Explantion needed - weather compensation.

[Dave Plowman (News)]

I'm hoping someone can explain how it works. According to one source you can use it with no form of heat sensing from inside the house at all. Seems weird to me.

[Roger Mills]

AIUI, it only works in an open-loop sort of fashion. A friend of mine has still got a system - installed many years ago - using a Drayton Theta controller. This controls the temperature of the water going to the radiators by mixing boiler output water with return water - and creates a flow temperature which is a function of outside temperature. The aim is that, by so doing, the radiators will emit the right amount of heat to balance the heat losses under all ambient conditions. But it's rather crude, and takes no account of *actual* room temperatures - and so relies on the radiator sizing, and the gearing between external and flow temperature being correct.

There may well be modern systems which do something similar by modulating the boiler without any need of a mixing valve.

[PCPaul]

We had an expensive new building at work with 'weather compensation'.

Apparently it means that when it's cold outside, you get really cold inside as well. If it's hot outside, it's sweltering inside.

That's how ours worked, anyway :-(

[Andrew Gabriel]

I'll have a go. I'm not familiar with current commercial systems, but I know well the system my dad installed around 1959, and the system I designed myself and benchmarked about 4 years ago, but haven't got round to installing properly.

That's how my dad's system worked. The controller was called BMT (don't know what that stands for). They were quite common in very large commercial heating installations, but very new to domestic heating systems at the time. It was promoted by the Coal Board, and dad had a coal-fired boiler at the time, although it was retained for the heating control with the two subsequent gas boilers dad installed. It was retired in 1999, mainly because dad had a CORGI to install the 3rd gas boiler and the bloke didn't have a clue what the BMT was or how it worked.

Anyway, on to how it worked... You had to do heat loss calculations very much more accurately than is done today. Maintenance of room temperatures relied on knowing exactly what the heat loss would be at a given outside temperature, and sizing the radiator to match that exactly. Then the BMT monitored the outside temperature and adjusted the radiator flow temperature based on how much higher or lower the outdoor temperature was than the temperature on which the calculations were based. This was done by mixing water from the boiler with return water in varying proportions inside the unit. It had an outdoor thermostat phile and sensed the flow temperature (so it didn't care what the boiler temperature was, providing it was higher than needed). It was powered by the water pressure across the pump (a thin pipe was required from it to the other side of the pump for the "power"). The pump therefore ran continuously, and the radiator water gradually changed temperature as the outside temperature changed. It couldn't respond to indoor events, such as someone leaving a window open, but that was generally a good thing. Some manual controls on the BMT allowed you to shift the temperature scale, either to change the indoor temperature, or to boost the system whilst heating the house from cold.

My much more recent system worked along the same lines, but uses computer control. The radiator flow temperature is set by dynamically adjusting the set temperature of the boiler and having it modulate to meet this. The ideal is that you reduce the radiator temperature such that the room stat (if you have one) almost, but doesn't quite switch off. This is the coolest you can have the radiator water circulate and get sufficient power from the radiators, whilst having the condensing boiler operate most efficiently. This temperature again depends on the outside temperature, and therefore on how much power you need the radiators to give off. A modern system will also monitor the indoor temperature, and will boost the system when the indoor temperature is far short of the set temperature, which will speed up heating up from cold, and compensate for things like having the front door open.

Once you have that level of computer control, it's easy to also have automatic adjustment of the set room temperature, e.g. setback at night or when the house is not occupied. This is strictly outside the scope of weather compensation though -- it's really occupancy prediction or occupancy sensing, depending on how you do it.

[Dave Plowman (News)]

Thanks for the explanation, Andrew.

To explain I've just fitted a Viessmann boiler and included this option. Which is reversible. I also fitted a remote extension programmer which includes a room temperature sensor. With the lack of any real information on their site I sort of assumed they worked in conjunction - but their technical guy says it's one or the other. Of course until the cold weather arrives I can't make any decision - and the options are all in software - but I've a feeling my lifestyle means I won't want a system that has to be on 24/7.