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OSO make a cylinder with a air bubble (no external expansion vessel) but does not have a floating baffle.

Did changing the setting not do anything??

Yes, try it on that, I is the lowest speed/head, if Rads not heating up fully, try it on II, if nothing changes install a new pump.

??? something like one of these. Is the pump downstream of the deaerator?

Yes, I suppose, no harm in spending a few quid now after 20 years or so, would suggest a DAB Evosta 3 (not the Evosta 2) or, like my own, a 6M Wilo Yodos Pico, both of which display the power in watts and the flowrate in m3/hr, very useful. What setting is your present pump on.

You said you pump is the Alpha + so probably a fairly old pump with no LED display etc, if its set to either II or III on the fixed speed mode then should be plenty of pump head to circulate the water. You could shut the pump isolating valves (but only if they are the gate type), remove the pump...

I suppose, ideally, yes, especially with a preheat combi but suggest you keep a eye on it for a day or two, I think the boiler heat exchanger is OK up to 10bar, but there are always some traces of air in the system which will probably absorb the pressure, you're probably only talking about 50ml...

The PRV acts like a non return valve so the expanded water rises the pressure, some/most combi manufacturers now specify a small expansion vessel on the cold mains at the boiler due to failing plate heat exchangers and don't think they are covered under warranty unless the EV is installed.

How old is the Alpha, if fairly modern it will display the power, watts, as W or/and some display the flowrate in m3/hr, so check for this and whether they are displayed or not, post a photo of the LEDs (running!!) and also check for the pump model like below, it will/should be either 45W or...

From my Archives Drop Tight PRVs Caleffi 535 Reliance 312 Series Honeywell D05 Honeywell D06 NON Drop Tight PRVs Caleffi 533 Reliance Predator (PRED800015) Honeywell D04

Don't forget to look at the minimum output on CH, very important to prevent excessive/any cycling at low heating demands.

The boiler HEX and PHEX contents + the PHEX mains (secondary) contain probably only around a very max of 10L requiring 10*(35-10)/860, 0.29kWh and if 25kW available then 0.29/25*3600, say 41 secs for water to reach 35C at the boiler outlet, this assumes that you only open the hot tap enough to...

Glad to hear you've found the problem. You might consider purchasing and installing one of these on the new EV, saves draing down the system when/if checking the pre pressure or renewing the EV, around £20, I've installed them on 2 EVs I've recently replaced on relations oil fired systems. You...

Wonder in the cases where the leaking Grants are < 5 years old as to what the boiler return temperature is/was. It could easily be less than 30C if UFH only used and lead to premature corrosion of the primary? HEX. Even all rad systems now regularly return temperatures down to 35C or less, the...

I have one of those pumps and every time they get a run signal, they flash the setpoint pressure for ~ 5 secs and then show the power in watts, W, like yours, all this is telling me is that the pump and presumably the boiler is getting a run signal but only for 5 secs or less.

There are still quite a number of 35 year old Firebird SE boilers around here, I installed a failed EV on one for a neigbour recently, and also a 2 bar PRV to replace the 7, yes 7Bar original, I presume.

The boiler is probably under ~ 0.35bar head pressure as is so if you installed a good big EV of say 18L, prechrged to 0.35bar and filled to 0.5bar then the final pressure when hot with that small 4 rad system volume will not rise above 0.6/0.65bar, install a 1.0bar PRV, can'e see any problems...

Sealing it might cure the problem, it would be easy to do a simple if temporary mod with the F&E cistern still in place, and require minimal drain down, just install a swing check NR valve on the cold feed where it exits the cistern and immediately below it, tee in and install a 6 or 8L...

Danfoss don't seem to use them, what does the spinner look like, is it shaped like a mini fan impeller??.

Triton make the Noval SR, don't know the price range but I do know someone whose Dublin company installs ~ 1000 showers/year and he now will not install any pumped Mira shower except explicitly requested by the customer due to problems, he has no affiliation to any of the manufacturers.