If water has to speed up to bass a bottleneck it will in general dump less energy on the way.
The whole argument is rather academic though, since the total rated power of most radiators is usually far in excess of the boilers ability to supply that power. I.e. the designs are such that most radiators will be throttled back by the balancing valves or TRV's.
So 'radiator efficiency' is only relevant when trying to get the most out of a small area. You aren't wasting fuel by having low 'efficiency' radiators.
w figures (in the CIBSE magazine ISTR) circa 1984 that said BBOE > TBOE, bu t think I've seen contradicting numbers since. I don't think it's a precise science and the differences are marginal.
I imagine Manufacturers will have been modifying the designs to be good for BBOE as that is what people like to use.
I connected all mine TBOE to I want that to be the best!
The difference arises not because of flow rates but the distribution pattern of the water. With diagonal connections, the water is more uniformly distributed and there are fewer "dead spots". Pumped circulation or gravity, makes little difference. Assuming a standard radiator with "manifolds" top and bottom.
Surely the something else is convection isn't it? When the hot, light water goes in at the bottom, it'll float up to the top of the rad, flooding through the top "tank". Flow rates inside the rad due to the pump are low, so convection can dominate and the hot water floats up through the first few channels. If it didn't, BBOE would never work. The cooler water already in the rad will pool at the bottom, where the exit ought to be. If the exit is at the top, then it'll be the hottest water in the rad that gets pushed out by the incoming water - obviously not what you want. A notional BTOE (as opposed to TBOE) rad will only get hot up one end and along the top.
No. I would expect convection to be the driving force in deciding which channels the water flows through. If hot water finds itself at the bottom of a cooler channel, it'll flow strongly up it. With the exit at the top, a large pool of cold water will accumulate at the bottom of the rad.
I've seen this very effect on a system I was balancing for a pal. All rads TBOE but one wouldn't balance correctly and room temp was down. Large pool of cool water from the bottom up was exactly how is manifested and swapping the flow solved it immediately.
Also, whacking more flow through makes little difference, you get a little more mixing but half the rad is still cold so I wouldn't expect more than half output from it and that would be geometry dependent.
Time (spent in the heat exchanger), Turbulence, Temperature (difference)
The more you get of any of them the greater amount of the available amount of energy is transferred per unit of water.
So in fact if the water spends longer in the radiator, it will come out cooler.
However the total heat emitted will be greater as the flow increases. (The temperature difference will be less between inlet and outlet so the average surface temperature will be greater.)
You're forgetting probably the most important one - the effective surface area of the heat exchanger. If the rad has a big cold patch, then its effective surface area for heat exchange is obviously much smaller than it should be.
If the outlet is at the top, then increasing the flow will increase the output power slightly, but then you'll end up with a badly unbalanced system. The rad in question will be a huge flow-hog and the boiler won't be able to achieve its rated output because of all the still-hot water coming back on the return.
The average surface temperature isn't the average of the inlet and outlet temperatures. With the outlet at the top, it'll be well below both of them.
I still don't agree with that. Please note that I'm not saying it won't h appen, I'm saying it wouldn't anticipate it happening.
The convection just generates a pressure difference due to the different masses of the H&C water columns.
With BBOE and no convection (air, rad, water at the same temperature) wat er will still flow up the first channels and down the last, the flow rate b eing such that the frictional pressure loss equals the pressure difference across the inlet & outlet provided by the pump.
With BBOE plus convection, it works as above, but pump and convection for ces work together.
With TBOE (wrongly connected, flow in bottom & return out opposite top) t he pump and convection forces are opposing one another. The pump will IMHO overwhelm the convection forces and ther will be little difference. It will be a parallel flow heat exchanger and the air movement will be reduced.
Domestic hot water cylinders are connected F in bottom, R out the top, bu t no-one has ever claimed it stops them working.
happen, I'm saying it wouldn't anticipate it happening.
masses of the H&C water columns.
will still flow up the first channels and down the last, the flow rate being such that the frictional pressure loss equals the pressure difference across the inlet & outlet provided by the pump.
work together.
Convection forces are very important. The hot water rises instantly in the first channel, and the rest of the radiator behaves as TBOE, with the hot water descending uniformly down all the other channels, as it cools, and remains stratified almost equally across them all. See
formatting link
pump and convection forces are opposing one another. The pump will IMHO overwhelm the convection forces and ther will be little difference. It will be a parallel flow heat exchanger and the air movement will be reduced.
no-one has ever claimed it stops them working.
The heating coil isn't. But neither are they much of an analogy to a radiator. If you are refering to the outer cylinder, then that's the same as the air convecting up a radiator, with the cold air going in the bottom, and the hot air coming out the top, again by convection.
happen, I'm saying it wouldn't anticipate it happening.
masses of the H&C water columns.
I wouldn't expect that. The vast majority of the water will go whistling straight through the bottom tank and out of the other end. To get to the top of the rad, it would have to fight the restriction of one of the channels, both up and then back down. in BBOE, it needs convection to have a reason to do that.
