swaped cylinder now water wont circulate ????

[Agile]

The coil in many unventeds spirals down and then back up in a larger diameter on the outside of the inner coil.

The purpose is to keep the main part of the heating coil lower down to heat more of the tankfull.

Because it goes down and then up it does not encourage thermal circulation. If you could give it a little push it might work a little!

The Unvented Qualification is not related to CORGI. It is issued by the CITB and they issue a silver photocard with WATER on the front and the unvented on the back with the date taken and any expiry date.

Many CORGI registered people have the unvented certificate but there are also some non CORGI who have it as well.

Unvented cylinders are notifyable under the Building Regs and the holder of the certificate can notify without normally incurring any charge. So far CORGI have not included it with their boiler notification scheme ( Yet ! )

Tony Glazier

 

[breezer]

thanks.

i think i get it.

you mean in a "ordaianiry cyiner" the coil being at the bottom causes the now hot water to rise through convection (to the draw off pipe at the top)

but an unvented one the coil is at the top for the h/w and one at the botto for c/h and an air bubble inbetween? (or have i still not quite grasped it)

 

[Agile]

Er.. No !

The "coil" is the heating coil of copper tubing which the boiler water is pumped through to heat the water in the cylinder.

The traditional coil started near the bottom and spirals up towards the top with a connection at low level and the other connection further up the cylinder. This will allow some thermal convection current if the pipework is at least 28 mm.


Have you been drinking?

Tony

 

[chrishutt]

The traditional coil started near the bottom and spirals up towards the top

Tony, surely it has always been normal practice to connect the flow to the top connection and the return to the bottom connection, which seems to contradict what your saying.

 

[Agile]

I said nothing about the flow connection, Chris.

It has been traditional in the past to connect the flow at the top.

Connecting the flow at the bottom takes marginally longer to heat the water at the top but heats more water.

Connecting the flow at the top heats less water more quickly but heats less water.

The traditional method makes expelling air easier in some peoples view.

The double coil usually has connections at or near the same level.

My personal preference would be for an extended ( stretched coil ) fed at the top. This heats the top of the tank quicker which is what most people want.

Tony

 

[chrishutt]

Connecting the flow at the bottom takes marginally longer to heat the water at the top but heats more water.

But surely a bottom connection of the flow pipe and top connection of return would reduce the heat transfer rate from coil to cylinder since the return water temp. would never fall below the cylinder water temp. at that higher level.

You could even have flow water at say 70 C entering at the bottom, cooling to say 50 C then reheating to say 60 C before exiting at the higher return. Wouldn't the result be a substantially extended heating time and therefore greater heat losses from boiler and primary pipework?

 

[Agile]

Yes, thats correct and why I said it takes marginally longer.

Its not really very important which way you connect it in the real world. Its pretty insignificant in reality.

Some people think that all pumped primaries should have the flow at the bottom because that gives more heat transfer. Thats necessary because its heating more water.

Tony Glazier

 

[chrishutt]

I can't see it working like that.

The cylinder water will heat up until such time as the cylinder stat is satisfied. The time this takes depends entirely on the heat transfer rate from the coil, which you've already agreed is higher with the flow connected to the top coil tapping.

At the end of the heating period, when the stat is satisfied, the amount of water heated will be much the same, albeit with slightly different levels of stratification. The end result is that the only significant difference is that the top coil to flow connection heats more quickly.

 

[ChrisR]

Total energy transfer is a bit higher if the primary water enters lower.
It's because the convection current's speed in the water, which is conducting the heat away, is disproportionately higher with increased temperature difference.
It's a positive polynomial in the delta T, so if you integrated over the coil length you'd get a higher sum than with smaller peak delta T's. You might remember it from skule - area under the graph an' all that.

 

[bmx]

just an update on this.

spoke to our HR departement today, and they must have pooped there pants because they have me and 2 others going on the course for unvented cylinders ASAP £260 each bloke and its a 2 day course. so thats good news.

the other thing is the guys gravity fed hot water is now working perfectly on the said cylinder. strange....

cheers al

 

[ChrisR]

now working perfectly on the said cylinder

You're lucky, imho!
It could have been air trapped in the coil inside, perhaps. Normally the pump would shove it out. Would only take a bit of horizontal pipe.

I went to a normal cylinder which had been used horizontally, and had worked on gravity. But it had stopped. Consensus thought is that the tops of the coil loops now have air/nitrogen trapped so circulation has stopped. All visiting plumbers have said "how interesting" and walked away!

 

[Agile]

And what was the outcome of the cylinder Chris? Did you walk away or bite the bullet?

The clear outcome of this discussion is the Company is now ensuring that its staff are being properly trained and certified for the work they are doing!

The odd cylinder is irrelevant but having the staff properly qualified is very important. Suppose an unvented exploded, the insurance would not cover the loss because the staff were not trained for the work they were doing.

The staff could not be blamed either as the Company had not ensured they were properly qualified.

Perhaps the Company should be asked to consider making a £200 donation to ARGI for highlighting this omission!

Tony Glazier

 

[chrishutt]

Perhaps the Company should be asked to consider making a £200 donation to ARGI for highlighting this omission!

Such an "invitation" could sound distinctly menacing -

"you wana maka "donation" to ARGI? We dona wana hava to tella thosa nasty boysa froma HSE, do we?

 

[ChrisR]

And what was the outcome of the cylinder Chris? Did you walk away or bite the bullet?

Weeeell, the wife wants the bathroom finished, sorry......

 

[kevplumb]

haven't you finished that bathroom yet jeeeeeeez mine only took 18 mths