom
I need to replace the boiler (an awful Baxi Bermuda back boiler) and have had many differing opinions on the best replacement. I'd be grateful for some advice.

We have a system with tanks in the loft now, and a pumped Aqualisa shower, with great pressure.. We don't want to have any reduction on pressure for the shower, and understand this type of shower won't work with a combi / megaflo, so will probably have to replace it. Fine as long as pressure is good.

We plan to extend house (3 beds, 1 bath to 5 bed, 2 bath & 1 shower).

So boiler must be able to cover these extra requirements at good pressure.

Various heating engineers have told us
1) A large combi would suffice
2) A combi won't suffice and we need a Megaflo-type system
3) We have enough water flow for a Megaflo
4) We don't have enough water flow for a Megaflo so need to mole a new main
5) We don't have enough water flow for a Megaflo so need an accumulator tank in loft.

No one has actually measured the pressure accurately, other than putting their hand under the kitchen tap.

I today measured
7.5l/min of cold water at kitchen tap with no other taps open, and
14l/min at garden tap with no other taps open

I have no pressure gauge - so don't know the pressure.

After much thought I decided to go the Megaflo because if the large combi disappointed with feeble shower pressure I would kick myself so got someone to come and quote for a Megaflo and he recommended a combi + accumulator - something else again. I am just confused now.

I have asked for Mains replacement quotes and await their replies. Affinity Water website suggests £1000-2000, a moling co website suggests £650 or so.

Opinions we have received on what we need:
a) Combi only (Ideal 30kw) £2850
b) Heat only boiler + unvented cylinder Ideal (24kw, 200l Telford) £3600
b) combi only Worcester 29cdi £4430
c) a combi + accumulator - £4700
d) boiler + Megaflo + mole new main - no quote back yet for this

I really don't mind which, I just want it to work. Both c) and d) systems require a storage tank so take up around the same room.

I think an accumulator is around the same cost as moling a new replacement mains - is this right?
An accumulator is more likely to go wrong than a new pipe and takes up loft space.

Please could you knowlegable folk give me advice on
combi + accumulator
vs boiler + Megaflo + mole new main
vs mole new main + large combi - so good mains flow but no storage tank

And the need to replace the current great shower.

Or whether advice is impossible without an accurate pressure reading.

Many thanks.
(Sorry for the longwinded message)
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Forget combis if you plan to have the extension.

Unvented cylinder and accumulator is subject to your incoming pressure. I have this system at the moment, mainly to give a store of water should the local supply go pop again like it did at Christmas. But I have phenomenal incoming pressure and a decent sized water main making it unnecessary really.

Not liking your choice of product line up though. Budget crap.

Megaflo as a brand though are overpriced crap.

See what the pressure is like and what the new main will cost. Then post back.

Have to go back to an accumulator installation soon for its annual service. I have a video with it working on my You Tube account. Must remember to take one with the accumulator disconnected so people can see the difference.

I thought the customer was going to offer me her first born when we fired it up for the first time. :LOL:

That is running at 3 bar off a 15mm water main.
Thanks for responding so quickly.

So you have unvented AND an accumulator?
I didn't know that was even possible.

The products are just what the plumbers suggested - we haven't expressed any preference.

Rather than calling s/o else out, I'll try and measure pressure myself - where/how do you measure?
Please recommend cheapest possible gauge etc - I'll probably only ever use it once, for about 30 seconds!

I'll measure and post back, and also the mole quotes.

Thanks again.

love that pressure on the vid.
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Which ever way you go the best option and the one least likely to break down is a.mains upgrade,this is just a new pipe and doesn't involve any type of pump or accumulators,so there won't be anything to breakdown in the future
I always recommend a mains upgrade where possible instead of the other options for this reason
Happy Plumber - thanks - that makes very logical sense - the simpler something is, the least likely it is to go wrong.

I just bought a gauge and measured as follows:

Measuring garden tap alone - 4 bar, and when I turned on kitchen tap, the garden tap pressure went up to 7bar.

What are my options, pro and cons, with this pressure?

And just in... first confirmed quote from a Moling Co for 15m length of mains replacement. £630
Thats about right and you won't get pump set up for that ,google the price of accumulators and grundfos whole house pump set and you will realise
So if we got the mains pipe replaced, then what are our options?

are they still:
-Just a big combi alone
-heat only boiler + megoflo type unvented tank
-combi + accumulator tank

Measuring garden tap alone - 4 bar, and when I turned on kitchen tap, the garden tap pressure went up to 7bar.