Actually, the pump would work against convection with BBOE, but the effect is very small because the velocity of water in the rad due to the pump is so low. Convection easily overwhelms it.
I think you're over-estimating the velocity of the water inside the rad due to the pump. For example, a 1kW rad might need about 1.2 litres per minute at "normal" temperatures. The top and bottom "tanks" might be similar in size to 28mm pipe, so that's just 3ish cm/sec even where
*all* of the water has to flow through one of the tanks (near the inlet or outlet). Once it splits up and goes through the numerous channels, it's just barely drifting along. Convection will have no trouble reversing any flow pattern the pump might tend to cause inside the rad.
The coil in mine has flow into the top. The flow velocity due to the pump is much higher than in a radiator in any case.
't happen, I'm saying it wouldn't anticipate it happening.
ent masses of the H&C water columns.
It definitely does do that. It's the electrical resistance analogy with mu ltiple flow paths. You could work it out from the same principles, i.e., fl ow in = flow out, differential across all the flow paths is the same, flo w along any one path is determined by the resistance and the differential.
Convection assists the pump, driving more of the flow up the alternative fl ow paths, along the top header, rather than straight through the bottom hea der.
won't happen, I'm saying it wouldn't anticipate it happening.
fferent masses of the H&C water columns.
h multiple flow paths. You could work it out from the same principles, i.e. , flow in = flow out, differential across all the flow paths is the same, flow along any one path is determined by the resistance and the differenti al.
flow paths, along the top header, rather than straight through the bottom h eader.
If you have ever had a system run purelyb y convection you will realise what a puny force it is. In a pumped system, it is neither here nor there.
It can be a significant factor, but then I could work it out if I felt inc lined.
The differential pressure, provided by the pump, across the radiator is throttled down in the course of balancing until the flow is adequate.
Most radiators can work adequately on a one pipe system with no diverter tees (Harry will have to google for that), with convection alone driving ne arly all of the flow through the radiator. That suggests to me that the dif ferential pressures across the rad, whether provided by pump or convection, may be of a similar order of magnitude.
It's a hypothetical question since no-one would knowingly connect a radia tor TBOE with the flow in the bottom. I don't see why it wouldn't work, is all.
It was a useful exercise, since my pondering of this gave me a bloody good idea about something else.
radiator TBOE with the flow in the bottom. I don't see why it wouldn't work, is all.
We've already seen lots of people saying that with BBOE the water comes in at one end and convects to the top in the first channel or two. The rest of the rad is then full of cooling water, which eventually makes its way to the bottom tank and then out.
In BTOE it'll come in at the bottom, convect straight up to the top tank in the first channel or two - and then have no reason to ever go back down to the rest of the rad, as it can leave from the opposite top.
happen, I'm saying it wouldn't anticipate it happening.
masses of the H&C water columns.
That's why I said the vast majority would flow through the bottom tank. Not *all* of it. The top and bottom tanks generally have a much wider bore than the vertical channels. Any path in BBOE that includes a bit of top tank, also has two vertical channels in it, along with the same amount of tank as the straight through path and four right-angle turns for good measure. The overall resistance to flow of a top-tank path will be a lot higher than the straight-through path, so the flow will be much lower.
Let's be charitable and assume that, say, 30% of the water does manage to go through the top tank. The 70% that completely bypasses the core of the rad would be absolutely disastrous in terms of system balancing. Luckily, with the boiler turned on, convection sorts BBOE out.
The pump-only flow pattern has most of the flow going straight through the bottom tank. Convection virtually completely wipes out that flow pattern and replaces it with one where virtually all of the water goes via the top tank. Hence convection fights the pump - and wins comprehensively.
Note that this only applies *inside* the radiator, not in the pipes.
If all the water tries to pile up the first few channels, the resistance of that flow path increases and other channels become a path of less resistance.
It balances out so that all possible flow paths have the amount of water flow that makes their resistance equal. You could get all the flow going up the first few channels if they were very wide.
It is puny *outside* the radiator. The pump can easily overwhelm the few mm of head it generates. *Inside* the radiator, it's a completely different story. The entire radiator has a very low flow resistance and it only requires a small flow, so the pressure drop across it is very small. Water drifts very gently through the channels and convection can easily hold sway. I'm glad it can or my all BBOE system wouldn't work.
Yep. That's the crux of it. In a properly balanced system, the flow through each rad is pretty small. Coupled to low end-to-end flow resistance of the rad, that makes for a very low pressure differential across the rad itself (not the tails) due to the pump. So it turns out that the effect of the pump inside the rad is very feeble, just gently sweeping the water from the inlet to the outlet.
I wouldn't be surprised.
Well, a couple where that has accidentally happened have come to light on this thread, and they've both shown the same symptoms. Of course, you could argue that there my be thousands of others where it hasn't mattered, but I know where I'd put my money...
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.