Unvented Undersink Water Heaters. For D.I.Yers.

Oooowwwhhh heck

The last and only unvented under sink water heater I fitted came with an expansion vessel. It never occured to me that you could leave it out. but then that was 35 years ago.

Legh

I’ll let you in on a little trade secret, learnt by experience, but totally obvious when you see it.

Female adapters come in two versions, with long or short threaded sections, if you buy the long version to fit an Ariston heater you have to rely purely on PFTE tape wrapped around the thread, however if you buy the short version you can install a decent quality fibre washer to seat the female adapter onto the plastic fitting on the heater as well as running some PTFE onto the thread.

Before now I have called at several plumbers merchants to get the right female adapters, but it’s worth the time it takes.

https://www.tradingdepot.co.uk/end-feed-straight-female-adapter-15mm-x-1-2

When I have not been able to get the short version I have cut some down with an angle grinder, I did this last year to replace a shower for a customer the other way around, I ended up cutting a male adapter down to get the new shower on the wall when the original fittings were too far back in the wall newly tiled with limestone.

Regards your link to the Screwfix forum, obviously the people posting either did not read or could not understand the installation instructions when they decided to use Speedfit plastic fittings and flexible connectors.

The instructions say:

Only the use of copper pipe is recommended for connection to the heater. If any other material is used it must be able to withstand 90°C at 7 bar pressure for long periods.
 

A couple of cheap flexible connectors from Screwfix is not a good choice of installation materials, you need to apply the same rules as combi boilers, no plastic pipe or fittings within at least a metre from the heater, ideally more.

I have seen Speedfit pipe fail when installed within a metre of a combi boiler.

The Ariston installation instructions are detailed and need to be followed, you cannot wing it.

The best DIY advice for non vented electric water heaters is IMHO, DONT !

Too much to be misunderstood or go wrong, use a vented type instead.

broadgage: 
 

The best DIY advice for non vented electric water heaters is IMHO, DONT !

Too much to be misunderstood or go wrong, use a vented type instead.

Agreed, but some oversink vented water heaters have an annoying habit of dribbling hot water from the spout into the sink due to the water expansion.

Z.

About 3 years after our Worcester Bosch floor standing combi (gas) had been installed by one of their accredited installers we had a leak. This was rectified but had various other leaks - all o-ring failures for the next few years. Each time requiring attention by WB.

Then the next attending engineer after fixing the leak, asked if we were on a water meter?  On the answer being yes, he asked where the expansion vessel was…… There wasn't one. Yet in the Installation manual it says that one is required. 

So why don't they fit one internally?

This must be the analogy of PME by stealth……  ?

Clive  

Zoomup: 
 

broadgage: 
 

The best DIY advice for non vented electric water heaters is IMHO, DONT !

Too much to be misunderstood or go wrong, use a vented type instead.

Agreed, but some oversink vented water heaters have an annoying habit of dribbling hot water from the spout into the sink due to the water expansion.

Z.

Mine does that, I simply accept it as being a very small price to pay for a simple and reliable way to heat water.

The water heater came with the house and is suspected to date from when electricity was first available in the locality in about 1963. 1 kw loading, connected via 3 core cable with red, white, and blue cores. White core used for earth.

From the Speedfit website, remember the Ariston pressure relief valve operates at 6 bar:

6. What temperatures and pressures are your pipe and fittings approved to work at?

Our pipe and fittings are approved to work at the following:

– 12 bar at 20°C
– 6 bar at 65°C and,
– 3 bar at 82°C – 105°C max, but they can withstand 114°C intermittently for short periods in case of thermostat malfunctions.

Fittings that are not suitable for central heating have a maximum temperature of 65°C.

So why don't they fit one internally?

Wall hung combis usually do have an internal expansion vessel - floor standing ones often don't. I think the problem is that the expansion vessel need to be sized according to the overall heated water content (e.g. including all the radiators and pipework, not just the boiler internals). A small (wall hung) boiler is likely to serve just one ordinary sized house so the same relatively small expansion vessel will serve 99% of situations (although the instructions usually say to add an external one as well if the heating circuit exceeds a certain capacity). Large (floor standing) boilers are likely to serve much larger and more varied areas - so you might need a large, very large or huge expansion vessel depending on the installation - so better let the installer select whatever is needed rather than include one that'll either be too small or too expensive/bulky for many situations.

    - Andy.