A city-centre flat on a heat network with no hot water. The fault was the flow switch inside the HIU, and a like-for-like replacement put the tenant back in hot water.
A tenant in a Birmingham city centre flat on a heat network had no hot water. Her heating and hot water come from a heat interface unit rather than a boiler, and after testing the unit we found the flow switch had failed. That is the part that tells the HIU a hot tap has been opened, so when it fails the unit never starts heating water. We ordered the correct replacement, returned and fitted it, and tested the unit. The tenant has her hot water back.
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The call came from a flat in Birmingham city centre. The tenant had no hot water. The heating was fine, which is a useful clue on a heat interface unit, because heating and hot water go through different parts of the unit.
Flats like this one are on a heat network: a central plant heats water for the whole building and each flat has a heat interface unit, or HIU, that takes heat from that supply and uses it to warm the flat's own radiators and to heat hot water on demand through a plate heat exchanger. There is no boiler and no gas in the flat, so a "no hot water" call on a HIU is a different diagnosis to the one on a combi.
Hot water on a HIU works like this: when a tap opens, a flow switch on the domestic water line detects the movement and tells the unit to start heating. If the switch does not register the flow, the unit sits there doing nothing and the tap runs cold, even though there is plenty of heat available from the network. After testing the unit we traced the fault to exactly that: the flow switch had failed. Everything downstream of it was fine.
A flow switch is not a part we can carry for every HIU, because the makes and fittings vary. We identified the exact model from the unit, ordered a like-for-like replacement, and returned to fit it. With the new switch in, and fresh washers on the connections, we ran the hot taps and tested the unit through its hot water cycle. It responded straight away, and the tenant has her hot water back.
Two things are worth knowing if you live in a city-centre flat on a heat network. First, no hot water with working heating usually points to something specific inside the HIU rather than the building's plant, so ringing the managing agent about the boiler room will not fix it. Second, most HIU faults come down to a small number of parts: strainers, sensors, actuators, valves and switches like this one. Diagnosed properly, they are repairs rather than replacements.
If the unit has never been serviced, a HIU service catches most of these before they leave you without hot water. If you are not sure what you have, send us a photo of the unit and its data label and we will tell you.
An emergency HIU repair is £225, which covers the callout and up to three hours of labour to find and fix the fault. Parts are quoted before we order or fit them, and where a part has to be ordered, as here, we say so before we leave. Annual servicing starts at £95. The full detail is on our HIU service and repair page, and there is a plain-English guide to why a HIU stops heating water in our advice section.
It sits on the domestic hot water line and detects water moving when a tap is opened. That signal is what tells the unit to start heating water through the plate heat exchanger. If the switch fails, the unit never gets the signal, so the tap runs cold even though the heating side still works.
Because the flow switch had to be ordered. HIUs use a range of makes and fittings and we cannot carry the right switch for every unit. We diagnose on the first visit, confirm the exact part from the unit, and come back to fit it. Where we can carry the part, as with strainers and common sensors, it is one visit.
Almost certainly not. On a heat network the building's plant supplies heat to every flat, and if it were down the heating would be off too. Working heating with no hot water points to the hot water side of your own HIU. Ring us rather than the managing agent for that one.
Yes. The city centre, the Jewellery Quarter, Digbeth and the canal-side developments are a short run from our Handsworth Wood base, and a large share of our HIU work is in exactly those buildings. We also service HIUs across whole blocks for managing agents in one visit.
Job attended and photographed by our engineers in Birmingham city centre, May 2026. Written up by Saeed Iqbal, the engineer who leads Synergy Gas & Plumbing (Gas Safe registered 594385). More recent jobs in the area are on our Birmingham page, and every job we write up is on the case studies index.
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