HIU double check valve leaking

10 Jan 2021
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United Kingdom
I am currently in a process purchasing a property and just realise it seems to be a leakage on the pipe connecting the HIU (Heat Interface Unit, transfers communal boiler to individual apartment then reheat the water).

I have done some research and it looks like the double check valve. You may see the rust on the wall besides the pipe and the water marks down to the wall edges.

Is it a serious problem and how can it be fixed? Are there other potential issues that I should arrange more testing on?

Thank you so much for your help!!

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The heat interface unit that receives the hot water from the communal heating
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Is the leak from the black item below the boiler (which I assume to be a tundish)?
HIU are specialist units that very few heating installers will have come across and are familiar with.
What's the arrangement between flat owners and the freeholder regarding the maintenance of these units.
Who does the current owner use for maintainence although water emerging from the tundish actually indicates a lack of maintenance (it's probably the safety valve dripping).
Make and model of the HIU?
Thanks @Gasguru!

The leak seems to be from the white pipe below the black item as the rust starts from there, I am not quite sure about the operation behind though.

I will have to arrange another visit to the apartment to confirm the make and model, while I have also asked the agent/seller to fix it if possible. Flat owners are responsible for almost everything except the meter itself is maintained by the management company.

Is this something serious that I should walk away from the transaction? Does it look like the HIU's problem or just the pipes? Thanks!
Are the heat meters in a separate location or located within the HIU?
Without a make and model it's guesswork but it's normal for the flat side of the unit to be an isolated and pressurised system and if that pressure is too high the safety valve will open and that appears to be the issue here.
The white item is a https://hepvo.com/ trap.
Rusty stains indicate a poorly maintained and corroding system assuming it's radiators...not good.
Your solicitor will request service info from the vendor and the vendor will be given a document where they can indicate whether the heating/hot water system is in good order.
Often vendors tick to say yes regardless of the condition so you need to resolve this before completion.
The Heat Interface Unit is the bridge between the district heating hot water supply and the home heating system. Normally it is 1 or 2 heat exchangers, a usage meter, pump and expansion vessel/prv.


If it is leaking at the black item underneath then invariably the system is overpressurising (EV needs pressurised probably), the pressure release valve is opening and dumping water, that water is then leaking from the tundish (the black thing) or lower down at the trap (white thing below it).

The fact that it is rusty water probably means the system water is full of corrosion. As suggested it all needs looked at and sorted before you would buy the property. Good thing it showed up now actually as it means it can be sorted by the vendor before you purchase.
Thanks @Gasguru and @Madrab !!

Is this a huge work as per your experience?? Would this issue affect the radiators, or any other potential related issues should I be noticed of? I am looking to complete the transaction asap, yet would definitely wait for this issue to be resolved.

Thanks again for your kind help!!
I wouldn't say it's a lot of work, just needs an experienced engineer to give it a once over and sort the stuff out.

Worst case may be the system drained, cleaned, flushed and then re inhibited. A check as to why the release valve is lifting and sort that out (expansion vessel maybe) and find why the tundish/trap joint is leaking. Not a huge amount of work I wouldn't think.

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