In the ‘old days’, when room temperature was controlled by an on/off thermostat, and central heating boilers were not capable of modulating the radiator water temperature, the room temperature control was of the ‘bang-bang’ variety. The boiler delivered water to the radiators at a constant (high) temperature until the roomstat switched it off, normally at about half a degree above the set target temperature. The roomstat would switch the boiler/pump on again when the room temperature dropped to (say) half a degree below the set target temperature. This is not an ideal control system. The (essential) ‘dead-band’ or hysterisis in the roomstat causes the room temperature to overshoot and cycle up and down periodically and wastes energy.
The modern use of a temperature sensor (e.g. thermistor) to measure the room temperature, coupled with the ability of modern boilers to modulate the central heating water temperature, enables a much more efficient ‘proportional’ control system to be used. The room sensor tells the boiler what the difference between the actual temperature and the target desired temperature is, and the boiler adjusts the radiator water temperature accordingly. As the desired room target temperature is approached, the radiator temperature is reduced until a balance is achieved and the room temperature settles at the target. The pump continues to run, and the boiler maintains the radiators at a (lowish) temperature. The pump and boiler do not completely switch off unless the room temperature rises significantly above (say 1 degree) above the target temperature, or when the programmer reduces the target temperature significantly. This proportional control system works remarkably well, and in the case of my Climapro and Flexicom 18HX boiler, keeps the room temperature remarkably stable. There are two clear benefits of this proportional control system:
1. The very precise room temperature control, and
2. the greater (condenser) boiler efficiency working at a low water temperature for much of the time.
However, there are a couple of potential pitfalls associated with this system relating to the Domestic Hot Water / mid-position valve, and the use of an outside sensor.
Domestic Hot Water cylinder and the mid-position valve
Clearly it is not possible for the boiler to deliver water simultaneously at two different temperatures. The central heating system might be close to the desired target temperature and be demanding water at (say) 40 degrees, whilst the DHW requires water to the indirect cylinder at (say) 80 degrees in order to get the cylinder rapidly to a 60 degree DHW temperature. The proportional CH control is therefore incompatible with heating a hot water cylinder and, to work properly, the system cannot use the mid position of a diverter valve. The system needs to be set to disable the mid-position of the diverter valve and DHW priority.
This is the default setting on the Glow-worm / Climapro boiler systems, but I don’t think the Climapro / boiler control logic prevents one from selecting the combination enabling the mid-position valve and modulating the water temperature. Perhaps it should, because selecting this combination will result in the DHW often failing to reach the desired temperature.
If you switch off the modulating water temperature feature and enable the mid position valve then, whilst the system will be able to heat CH and DHW at the same time, you will lose the accurate and efficient room temperature control because the Climapro room sensor will revert to a simple on/off “Bang-bang” control, just like an old fashioned room thermostat.
DHW priority has the slight disadvantage that the CH will be overridden by a DHW demand. However, this is seldom noticeable in our house. It is only really noticeable when switching the system on returning to a completely cold house. It takes the system a while to heat the DHW cylinder before the radiators can start to warm the house. In the morning it is a good idea to set the programmer to turn on the DHW a while before the central heating switches up to the ‘comfort’ temperature. This will normally ensure that any DHW demand is satisfied before the CH starts to try to warm up the house.
The Outside Temperature Sensor
In principle it should be possible to design a boiler control logic which makes use of both the sensed room temperature “error”, AND the outside temperature, in order to decide what temperature to heat the radiators to. However, at least in my version of the Glow-worm Climapro and Boiler control software, it seems that the designers have not attempted to do this. In my Glow-worm system, if you connect an outside sensor, then control of the boiler temperature seems to pass completely to the outside temperature curves shown in the Climapro manual, and the Climapro itself reverts to a simple roomstat “bang-bang” operation. This means that, even if the room temperature is close to target, if the outside temperature is very low, the boiler will continue to work at high temperature until the Climapro roomstat mode switches if off (at about 0.5 deg above the target). This is unfortunate and inefficient, and rather dumb control logic. I therefore soon disconnected my outside sensor!
I think the signal from the outside sensor should perhaps be used to set the maximum boiler temperature, but leave the Climapro room temperature sensor to control the actual boiler flow temperature thus retaining the much superior proportional control. Glow-worm please note!
I fitted my outside temperature sensor on the recommendation of a heating technician, and didn’t think too deeply about it at the time. However, when I saw how the Climapro and Boiler control behaviour were changed by introducing it, I started to think more deeply about the potential benefits. I now think there can be very little benefit from an outside sensor if you have a room sensor. For example a feature to turn up the central heating early when there is a low outside temperature (not a Climapro feature I think), would work just as well if the room sensor temperature were compared with a programmed new target temperature anticipated in the near future. A large discrepancy between the two could be used to trigger an earlier switch on.
I am sorry this is rather lengthy, but I hope that some may find it useful in understanding the behaviour of these quite complicated CH/DHW control systems. My apologies if you think I am ‘teaching grandma to suck eggs’!
Regards