Author Topic: hysteresis in thermostatic valves?  (Read 857 times)

hysteresis in thermostatic valves?
« on: December 16, 2019, 08:02:56 pm »
I apologise for seeming to hog this section.

The house has thermostatic valves on the radiators but they appear to cool down a long way before opening and then get hot before closing so the room temperature cycles between too cold and too hot.

They appear to be a brand called "Bulldog" which immediately makes me think they are probably crap!

Do more expensive valves work better with less hysteresis?

On a related note they have put in a wireless honeywell thermostat (one for a 4 bedroom house!!!).  would I benefit from wireless thermostatic valves and eithe Hive or Honeywells equivalent?

We have 2 bedrooms that are only used in the evening and a loung which is used less in the day and then my wifes study which needs to be a perfect working temperature for most of the day 5 days per week?

I am not really interested in cost efficiency as much as temperature maintenance efficiency.

Thank you


Re: hysteresis in thermostatic valves?
« Reply #1 on: December 16, 2019, 08:33:48 pm »
At the risk of spamming your thread, for which I apologise profusely, that's gotta be one of the nerdiest thread titles on YACF, and I'd like to congratulate you on that - it's not an easy title to win round here.

FifeingEejit

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Re: hysteresis in thermostatic valves?
« Reply #2 on: December 16, 2019, 08:40:41 pm »
Is the fancy thermostat in the room that cycles between too hot and too cold or is it in another room?

Re: hysteresis in thermostatic valves?
« Reply #3 on: December 16, 2019, 08:43:14 pm »
Not sure what it adds but I was under the impression that in the room with the thermostat you should leave the TRV fully open so the thermostat can do it's thing.

Mrs Pingu

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Re: hysteresis in thermostatic valves?
« Reply #4 on: December 16, 2019, 09:00:45 pm »
And if the study is the one which needs to be warmest I'd say the thermostat would be best in there.
Do not clench. It only makes it worse.

Kim

  • Timelord
Re: hysteresis in thermostatic valves?
« Reply #5 on: December 16, 2019, 09:05:50 pm »
Or take the thermostat with you as you wander around the house.  Wirelessness isn't just for lazy plumbers.
Careful, Kim. Your sarcasm's showing...

Re: hysteresis in thermostatic valves?
« Reply #6 on: December 16, 2019, 09:09:40 pm »
Or take the thermostat with you as you wander around the house.  Wirelessness isn't just for lazy plumbers.

This.

You can also vary the hysteresis by putting it nearer or further from a radiator.

Mrs Pingu

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Re: hysteresis in thermostatic valves?
« Reply #7 on: December 16, 2019, 10:08:34 pm »
Or take the thermostat with you as you wander around the house.  Wirelessness isn't just for lazy plumbers.
That too. My wireless stat lives in the bedroom cos we have the stove in the lounge but if the stove went out of service for any reason I'd move the stat back in there.
Do not clench. It only makes it worse.

Re: hysteresis in thermostatic valves?
« Reply #8 on: December 16, 2019, 10:12:26 pm »
I perhaps did not explain myself very well.

All rooms except the bathroom have TRVs on each radiator.
There is a thermostat in the hall which controls the boiler.

The individual rooms cycle cold-hot-cold over a bigger range than I am used to.

This may be due to uninsulated walls,(cannot be altered)
Or
Poor radiators some 30-40 years old.
Or
The TRVs taking a long time to cycle from open to shut and back again

In our own house all the TRVs were digital timed TRVs which maintained a very steady temperature.

I am wondering if I need to go down that route in order to get a smooth temperature control.

Re: hysteresis in thermostatic valves?
« Reply #9 on: December 16, 2019, 10:30:39 pm »
All rooms except the bathroom have TRVs on each radiator.
Yup bathroom is ye olde bypass (and therefore warms towels)

There is a thermostat in the hall which controls the boiler.
But has a TRV so they fight, bypass rad is usually fixed stat location rad

The individual rooms cycle cold-hot-cold over a bigger range than I am used to.
Some brands do better than others

This may be due to uninsulated walls,(cannot be altered)
Easily/cheaply you probably mean

Or
Poor radiators some 30-40 years old.
May have been undersized, but if when 'hot' they are OK fine but generally a condensing boiler won't like them

Or
The TRVs taking a long time to cycle from open to shut and back again
Some brands do better than others

In our own house all the TRVs were digital timed TRVs which maintained a very steady temperature.
Some brands do better than others

I am wondering if I need to go down that route in order to get a smooth temperature control.
Only if you use the previously acceptable valves and the sensors haven't changed

Re: hysteresis in thermostatic valves?
« Reply #10 on: December 17, 2019, 12:59:39 am »
there is always going to be some hysteresis in a TRV because

a) without any the system wouldn't work properly (eg with the boiler short cycling) and
b) the wax in a TRV has an appreciable latent heat of fusion, so won't change phase very quickly almost regardless.


The wax sees heat from both the air in the room and more directly via conduction and radiation from the body of the radiator/pipework it is attached to.
If the contribution from the latter sources is large and/or continues for long after the water flow through the radiator shuts off, you can get some odd effects.  Worse yet if the room cools a lot faster than the radiator itself (eg because the room is cold and draughty, and/or the radiator/TRV etc stays warmer than the room for a long time then the air in the room can be quite cold before the heating kicks in again.


So I'd experiment with a few things; in each room  look at how the TRV is sited, and try to allow the TRV  to see more average room air and less heat from the radiator and the hot water circuit.  In practice this often means insulating the hot water pipe run if it is beneath a TRV and maybe putting a  insulating baffle between the radiator body and the TRV, all whilst not obstructing airflow over the TRV itself.

There is also an argument for turning the boiler down, so that the water coming out is not as hot; this will cause the TRV temperature not to overshoot (get too far past its normal closing point) on each cycle, and will probably mean that the TRV will not take quite as long to open again. It will also make the whole system less prone to short cycling via the main system thermostat.

BTW if the whole boiler shuts off when the main thermostat senses the set temperature, yet this location doesn't cool as fast as the rooms you are in, they can get rather cold before the system fires up again, andin this case it is probably little to do with the TRVs

cheers

Re: hysteresis in thermostatic valves?
« Reply #11 on: December 24, 2019, 11:50:41 pm »
There are indeed differences in trv head performance. Wax is cheap and liquid more expensive with faster reaction.
I fitted a danfoss in our new shower room and the whole thing was quality. Temperature adjustments is easier.

Re: hysteresis in thermostatic valves?
« Reply #12 on: December 25, 2019, 01:31:01 am »
There are indeed differences in trv head performance. Wax is cheap and liquid more expensive with faster reaction.
I fitted a danfoss in our new shower room and the whole thing was quality. Temperature adjustments is easier.
Thanks

Mrs Pingu

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Re: hysteresis in thermostatic valves?
« Reply #13 on: December 25, 2019, 09:56:11 am »
Might be worth getting some cheapo thermometers and putting them in your rooms and the hall next to the stat so you can see how the temperature changes in each. Could be the hall is lagging behind the other rooms.
Do not clench. It only makes it worse.