noob question about how water flows through a rad!

25 Jul 2020
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United Kingdom
i've recently balanced my system and learned a bit about inlet/outlet flow, TRVs and lockshields, but something is still bugging me. assuming a system is well balanced, and you're using the TRVs on a towel rail, if you set the TRV to 1 (therefore limiting flow) does this mean that the top rung of a towel rail won't get hot? and if you have the TRV set to 5 then it pushes more water up the towel rail so all rungs are heated? in other words, you can't have evenly spread LOW heat across the whole towel rail? or is there something i have misunderstood?

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The TRV just measures the temperature of the room and opens/closes accordingly. Set to 1, it will go off quicker than if set to 5. That’s how I understand it.
Wot they said 2 ^^^

- On 1 the TRV shuts at around 10degC, on 5 it shuts at around 30DegC, that's only a guide though as they aren't really that well calibrated.
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As above more or less, the TRV measures the room air temperature and depending on its setting might be fully open, 50% open or practically closed etc, fully open and assuming a boiler temperature of 75C then the return may be 65C so the mean rad temp will be (75+65)/2, 70C, as the room temp rises the TRV will start closing and throttling the flow of water through the rad and eventually might end up with a return temp of say 45C giving a mean rad temperature of (75+45)/2, 60C, which is sufficient to maintain the room temperature at its desired level. The only way to keep a rad or towel rad flow and return temperatures as close as possible is to keep the flow rate as high as possible and reduce the boiler flow temperature ie fully open as above mean rad temp 70C and by reducing boiler flow temp to 65C might give a return temp of 55C to give a mean rad temp of 60C but with only 10C difference between flow & return vs 30C above when throttling the flow with the TRV to give flow/return of 75C/65C for the same mean temp of 60C.
Hot water entering the towel rail always rises to the top, (convection) but generally all bars will get to more or less the same temperature.
The TRV limits water flow depending upon its setting and the room the temperature
If the TRV permits high flow the towel rail will be hot; if the TRV throttles flow the towel rail will be cooler.
Here is a screenshot of various temperatures I took from a 1.7kw, 1400X400 Rad a few years ago.
The rad is plumbed normally, ie BOE, (bottom, opposite ends for flow/return piping).
You can see the temperature gradients in 3 places on the 5 vertical planes, you can see the water shoots straight up in the first column, then moves along the top and falls to the bottom.
The TRV was fully open for these tests.

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