X-Received: by 2002:adf:f84e:: with SMTP id d14mr16346578wrq.390.1607278629891; Sun, 06 Dec 2020 10:17:09 -0800 (PST) MIME-Version: 1.0 Path: ...!news-out.google.com!nntp.google.com!proxad.net!feeder1-2.proxad.net!weretis.net!feeder8.news.weretis.net!eternal-september.org!reader02.eternal-september.org!.POSTED!not-for-mail From: Pamela Newsgroups: uk.d-i-y Subject: Re: Externally mounted replacement expansion vessel on heating boiler? Date: Sun, 06 Dec 2020 18:16:50 GMT Organization: A noiseless patient Spider Lines: 152 Message-ID: References: Injection-Info: reader02.eternal-september.org; posting-host="b9c8737720ab054edb75105f2fd4ec10"; logging-data="5029"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX18CbWFEwC97Zt/3xINzdVLQZXzmKneqeqY=" User-Agent: Xnews/2009.05.01 Cancel-Lock: sha1:8mCRJ6/S4uySMjRKKDx6kcOXVmM= Bytes: 8919 On 17:27 4 Dec 2020, John Rumm said: > On 04/12/2020 15:19, Pamela wrote: >> On 08:01 4 Dec 2020, John Rumm said: >> >>> On 03/12/2020 22:00, Pamela wrote: >>>> On 19:50 3 Dec 2020, John Rumm said: > >>>> Someone stayed with me and from some genuine OCD washing habit >>>> they turned the hot tap on and off repeatedly about 100 times >>>> ever few seconds. Maybe more. >>> >>> That's quite an achievement! >>> >>> However the use of the hot tap will not have any practical >>> influence on the expansion vessel which is connected to the >>> primary water circuit (i.e. the water that flows through the >>> rads) >> >> What else than the exapansion vessel could her "routine" have >> affected? > > Well the hot water pathway is basically mains in, secondary heat > exchanger, hot water out. > > A combi like the ISAR has two heat exchangers (HX), the main or > primary one is directly heated by the combustion of gas, and the > wet side of it is connected to the primary heating circuit. That > is usually the radiators. > > There is also a secondary heat exchanger (usually what is called a > "Plate Heat Exchanger" (PHE) that is used to heat the hot water. > This is a water to water HX - so clean mains water is fed through > one side, and the dirty primary heating water is fed through the > other. So there is never any contact between the wholesome hot > water in the house, and the primary heating circuit water that is > potentially full of rust, and inhibitor, and sludge etc. > > There is a flow switch that detects when the hot water tap is > turned on, and that will do two things: cause the boiler to fire, > and activate a diversion valve in the boiler. The valve will > redirect the flow of water > so that instead being pumped around the radiator circuit, it is > instead pumped through the primary side of PHE, and its this that > indirectly heats the incoming mains cold water passes through the > secondary side of the PHE. > > So the expansion vessel is not in any way connected to the DHW > side of the boiler. Even if you cycle the hot tap, the boiler will > not fire on an off at the same rate - it will typically need a > period of demand to get running, and then will run for a few > seconds after the demand goes. > >> Afterwards perhaps 100 ml of water was splashed around the >> internals of the boiler and dripped out the bottom. >> >> In the past, the combi would fire up at night (presumably to heat >> its internal hot water reservoir) and make an almighty banging >> sound which I thought was coming from the neighbour. Turns out >> to have been from the boiler -- without any heating on. Are you >> saying this can't possibly be from the expansion vessel as it's >> on a separate CH water circuit? > > The banging is usually what they call kettling - small cavitations > or expanding bubbles in the water in the primary HX. This can > occur if the main HX is not completely full of water, or if the > flow rate through it is too slow, or sometimes if the primary side > of the main HX is badly scaled up meaning the surface temperature > has to get much higher to effect the same amount of heat transfer. > > Now kettling will cause lots of shock to pass through the primary > circuit, and the expansion vessel will see that. If its > functioning normally, then you would expect it to absorb that > shock without any problem. However if deflated, then the system > will "feel" that percussion more acutely. > >>> As a temporary workaround for a failed expansion vessel, you >>> can bleed some water from a rad, so that it has an air pocket at >>> the top. That will work as an expansion space. >> >> How would that work? Why would CH water expand into the air >> inside a radiator in preference to the expansion vessel, which >> presumably has little gas pressure now? > > If you think it through from the basics, it hopefully makes > sense... > > (apologies if I am teaching granny to suck eggs here!) > > Water will expand in volume as its heated. Now on old vented > systems, this just means that some of it would push back into the > header tank in the loft. However with a sealed system like yours, > there is no header tank. So there needs to be another way of > allowing that expansion to happen without blowing the pipe joints > apart! So sealed systems basically include a bag of pressurised > air inside an expansion chamber. > > When you fill the system to the typical 1 - 1.5 bar, that bag of > air gets squashed a bit, but there still remain a good few litres > of space. As the system fires and warms up, the water expands, and > the pressure will rise. This will compress the air in the bag > more. So a hot system pressure will perhaps rise to 2 to 2.5 bar > depending on the actual temperature and the total volume of water > in the system, and that will cause a matching reduction in volume > of the bag of air. > > If however that bag of air is not there, then the water will still > expand, but now its "pushing" against rigid pipework and hence the > pressure rise will be much higher. To prevent something being > damaged the boiler also includes an emergency pressure release > valve (PRV), that will allow some water to be expelled from the > primary heating circuit to a pipe that discharges outside. The PRV > will normally be set to "let go" at 3 to 4 bar. > > If the PRV does activate, some of the system water is let out, > dropping the pressure back to a safe level. However once the > system cools, that volume will shrink again, and the pressure will > now be lower than the normal resting pressure that was set by the > installer. In some cases this may be so low that the boiler > detects an low pressure error and locks out. (there is also the > risk that once the PRV has operated, it can get dirt into it, that > prevents it sealing again properly in the future). > > So, to your question about expansion into rads etc: > > The rise in pressure due to heating will be "felt" everywhere in > the primary system, and hence all the things connected to it. That > includes the expansion vessel, but also the rads, the pipework, > and the boiler itself. So the extra water volume *will* find a > place to go. Normally it would be the expansion vessel, but if > that can't take it, then it will go elsewhere - say causing the > PRV to operate and release some. If however there is a pocket of > air in a rad, then that will be compressed, instead, and that will > prevent the pressure rise being large enough to operate the PRV. > > If the expansion vessel is working normally, *and* you have a > pocket of air in a rad, then they will both share the expansion. > > The bubble in the rad solution is not however a good long term > fix, since there is no separation between the air and the primary > water - so over time it will diffuse into the water, and that > extra oxygen will cause more corrosion. Thank you for the very useful explanation. It took a while to digest but now makes a lot of sense. There's more going on than I realised and the online videos I saw were too superificial to account for the things you describe. If this boiler makes it through winter, I'll get it replaced in the spring. What would be good make to get? People say Worcester Bosch are good but others point out their aluminium heat exchangers (like the Ideal Isar) are inherently troublesome.