vent pipe run

[Softus]

As i said you only have to bridge the mains cold with the hot and even you will see where the water comes out.

Yeah right - go tell this guy your amusing theory.

You're losing credibility at an alarming rate.

 

[chrishutt]

There's a hundred quid to a charity of your choice say it pumps over

OK DIA, you'd better get the cheque book out. I'll let you choose the charity (ARGI?) since it's your money.

The pumping over question is easily answered by a simple calculation as follows:-

1. Output of Viessmann boiler is 22.3kW (=22.3 kJ/sec). Temperature drop across system should be 20°C. Specific heat of water at 70°C is 4.190 kJ/kg.

2. The flow of water around the main circuit will therefore be 22.3 / (20 x 4.19) = 0.27 kg/sec.

3. Head loss in 22mm pipe with 0.27 kg/s flowing is around 40mm per metre run (from tables).

4. Preferred distance from vent connection to feed connection (neutral point) is less than 150mm, but allowing for Viessmann drawing which suggests larger gap (another mistake!), let's call it 1 metre (equivalent length) to be generous to you.

5. Therefore water level in vent pipe will be 40mm above level in feed tank. (Or only 6mm if we take the 150mm figure referred to above)

4. It's clear from the Viessmann drawing (and standard practice) that the vent pipe invert should be well above the water level in the feed tank - but let's assume this is only 200mm (I'm feeling really generous).

5. 40mm is less than 200mm, therefore no pumping over, therefore your bank account is less £100.

 

[ChrisR]

Doitall you're wrong. It wouldn't pump over.
To do that there has to be a circuit between the feed pipe and the vent, with a pressure difference between them at the "t"'s. So it doesn't matter a toss where you put the pump - as long as it isn't between them. As ChrisH has explained the pd due to water flow along the less-than-150mm pipe is as I said earlier, negligible.

Also it's about time at your age you stopped calling the vent pipe the "expansion"! The level of the water in the vent and the feed are exactly the same until the pump starts, so the majority of the expanding water goes up the feed pipe not the vent.

ChrisHutt - "Neutral point" is another term I object to! Neutral with respect to what? The vent and feed are just two vertical pipes. When the pump starts one goes up and one goes down. Different amounts, yes. Nothing magic abut the feed pipe "point" - especially when things go wrong.

You show the red line (pressure = atmospheric) passing through the radiators. Assuming equal resistances on flow and return pipes, and no resistance between pump and cold feed, the head at the rads would be -0.5 and the red line would come upstream of the rads.

Red line comes somewhere in the the "load", which is represented by the rectangle. Where exactly atmospheric pressure comes, would depend on what's connected.. Perhaps I should have drawn the rectangle wider! The biggest resistance (and therefore pressure drop) might be a rad valve, so it depends which end of the rad... Which is why an air vent can suck air in if the pump's running.

 

[chrishutt]

ChrisR, I take your point about your rectangle representing resistance load rather than the rads specifically. I assume you agree with the other points I made in that post.

With regard to neutral point, I thought it was standard practice to use this term. The water level in the feed tank will only be altered ultra-marginally by pump action, according to how much water is accommodated in the level rise/fall in the vent pipe (assuming no air pockets). Even the water held by a 100mm length of 22mm would amount to nothing when spread over the surface area of a feed and expansion tank.

So for practical purposes the feed tank level is only altered by temperature induced expansion/contraction of the system water. It therefore makes sense to use this level as a reference point for pump induced pressure variations around the circuit, as you did in your diagram.

 

[corgiman]

I have been giving this some thought

and logically the "neutral point" can only exist in the middle of the pump cant it???

 

[ChrisR]

Depends what is meant by "neutral", innit! Neutral in what way exactly, when, with respect to what...?

I assume you agree with the other points I made in that post.

More or less! Was intended to show principles rather than account for all resistances.
When the pump starts you can get a pulse of full pump pressure before it drops off - if a bubble of air, etc.

 

[chrishutt]

Neutral means subject to neither positive or negative pressures. In a diagram of a pumped circuit one can show a series of +++++ downstream of the pump to signify increased pressure and a series of - - - - upstream to signify decreased pressure. The +s and -s continue around until they meet at the feed tank connection. At this point the pressure is neither increased nor decreased by pump action and is therefore known as the neutral point. This is true even if the feed connection is very close to the pump.