The spec flameport posted indicated that, as a feature, the mixer would 'shut off' if the input hot water temp was not at least 12° above the output temp of the mixer -= something which could obviously only be achieved if both hot and cold water were available and that at least some cold water were being mixed with the hot.
I'm not sure what you mean by "latent heat expansion" but it certainly is (from the heater's point-of-view) an 'unvented system' (since water flow controlled by downstream taps/valves. However, unless there is a non-return valve in the cold fed to the heater,the pressure within it presumably can't rise to significantly above that of the cold supply? ... although I have to say that the instructions for some of these heaters I've seen indicate that a 'pressure relief valve' should be installed in the cold feed.
Fair enough, but you've seen it now, as well as reading my precis of the relevant bit, so should be aware of this 'safety feature' on at least one of them, and its 'consequences'.
All true - but none of them I've looked at so far say anything about a pressure vessel - and, on the contrary incude diagrams which show only heater, pipework and taps/showers. As above, in the absence of a non-return valve the pressure should be limited by supply pressure, if not also by an explicit PRV on the input supply.
No comment - but we certainly did the opposite (imploding a 1-gallon oil can) in the 'collapsing can experiment' at schoolI don't know about you but I never heated a sealed 1 gallon oil can with a blowtorch to see if it split
Just to reiterate, any system where an outlet can be under standing water requires a NRV to prevent any contamination to the public supply and I believe that is by law.
Indeed - but in relation to indoor plumbing, my understanding is that everything is meant to be arranged so that situation cannot arise - e.g.. taps must be above bath/basin/sinks (or their overflows), and the equivalent for shower hoses,etc. 'Outdoors' introduces more potential for backflow - but I presume even that only if supply pressure drops to a very low figure.
And a vessel full of latant heat doesn't create a back flow?
Again, I don't really understand what you mean by 'latent heat' - a term which really only applies to situations such as 'phase transitions' (i.e. from solid to liquid or liquid to gas, or vice versa).
For what it's worth, there is definitely no such requirement mentioned in the installation instructions of the heater I'm looking at, nor in any of the instructions for other similar items that I've looked at in recent days.
As far as I'm aware a check valve is only a non return valve whith a specific minimum pressure differential but in truth this isn't really my field of expertise to be able to quote chapter and verse. For example I know a check valve is fitted as a bypass in heating systems when motorised valves close to allow the pump to 'run on' until the boiler reaches a lower temp.
See below.
That is consistent with what I wrote. The rules/regulations/laws in relation to prevention of backflow into the public water supply network have no interest in what happens within heating systems, so it's perfectly OK to use a 'NVR', rather than a 'check valve', in that situation. That is confirmed by statements one finds on-line such as ...
If you ask Mr Google about this, you'll probably get driven mad (as was I), since it is apparent that 'everyone', from manufacturers through retailers to users tend to use the terms "non-return-valve" and "check valve" interchangeably - although it seems that they probably shouldn't, since one also finds seemingly more authoritative (apparently from 'a man at WRAS') information such as (click here) :
That makes sense, although,as above, it looks as if, when such anti-backflow precautions are required, UK law/regulations requires it to be achieved by use of a "check valve", rather than a "non-return valve". In my situation, the volume (heater plus pipework) of the 'pressurised hot water system' would probably be no more than about 1 lrr at most.
No not quite, what is required is a non return valve with a distinct pressure setting, otherwise it could constantly bypass the motorised valve (or other position it's installed in). However Having just googled I see the device offered is generally a "By-Pass Differential Valve" although I know Check valves are used in this situation - as in my home CH system, I have also known of them being used as PRVs (Basically both can be similar items)
Having made my statements, much info just shared and identified makes a lot of sense, as I mentioned this is not my field of expertise despite working on significant heating systems controls containing these parts.
I think you're referring to what would normally be called a 'pressure relief valve", common in heating systems, and with no 'non-return' functionality really needed.
No I'm describing a bypass device shown here in red, the only purpose of which is to provide a route such that with the motorised valve closed the pump can continue to circulate water through the boiler until the latent heat is cooled sufficiently. It requires a pressure relieve type function, ideally a pressure differential function typically set to something in the order of 200 to500mbar otherwise the the bypass would be constantly open when the pump runs. A check valve is sometimes used.
All of the check valves I've encountered are spring loaded which means they cannot operate in the 'ideal' way you describe, there has to be some resistance to overcome and that setting may be part of the description/part number/spring type/setting etc. In the Plant rooms I'm accustomed to working in (very much not domestic) there are frequently spares and as I mentioned before there are common parts between check valves, NRVs, strainers etc.
Indeed - that's what I was talking about, and common in heating systems.
That's what I said - which is why they are commonly known as 'pressure relief valves - they open, and allow flow, if the pressure differential across them rises to a certain level.
As I said, a standard 'check valve' would not work in that situation because it would do the very thing you mention - i.e. "be constantly open whenever the pump runs"
As I said, "Ideal" (aka "not achieved in the real world"
However, although I suspce that most (all?) of the check valves we sre used to seeing/using probably do, indeed, have (weak) springs, that is not the only possible design, since some seemingly just have 'flaps' which, in the event of any attempted backflow, remain closed just by virtue of the 'back pressure ...
redfluid.es
Yep, same here - and it is one of that type for which I found the spec which indicted an opening pressure of 0.013 bar.
That would be about 0.07 bar - I presume far too low for the CH application you mentioned?
Quite so. It's totally confused, and it's clear that a high proportion of people, from manufacturers downwards, are using the terms interchangeably.
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
