We used to have an old bimetallic Honeywell thermostat but it always seemed wildly inaccurate and slow to switch on or off.
I thought my problems would be solved by going digital with one of these.
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Today however, it was getting uncomfortably warm in our hall. The thermostat was set to twenty, it was displaying 20.5 and was still calling for heat ( flame symbol in display).
Shouldn?t it have turned itself off at 20 or are digital ones designed to keep calling for heat for a period after reaching the target temperature?
On mine can set the accuracy to 0.5, 1.0 or 2.0 degrees. Probably depends if you've got a cast iron lump or a modulating boiler which you should choose ...
Cannot you use the buttons to manually adjust the target temperature to find at which point the thermostat stops calling for heat? If its anything like my thermostat the first press of the button will display the actual target temperature, the second press will then reduce the target by 1C. At this point leave it for say 30 seconds to see if call for heat turns off. Repeat with lower temperatures if still calling for heat.
Or, perhaps it is the function of the proportional bandwidth facility (setting) on the thermostat that is attempting to modulate the firing of an older boiler as the temperature reaches the target and the thermostat tries to be clever to maintain that temperature that is causing the problem
The following thread may give some insight - read to the 9th post in
Thanks, that was very helpful. It?s probably just operating as designed then. I?ve dug out the instructions and see that it is possible adjust the proportional band width. I might have a fiddle. ;-)
Digital ones also have to have hysteresis too, otherwise your heating system would be switching on and off too rapidly.
There are better ways - You can buy integrated systems which are able to predict how much heat input is needed, so as to just hit the set temperature, then as that is neared, they ramp the boiler output down and eventually off. When more heat input is needed, they will run the boiler at a suitably low output, that they don't over shoot the setting.
I have such a system and it makes the heating system unoticeable, no more temperature cycling as the boiler cuts in and out, no noise as pipes expand and contract.
I bought three wall heaters for work a few years ago, rather than the 10 quid plastic ones we'd normally dot around when it's brass monkeys I spent good money on some 'nice' glass panel digital ones.
They had a 5 degree gap between calling for heat and not. Set them at 21 they'd come on at 18.5 and go off at 23.5 so rarely a comfortable heat.
The timer on them was like a Japanese riddle, you (or I at least) could only set it by way of a fluke.
But it's OK because all 3 packed in and we're back to the pound job jobbies.
I should add to my prior post, everyone I live/work with* has no respect for a thermostat and will ramp them up to max in the bizarre hope that they will heat an area up more quickly.
Anyone who has had a gas oven will be familiar with the concept of a high gas mark (temperature) giving a higher power (gas flow) than a low one. In other words, from cold, gas mark 9 followed by gas mark 4 when it's up to temp will result in the oven getting up to the final temperature of gas mark
4 quicker than gas mark 4 from the start: a gas "regulo" controls both power and on/off cycling temperature. I presume once the oven is at temp, it alternates between virtually off (just enough to keep the burner alight) and full, in a similar way to an electric oven, but when it's heating up from cold, the rate of heating is controlled by the gas mark setting. I presume that's still the case; we've always had an electric oven, but my parents have gas and I'm basing my comments on how their oven behaved.
When we changed our previous house and our present house from (in each case) a conventional thermostat to Hive, we noticed that the temperatures remained a lot more constant. I think originally the temperature stayed within about
+/- 3 degC of the required value, whereas it's now within about +/- 0.5 degC (*). That's ignoring any additional temperature rise due to the sun (the room where the thermostat lives is south-facing) which sometimes causes a temperature rise that outweighs the fact that the central heating then doesn't come on as much.
I imagine that solid-state temperature sensors have no hysteresis at all, and the control circuit has to have a defined amount of hysteresis built in to avoid the boiler turning on/off too frequently.
(*) Looking at today's figures, the temperature on the Hive is set to change from 17 (overnight) to 21.5 (from 0800). The actual temperature overnight dropped gradually to a low of 18.5 at 0630, then rose to reach a steady state of around 21 by 1000, and from then until now it's varied from 21.1 to
21.9. With Hive, you don't have the boiler timed to go off overnight, you simply set a lower desired temperature overnight which means it usually never comes on (because actual > desired) but occasionally comes on if it's a cold night and the actual drops below the desired.
Nope. I?m almost certain that our old gas oven gave a ?full size? flame from cold whatever the temperature setting. The temperature control just changed the temperature at which the flame modulates down.
a gas "regulo" controls both power
How it behaved or how they ?believed? it behaved? The idea that turning a stat up to full will speed up the rate of heating with any sort of heating system is so pervasive I?m sure many folk believe it absolutely.
From the manufacturer's blurb it looks like it uses electronic proportional control logic instead of relying on a physical hysteresis. As the temperature approaches the set point it modulates the heat supply by progressively reducing the duty cycle in order to avoid overshooting the target. The call for heat indicator doesn't necessarily mean that the heater or boiler is actually running because the indicator remains on during both the 'on' and 'off' parts of the duty cycle.
Even at the set point, or even slightly above this, its indicator still shows a call for heat because a low level of heat input is still needed to balance heat losses. Our Honeywell CM927 works this way maintaining the temperature within +/- 0.5 deg C of the set point, and it does still show a call for heat when 0.5 deg above.
Incidentally with this type of thermostat the theory of temporarily turning it up to a higher value can in some cases actually result in heating the house slightly quicker because you don't get the reduced duty cycle during the last few degrees as you approach the temperature you want.
Maybe my parents' oven was unusual then, and I've extrapolated to thinking this was how *all* gas ovens behaved. Sorry, if that's the case.
I know that if you lit it (by putting a taper down the lighting hole at the front of the bottom of the oven), the size of the flame in the burners varied according to how far round the "regulo" knob was turned. There was no "full on or barely lit" with no gradation in between. Once it got up to temperature, *then* there was a sharp change from barely on to full on at around the temperature it was set to, but when it was cold there was a full range of flame heights, so starting it on maximum temperature *did* actually make it heat up quicker - unlike every other appliance which is simply on or off, so it would make no difference.
This was an oven that dated from 1962 (it was a wedding present to my parents) and it lasted until the mid 90s when they called out a gas engineer for some trivial fault and he condemned it as unsafe, so they had to have fish and chips that night and go looking for a new cooker the following day, living on microwave meals (no oven, hob or grill) until the new one was delivered and fitted.
I remember that it originally had a "gas taper" for lighting the oven: this was a thin flexible hose with a metal burner on the end, which you lit from the main pilot light (no piezo lighting!) or from a gas hob burner, and then transferred the little flame to the lighting hole in the oven. That hose got trapped so many times in the oven door as it hung down on the hinge side of the door when it was not in use, so it was removed and blanked-off when the Gas Board were converting everyone's cookers from coal gas to natural gas.
Likewise and obvious if you turn the control's setting. Set it to a mid setting, watch the flame and when it hits that setting the flame significantly reduces to a low level. Turn it up a notch, the flame goes to maximum. Turn it down the flame reduces to minimum, until the cools down to the new setting. All done by capillary tube and expansion inside the tube, controlling the jet's supply of gas.
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