Anyone using Hive?

but you don't need an internet controlled device to do that

you just need a good quality static thermostat

tim

Are normal thermostats are precise as that. With the static thermostat, we were getting cycles of about 3 degrees (+/- 1.5) through the day. I'd assumed that this was normal for mechanical bimetallic thermostats. No doubt a modern thermistor-driven thermostat, switching the boiler on and off by a little relay in the stat, would be a lot better - the same technology as Hive probably uses.

How accurate can the Honeywell with PID maintain the temperature, if it is better than Hive because that has "quite a lot of overshoot and hysteresis"?

Presumably any electronic thermostat can use whatever level of hysteresis that you like (user-settable). Is the advantage of PID that it can anticipate how long it will take for the radiators to warm up when the boiler is turned on and to cool down when the boiler switches off, and apply

Mechanical thermostats have a couple of degrees or so of hysteresis. Many seek to reduce that by incorporating "accelerator heaters" - resistors which warm up when the stat is in the ON state, causing the heating to be turned off a bit before the air temperature reaches the set point.

Hive doesn't even do that. The heating is only turned off once the set point is reached - so the hot radiators continue to increase the air temperature, causing overshoot. In cold weather, mine often overshoots by up to 1.5 degrees, then the heating is off for a long time while the rooms cool naturally before the heating comes on again.

The Honeywell is better because its P.I.D. logic causes it to cycle the heating on and off within the 'proportional' zone (a couple of degrees either side of the setpoint) with the result that it doesn't overshoot. Once mine has reached the setpoint, it just sits at that temperature, cycling the heating on and off as necessary. [It only displays the temperature to the nearest 0.5 degrees, so I suppose that there could be variations of up to + or - 0.25 degrees which wouldn't show up].

Sort of - see what I wrote above.

The Honeywell is also set up to prevent the boiler from short cycling.

Chris

Interesting that your Hive gives you such a large amount of overshoot. For me, both Hive's own temperature sensor and the one in my weather station show that the temperature remains constant within about 1 degree (so presumably +/- 0.5 degree of the required temperature). I imagine Hive must have *some* hysteresis, or else it would turn on the boiler every time the temperature dropped by one unit of the required temperature and turn it off every time the temperature goes one unit over the required temperature - for whatever granularity "unit" the temperature sensor has. Since Hive reports temperatures to the nearest 0.1 deg, that may well be the granularity of the sensor, so you've expect the boiler to by cycling fairly frequently if (desired-0.1) deg turns it on and (desired+0.1) deg turns it off. Given that Hive allows temperatures to go wider (eg +/- 0.5 deg) that is presumably the amount of hysteresis that it built in, to reach a compromise between accurate temperature control and frequent boiler cycling.

That's true - it has a configurable minimum 'on' time. I think the Hive has something similar, but you can't adjust it. At any rate, if you turn it on manually by pressing the button on the Receiver, and then try to turn it off again, it stays on for a minute before actually going off.

My Horstmann wall electronic thermostat does that with some clever internal logic which appears to learn. Looks like a normal thermostat with a control knob. Painless to install and operate.

It may not have intentional hysteresis - i.e. different on and off temperatures - but the overshoot provides 'effective' hysteresis because the temperature goes on rising in the 'off' state, and the stat doesn't switch on again until the temperature drops back down to the setpoint.

The amount of overshoot is probably determined by the capacity of the radiators and how well insulated the building is, coupled with the outside temperature - so won't be the same in all cases.

I only installed my Hive at the end of March this year, so it hasn't seen any really cold weather. It's quite possible that the overshoot will reduce when the heat losses are greater when it's really cold outside.