Something not quite right here. Connect the pressure gauge and measure the pressure with NO water flow. That reading is the static pressure ie. the water pressure in the street main.

Now open a tap and measure the pressure again. That's the dynamic or working pressure and will drop as the water flow is increased. Given your flowrates from before I'm guessing the pressure drops off to near zero indicating a blocked/partially closed internal or external stopcock and/or an undersized/blocked service pipe. There are other possible reasons...outside stopcocks frequently give poor flow due to freeze damage to the non return valves. Report back the figures/flowrates.

For a property your proposed size forget combis (of any type). I would always advise of the necessity of an immersion heater backup a cylinder gives. When combis fail you loose the hot water.

Until you provide pressure readings it's guesswork regarding the best setup.

An accumulator is a temporary store of high pressure water. They are useful where the static water pressure is high but the flowrate is poor. However, once the store is depleted the water flowrate drops back to the previous rate. For the little extra cost is is often more preferable to resolve the low flow issues ie. new service pipe etc.
IF you have 4 bar pressure coming in, and the cost of a new water main is £650 or so, then crack on with that option.

Accumulators are a compromise as much as pumps are.

Double check those figures though.
Thank you Gasguru and Dan.

Yes I thought it was odd that pressure increased at garden tap when I opened the kitched tap too.

I have just been out with a torch and re-measured!

1. Time 21:01:26 All taps in house off. I attached gauge to garden tap, and reading goes from 0 (garden tap off) to 4.4bar (garden tap on)
View media item 937592. 21:03.27 I go inside & turn on kitchen cold tap (leaving garden tap still open), return to gauge at garden tap, and it now reads 8bar-ish.

View media item 93760
3. 21:03:36 I go inside & turn on cloakroom cold tap (keeping garden and kitchen tap open), return to gauge at garden tap, and it now reads 7.8bar.
View media item 93761

Please let me know if these images are not showing - i've not uploaded photos before.

And please let me know if I am measuring wrong - I read your instructions carefully Gasguru but let me know if I'm missing something.

I wanted to measure at washing maching outlet too but there is a valve on that so no water comes out at all when gauge is connected it to, (and presses the valve).

Flow rates measured yesterday:
7.5l/min of cold water at kitchen tap with no other taps open, and
14l/min at garden tap with no other taps open

Would Megaflo require more than these readings?
Would doing the mole thing definitely sort it out? & which diameter pipe?
What other unvented system would you reccommend (if Megaflo are 'overpriced crap' as you say Dan!)

Gasguru - as you say you "would always advise of the necessity of an immersion heater backup a cylinder gives" ... so does an unvented Megaflo type contain an immersion heater too??

Many thanks to all.
Your outside tap's non return valve may be part of the problem.

Best to try and carry out the test from the washing machine valve or dishwasher valve. You might have to remove the appliances for this.

You have to plan for the future. A combi will be fine for what you have now, but terribly inadequate if you plan to use more than one outlet at the same time.

Upgrading to 32mm is recommended. Ideally with 22 litres a minute.

As for brands.... as long as it is stainless steel you should be gravy. Spend your money on the boiler and the controls.

All unvented cylinders come with immersions; you need to have them wired up though.

With 14 litres at the garden tap, I would suggest a more thorough check of the incoming supply as both the kitchen and garden taps can be choke points.

You haven't told us what your current pipe work is made of or how it is run.
Thanks Dan. I tried at the washing machine... disconnected it, dragged it out, nearly expired at the rubbish and toys and missing socks hiding behind.
but there was some kind of valve on the water outlet. When I attached the pressure gauge and opened the tap, the gauge measured 0.
So I took gauge off and found a valve in the outlet which pressed in and out. Pressed in... no water flowed let go... water flowed out.
I couldn't see a way to remove valve by hand.

I then tried under sink for dishwasher hoses, but had these metal connections looked like they needed a spanner, and couldn't be undone by hand. I'll have a go tomorrow.

Your other requests:
Where should I check what pipe is made of? Which pipe? Outside one in the drive.. leading into water meter?
What do you mean 'how is it run'?

(Novice here if you hadn't guessed. Sorry! )

Many thanks.

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