Hot water from boiler wrong direction through Hot Water Cyl

[croydoncorgi]

We are rapidly approaching Angels on Heads of Pins territory here...
But I always thought the logic went this way:

- start with a cylinder full of water that's stratified (coldest at the bottom) and in equilibrium (no convection currents)

- then heat up the coil with water from the boiler. If you put hot water into the top of the coil, you transfer heat first to the WARMEST water touching the coil. The water in the coil cools as it moves down (incidentally increasing the thermosyphon effect by getting denser) but encounters cooler water towards the base of the cylinder, thus maintaining the temperature difference demanded by the First Law.

- at the same time, convection currents in the hot water (caused by heating water towards the bottom of the cylinder) start to mix up the water and therefore average the temperature. If you heat from the bottom, the effect will be to start the convection from the bottom of the tank and therefore (by including the whole volume of water) create a lower average temperature than if only (say) the top two thirds of the cylinder get stirred up by convection.

The practical effect of feeding the top of the coil SHOULD be that the water towards the top of the cylinder (and therefore coming out of the hot taps) stays WARMER while the cylinder is recovering.

This (theory?) only works if the flow of heating water through the coil is relatively gentle. If it's pumped and flowing quickly, chances are that roughly the same delta-T between coil and cylinder contents will apply over the full height of the coil and therefore it will make no odds whether you feed from the top or the bottom. Furthermore, a fast-recovery system with a big coil and a big delta-T will create strong convection currents which will quickly mix up all the water in the cylinder, so as soon as heating starts there is no stratification of the hot water to make any difference anyway. Also, Megaflos tend to be linked to 'hot loop' systems meaning that the contents of the cylinder will be stirred all the time anyway.

 

[doitall]

To reach equilibrium the two temperatures become equal, to transport energy (work) to the cylinder the temperatures have to be different, for example a problem encountered with larger pipes was reverse circulation, when the heat source was turned off the flow reversed as the pipes cooled therefore taking the stored water from the cylinder.

Anyways the flow into the top of the cylinder will reach equilibrium far quicker and with say only the top third with hot water the gravity flow will stop, with the flow into the bottom the flow will continue until the whole cylinder reaches equilibrium.

Think about it

 

[felix]

OK ChrisR. I concede that heat transfer into a tank full of water moving only by convection is not the same as that in a contraflow heat exchanger pumped on both sides.

 

[croydoncorgi]

the flow into the top of the cylinder will reach equilibrium far quicker and with say only the top third with hot water the gravity flow will stop, with the flow into the bottom the flow will continue until the whole cylinder reaches equilibrium.

No - you think about it - unless you believe in perpetual motion machines...

The water in the primary circuit will remain hotter than the AVERAGE temperature of the cylinder until all the water in the cylinder reaches its temperature. Makes no difference whether the inlet is to the top or bottom of the coil. It's just that with the top inlet, the water right at the bottom of the cylinder will be heated by (cooler) water travelling downwards. Convection currents involving the water at the bottom will still happen and the cylinder contents will still mix up but at a slower rate.

To believe otherwise requires that you also believe that the thermosyphon will stop even though one leg of it is on average cooler than the other.

 

[htgeng]

if you're running a condensing boiler, my two-penneth would be that its more efficient to heat the cylinder in classic mode. this way your boiler will be running in condensing mode longer

 

[croydoncorgi]

Heat transfer is more efficient with the input at the bottom. You can prove it mathematically very easily. NB Megaflo's et al are configured that way for that reason

Hey Chris. Maybe you should tell Ariston about this. they obviously never heard. Or maybe disagree with you AND Heatrae Sadia! Risky!

 

[doitall]

Heat transfer is more efficient with the input at the bottom. You can prove it mathematically very easily. NB Megaflo's et al are configured that way for that reason

Hey Chris. Maybe you should tell Ariston about this. they obviously never heard. Or maybe disagree with you AND Heatrae Sadia! Risky!

So why do the house bashers always connect radiators to the bottom connections if they work better with TBOE

 

[doitall]

if you're running a condensing boiler, my two-penneth would be that its more efficient to heat the cylinder in classic mode. this way your boiler will be running in condensing mode longer

As Chris said earlier Megaflo have started putting the primary connections at the very bottom of the tank, this provides more stored hot water and a colder return ideal for condensing boilers.

 

[AF]

Thanks to all those who went to great lengths to help me make a decision about my water heating.

Appears to be less than unanimous agreement on some issues. I should have mentioned in origional post that I have a pumped system.

However with respect to my origional questions:

What effect is the reverse flow having on my hot water heating?
I gather probably not a lot

Is it causing the excessive boiler cycling?
NO, but I should replace the Hot Water Cylinder
thermostat which is inop

Should I get the reverse flow rectified?
Ideally YES but might not be worth the money

Does this seem more or less right?

Thanks again for all the help