Expansion loops ?

[Arfa Daily]

In our new burger joint (soon to open now !) the building owners fitted a new bathroom as part of the conversion process. In there, is a floor-standing unvented multipoint hot water unit. Not very big. Probably no more than 20 lt. The cold feed to it comes via a really convoluted route, looping round past the feed to the cold tap on the hand basin. Initially, I couldn't figure why anyone would run a pipe from the rising main, all the way round to the basin, then all the way back to the water heater, which is right by the rising main. I thought perhaps it was some 'Eastern European' way of doing it ... :-)

I then obtained a similar unvented water heater for our prep area, which is rather too far away from the heater in the bog, to sensibly run a pipe back to. When I came to study the installation instructions, there is a section about expansion, and it says that the unit uses the incoming cold supply pipe as its expansion means, so that some 4 m of uninterupted 15mm pipe run, must be allowed to accommodate this. So I guess this is the answer as to why the 'loop' is in the pipework in the bog, to provide whatever length is required to satisfy the needs of that one.

Fair enough. So I've done a similar loopback now on the one that I am fitting. The thing is, I can't for the life of me figure how this works. How is any expansion being taken care of, in 4 m of copper pipe, filled with cold water under mains pressure ? The pipe isn't going to expand, and you can't compress water, so what's going on ? There is a safety over-pressure valve supplied with the heater, which has to go on the cold feed close to the unit. This is preset at 6 bar, so the manufacturers are clearly not expecting the pressure to rise to anything near that, except under fault conditions. Anyone know the hydraulic principles involved here ? I'm sure it's something simple and obvious, but it's evading me at the moment ... !

Arfa

[Dean Heighington]

I don't know the exact science but I would expect that the manufacturers have done their tests to protect the unit and established warranty criteria. There will be an 'amount' of flexibilty in the 4m run from the cold feed to allow for expansion, minimal in cold supply, due to ambient temperatures(?) and perhaps a little heat exchange from the water heater element. I had to do the same thing with my Ariston water heater and ALSO fit a PRV so the pressure never reaches damaging levels for the heater as my mains pressure was higher than recommended for the unit.

[Andy Wade]

The expanding water is simply pushed back into the mains. Just imagine the water mains as being a big header cistern 60 ft. (or whatever) up in the air and the penny will drop. The safety valve will take care of the case where someone has turned off the stop-c*ck, preventing back-flow. The reason for the 4m leg (much shorter if you plumb it in 22 or 28 mm) is to ensure that expanded hot and possibly contaminated water doesn't get back far enough to reach a tap.

You need to contact your water company and ask what the maximum pressure is in the area. IME you have to persevere to find someone who understands that you are talking about max. and not min. pressure! If the supply pressure exceeds the maximum allowed in the instructions for the unvented heater (usually 4.5 bar for the 15 litre units) then you

*must* install a non-return valve and expansion vessel - usually available as a kit from the heater manufacturer.

Any unvented heater larger than 15 L subject to building regs.

[TheScullster]

"Andy Wade" wrote

I'm not familiar with these heaters at all, but this seems to contravene water authority principles. There's all ths business about not having showers hoses long enough to reach the tray so that water can't possibly back-siphon into the mains. Also outside taps with non-return valves to prevent the same. Yet here is a piece of kit designed to use the cold feed as a form of expansion space albeit with some proviso governing the length of feed pipe. Or am I misunderstanding the content of other posts?

Phil

[Arfa Daily]

OK Andy. Thanks for that. Makes sense now on the minimum lengths etc. The amount quoted for 22 mm pipe-work was indeed considerably less, as you suggest. So, if I'm understanding this right, whilst the heater is on, the pressure in 'my' bit of the main, will in fact be a little higher than it would when the heater was not on ? Or is the pressure 'upstream' actually a little lower, due to the fact that the heater is 'pushing back' the opposite way ? To be truthful, the mains pressure does seem 'quite' high compared to say my house, and my first thought was to just turn the mains stop-tap down, but there is a note in the unit's instructions saying that "turning down a stop-tap only reduces flow, NOT pressure ..."

So I guess that knocks that thought on the head. It does also mention that the manufacturers have available a pressure reduction kit, which I think it said has an expansion vessel as part of it, so if it ultimately looks as though the pressure *is* going to be too high, I guess that will be the way to go.

Arfa

[Andy Wade]

No, the flow rate resulting from expansion or contraction is tiny so the pressure remains constant throughout the system (other than the normal 'static head' variation due to height of -0.1 bar/metre). When you open a tap and draw water the pressure falls due to flow resistance in the pipe- and valve-work.

Yes, sorry, I forgot to mention the PRV last time. Follow the instructions, no cheating - unvented heaters can blow-up in an impressive manner if the expansion has nowhere to go...

[Andy Wade]

The Water Supply (Water Fittings) Regulations 1999

15.? (1) Subject to the following provisions of this paragraph, every water system shall contain an adequate device or devices for preventing backflow of fluid from any appliance, fitting or process from occurring.

(2) Paragraph (1) does not apply to?

(a)a water heater where the expanded water is permitted to flow back into a supply pipe, or

(b)a vented water storage vessel supplied from a storage cistern,

where the temperature of the water in the supply pipe or the cistern does not exceed 25°C.

Source:

[cynic]

The volume of cold water in the heater will expand when heated by a small amount. this will be pushed back into the heater supply pipe and I suspect the length quoted is an attempt to prevent the expansion volume reaching back far enough to mix with the rest of the mcws system pipework

[Arfa Daily]

Yes indeed. That seems to go along with what Andy said as well, so I guess that's the answer.

Arfa