F and e heating question

[Johnyc86]

Hi
In open vent heating (f and e) should the feed be tied into the return or flow?also is there a 6 inch rule for distance between vent and feed??and why is this

 

[SJB060685]

The position should be Vent, Cold fill, Pump and on the flow. The vent and cold fill should be a maximum of 150mm between centres, often the cold fill is brought down below the flow and then bends back up into it to prevent thermal drift. The reason the distance is 150mm maximum is because where the cold fill enters the system is the neutral point and you want the vent within this area.
There are other ways to position them but they are not ideal for system performance.
The above is the full proof way of doing it.

 

[Ric2013]

Although, on very old boilers that had low internal resistance to flow and no overheat thermostat, the convention was to put the cold feed on the return before the boiler, the vent on the flow after the boiler, and you could put the pump either after the vent or before the cold feed.

You'd only really do it this way in a legacy boiler/uncontrolled solid fuel situation though as modern boilers have high internal flow resistance. Otherwise you'd use John's method above.

My understanding of the 150mm rule is it's because you want to minimise any resistance to flow between the vent and the cold feed. As you'll note, the ancient method I've mentioned breaks this rule and MIGHT be prone to pumping over in some circumstances.

 

[SJB060685]

As I said above the proper way to do it is the method I first said, ie V,C,P. The cold fill can tee into the return but its not advisable. If you do that and put the pump after the vent, or before the cold fill into return you are pumping into the neutral point. Doing this is the same as pumping into the neutral point on a sealed system with expansion vessel. The pump won't be able to increase its discharge pressure, therefore it can only reduce suction pressure, this means you will still get flow the moment the pump energises yes but you will drop the pressure around the system to below ideal and atmospheric pressure. This will pull any dissolved gases out of solution as well as draw air in from any weak points and can cause cavitation.
That system will work but it's not ideal.

 

[Ric2013]

Yours is the best method, but surely not in every case? The talk about the 'proper way' ignores the fact that there can be different circumstances, although I've already stated that in most current circumstances, your method is the best.

The reason I am making a point of this is that the OP has not clarified in any way whatever what the heat source is. Given that a little knowledge can be dangerous, I thought I'd fill in the gaps, just in case.

Agree, almost always better to pump on the flow rather than the return for the reasons you have outlined.

However, an old boiler (e.g. a Thorn/Myson Houswarmer S) or an uncontrolled solid fuel system will surely be safer if the cold feed is installed into the boiler return. In fact, the Housewarmer S instructions insist on this, so you don't really have a choice if working around such an old boiler.

Obviously, in normal circumstances, the modern method would work very well with these boilers that have no overheat lockout thermostat, but would be unsafe under fault conditions, such as if the thermostat fails with contacts closed. If the system boils, and the boiler is between the cold feed and vent then the water has a clear circuit to follow whereas with the modern method this is not the case.

 

[SJB060685]

To be fair I cant really comment on requirements for older boilers, although I would like to see some literature stating it has to be that way? A lot of solid fuel appliances also require a quench coil, where if water gets too hot a valve connected to cold mains water opens and cools the unit down while dumping the excess heat externally.

 

[Ric2013]

I would like to see some literature stating it has to be that way

www.tinyurl.com/SJB060685 (not the S model, but close - see section 11)

Back to the OP, else we're off-topic. Out of interest, what boiler are you talking about, and has all the above conversation been in vain?

 

[Johnyc86]

The position should be Vent, Cold fill, Pump and on the flow. The vent and cold fill should be a maximum of 150mm between centres, often the cold fill is brought down below the flow and then bends back up into it to prevent thermal drift. The reason the distance is 150mm maximum is because where the cold fill enters the system is the neutral point and you want the vent within this area.
There are other ways to position them but they are not ideal for system performance.
The above is the full proof way of do

 

[Johnyc86]

What is thermal drift basically??

 

[SJB060685]

That would be hot water creeping/rising up slightly through the cold fill. You will still have expanding water doing its thing but you're trying to minimise heat loss to the system load/loads, although it would be very little energy lost. I was always told to try and do it that way.

 

[Johnyc86]

That would be hot water creeping/rising up slightly through the cold fill. You will still have expanding water doing its thing but you're trying to minimise heat loss to the system load/loads, although it would be very little energy lost. I was always told to try and do it that way.

Thanks man.

 

[SJB060685]

The cold feed on an open vented system acts exactly like where the expansion vessel tee's into the system on a sealed system. Google point of no pressure change and watch a video or two, there loads on there. You really do not want the pump discharging towards this neutral point, the videos will help you visualise why my friend.

 

[Johnyc86]

The cold feed on an open vented system acts exactly like where the expansion vessel tee's into the system on a sealed system. Google point of no pressure change and watch a video or two, there loads on there. You really do not want the pump discharging towards this neutral point, the videos will help you visualise why my friend.

Your saying expansion vessel should be on flow so??heard its better on return coz its cooler..is this waffle??

 

[SJB060685]

Ideally yes I would want it on the return as the cooler temperatures are less vigorous on the bladder. However, providing the position of the pump/pumps and vessel are right it will still function. Again though I personally would want it on the return.

 

[Johnyc86]

Ideally yes I would want it on the return as the cooler temperatures are less vigorous on the bladder. However, providing the position of the pump/pumps and vessel are right it will still function. Again though I personally would want it on the return.

May I ask you a question on slightly different topic?? Heating controls...??

 

[SJB060685]

You can and I will do my best to answer

 

[Johnyc86]

You can and I will do my best to answer

Ok thanks
Just need basic understanding of how electrics work with heating
Can't get my head around videos on YouTube
I'm still learning alot tbh
Right let's say programmer calls for heat.does it send signal via room stat which then tells zone valve to open which in turn sends a signal via switch to tell boiler come on.is that correct??also whats a permanent live to boiler mean...

 

[SJB060685]

The programmer when demanding CH or HW will send a common feed to roomstat or cylinder stat. If either stat requires heat it will send a signal to the motorised valve/valves, which in turn will motor open connecting a permanent live to a boiler live through a microswitch. Mid position valves, diverter valves and the rare C plan on gravity hot water valves are all wired differently but ultimately open a port to allow flow.
Permanent lives at boilers are for things like pump overruns etc, however I'm an oil man by trade and can't say how gas boilers behave, although you won't get that information on here if you're not gas certified.

 

[Ric2013]

although you won't get that information on here if you're not gas certified.

I think the line has become less strict on this forum now that it's international. He may well get the answer he wants (at least at the level he needs to know), but I think you've already covered it really - to me it sounds like oil boilers are much like gas, caveat that I'm not a Registered Gas Installer, so my knowledge on the latest models of gas boilers is limited.

 

[SJB060685]

I think the line has become less strict on this forum now that it's international. He may well get the answer he wants (at least at the level he needs to know), but I think you've already covered it really - to me it sounds like oil boilers are much like gas, caveat that I'm not a Registered Gas Installer, so my knowledge on the latest models of gas boilers is limited.

Oil is a hydrocarbon like gas yes and the science behind ignition is the same, however up until very recently oil boilers have always been very primitive, fixed rate burners, on/off controls with no modulation, at least in domestic properties they were. This is changing. There are already two stage burners on the market now and a fully modulating boiler is released very soon. Exciting times for oil engineers, especially when you consider the new HVO fuel.