Convection and continuity

With the coming of the colder evenings, we’ve had our new heat pump going. It’s quite a powerful beast, as it has a large volume to heat, and it comes with a plethora of different settings – temperature, fan speed, air flow angle and so forth. It’s taken some experimentation to get some decent settings on it.

One thing that’s clear is that the heat pump generates a lot of convection in the house. That, of course, is the point – that’s how the heat transfers from the indoor unit to the room – but in our case the air currents can be pretty strong. There are two ways this happens – ‘Free’ convection and ‘Forced’ convection.

Forced convection just describes the air flow due to the fact that there is a fan inside the unit – it’s blowing the air around. Given that air isn’t created or destroyed, this means that the air has to circulate around the house. Free convection describes the currents caused by warm air being a lower density than cold air. The warm air will be buoyant, and rise, which means (by continuity again) that colder air has to sink.  The two mechanisms are both in place, and they are both very evident. First of all, one can feel the air moving around (and see the light shade swing a little bit as the air blows past), and secondly upstairs becomes nice and toastie while downstairs is rather cooler.

So, the problem then becomes setting the unit so that we get the ‘best’ situation. One obvious problem is that the forced convection results in the airflow into the unit  being rather cooler than  the average room temperature. The pump measures the temperature of the incoming air, and then attempts to take this to the target temperature you set. So if you set the target temperature as 19 Celsius, it means that the air temperature of the majority of the area, especially upstairs, will get to rather higher than 19 C. That we can cope with, simply by setting the target temperature a bit lower.

We’ve also found that the angle of air flow leaving the pump makes a large difference to the distribution of heat around the house. If it’s too flat, it just scoots across the downstairs ceiling, and then heads upwards when the ceiling runs out – we end up with an oven upstairs and a fridge downstairs. If it’s angled too far down, it blasts into the dining room table – it keeps you dinner nice and warm but it feels like you are eating dinner in a Canterbury Nor’wester. Again, with some experimentation, we think we’ve got a reasonable setting.

So that’s all good for now, but there is a nagging doubt that when it gets much colder still the convection currents will change again and we’ll have to ‘re-optimize’.



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