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Andrew Smith
Andrew Smith, HVAC Technician
Category: HVAC
Satisfied Customers: 4021
Experience:  Heating and Controls engineer
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I have an OSO Unvented hot water tank. It makes a loud

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I have an OSO Unvented hot water tank. It makes a loud humming noise when I draw hot water. Can this be fixed?
JA: What's the brand and model of your water tank? How old is it?
Customer: It's an OSO Indirect 330. About 15v years old.
JA: How long has this been going on with your water tank? What have you tried so far?
Customer: About 6 months. No problem with the hot water, just the noise when I draw.
JA: Anything else we should know to help you best?
Customer: I have photos if that would help.
Customer: replied 4 days ago.
File attached (SM5SVT2)
It sounds like an issue with the pressure reducing valve (big grey head) can you put your hand on it when you run your hot tap?
Customer: replied 4 days ago.
Nothing to feel. But didn't make a noise when I turned the hot tap on! I released the other two valves shown in the picture for about 5 seconds each earlier, but hadn't tried since then. Not sure what these do..

Sorry at work currently.

We need to find where the vibration is coming from to start with. No rush, let's just wait until it makes the noise again and then feel every valve.

Customer: replied 4 days ago.
Well opening the other two valves (either of which allows water to flow through an exterior pipe) appears to have fixed the problem.

OK, so the pressure relief valve or temperature/pressure relief valve both discharge into the same pipe. These valves do not last forever and may be a simple swap.

Try and see which one is dripping next time you get the noise.

Andrew Smith and other HVAC Specialists are ready to help you
Customer: replied 3 days ago.
Looks like I missed your call
Sorry, not from me?

Just come back when you see one dripping.