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Why do we need pressurised systems?

Thermostats, Volumisers, Buffer Tanks, TRVs, Pumps, Piping, Valves, Radiators & Controls

Last Post by AllyFish 9 months ago

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Toodles

(@toodles)

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Joined: 4 years ago

Posts: 2727

Topic starter 31/07/2025 4:13 pm [#2405]

Having had a second occasion where the system pressure for the heat pump dropped to zero (under investigation at present) I go to wondering:

Why do we need a pressurised system in the first place? I have always assumed that a positive pressure is used to ensure a constant supply of water to the pump(s) thus reducing air bubbles which would be purged from the system during circulation and possibly also to avoid risk of cavitation in the pump impellers.

Have I summed it up correctly or am I nowhere near the truth - is there something else I have not thought of perhaps?

Curious Toodles.

Toodles, heats his home with cold draughts and cooks food with magnets.

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IaAck

(@iaack)

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Joined: 3 years ago

Posts: 60

31/07/2025 6:04 pm

A question for Inspector Watson?

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Toodles

(@toodles)

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Topic starter 31/07/2025 6:48 pm

@iaack I enquired of Wattson and the reply was very much along the lines of that which I surmised Holmes.

Toodles, heats his home with cold draughts and cooks food with magnets.

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IaAck

(@iaack)

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Posts: 60

31/07/2025 9:38 pm

@toodles, touché chez. /\

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Old_Scientist

(@old_scientist)

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Joined: 2 years ago

Posts: 405

02/08/2025 12:48 pm

Posted by: @toodles

↑

@iaack I enquired of Wattson and the reply was very much along the lines of that which I surmised Holmes.

@toodles Is that because the AI has searched the forum for information, and has found and paraphrased your own question and assumptive reply back to you, reinforcing your own assumptions (which may well be correct). Just surmising 🤣

Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
Solar generation completely offsets ASHP usage annually. We no longer burn ~1600L of kerosene annually.

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Toodles

(@toodles)

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Topic starter 02/08/2025 2:11 pm

@old_scientist If that were the case (and it might well be the case I suppose!) perhaps Wattson would be a bestie to a certain President 😉. Oops, Sorry! No Polyticking! Toodles.

Toodles, heats his home with cold draughts and cooks food with magnets.

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AllyFish

(@allyfish)

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Joined: 4 years ago

Posts: 518

05/08/2025 10:16 am

A good question @toodles. My old open thermal store and oil boiler chugged away quite happily, if expensively, unpressurised. But the header tank was sat right on top of the thermal store in the airing cupboard, with stagnant water sitting at a cosy 30-35degC breeding and growing all manner of questionable organic produce inside it. 🤢

Pressurisation and sealed systems eliminates the crusty & stinky stagnant header tank issue, and almost completely eliminates risk of pump cavitation. There's a net positive pressure at the pump suction inlet far higher than the minimum the pump requires, and at all points in the pipework system for that matter, making pump or system cavitation anathema. So it doesn't really then matter where in a sealed system the circuit the primary and any secondary pump are placed - they'll do what they do, pumping water. It also usually means a quieter running system with less air entrainment & gurgling.

A sealed system eliminates many corrosion risks. Air in systems promotes internal corrosion. Corrosion needs an oxygenated environment to thrive. No air = no corrosion, theoretically.

And yet installers still do strange things, such as fitting auto air valves on sealed pressurised systems. Open system + auto-vent - yes. But sealed system + auto-vent, er, why? Over time the vent will open, and pressure will be lost. I much prefer a manual air vent, which I can open up to let air escape and then check the system pressure on the gauge and top up slightly with the filling loop if need be. A month or two after system filling no more air separates out and, provided the system is holding pressure, the manual vent never needs checking.

This post was modified 9 months ago by AllyFish

👍

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