Sunday, 23 December 2012

The house sucks...

...air in when the extractor fan in the kitchen goes on. This makes the pressure drop, and there are two consequence. One is that the front door is difficult to open. It's not impossible to open, but can be quite hard work. The first time I tried to open the front door when the fan was on, I thought it was locked.

The other problem is cold air coming in through the bottom hinge of our big window. Already the floor seems to be a few degrees lower around it as cold air is leaking in, but when the extractor fan is on, you can feel a draft. I think the window could be fixed so there is no draft, but this problem perhaps seems worse because the rest of the house, including all the other windows, is so airtight, and the air has to come in somewhere.

I imagined that the ventilation system would be able to accommodate this somehow, so I've been looking at the controls again. The two pertinent settings, I think, are "Fixed pressure imbalance" and "Constant pressure off".

For the latter, the default setting is zero, "No". It can also be set to 1, which is presumably "yes". The manual explains, "This enables the determination whether the fans should run at constant flow rate at all times or whether, if a certain pressure drop has been exceeded, the fan changes to constant pressure."

I changed it from the factory default, zero, to one. Then I wasn't sure if that was correct. Presumably it was trying to balance the pressure before, but was not doing well enough. For a start the ventilation system is set to shift 160 cubic metres of air per hour, whereas the kitchen extractor fan can shift over 500. There's no way it can compete. At the medium setting, the kitchen extractor moves 380 cubic metres per hour, and at low it shifts 160. It also has a regular ventilation function which shifts 90, at a power usage of 18 watts. This may be useful in some seasons. Also, it will take a while before the "certain pressure drop", whatever that is, has been exceeded, so the ventilation system is not going to start compensating as soon as the fan goes on.

A few days later, with constant pressure off, the door is being sucked in, and is getting increasingly difficult to open. I guess what is happening is that the ventilation system is diligently pumping in and out equal quantities of air, but every time the kitchen fan goes on more is pumped out and the pressure is dropping. Previously this would have reached an equilibrium after the ventilation system realised the pressure was different. Now it does not care. 

The other setting, "Fixed pressure imbalance" was at the default of zero, so I'm thinking that this should perhaps be set to some positive number, so the pressure inside is slightly higher than the pressure outside. This would mean that any leaking air was going outwards, so drafts would stop coming in. All of the windows open outwards, so increasing internal pressure would probably strengthen the seals. The two doors open outwards, so they may become more leaky. With an over-pressure house, when the extractor fan went on, for a while it would just be bringing the internal pressure down towards the external pressure. "Fixed pressure imbalance" can be set anywhere between -100 and +100, but I wasn't sure what the unit was. Further reading suggests that it is the difference in cubic metres per hour of the fans blowing in and out.

As a  complete thermal system, less heat is probably wasted if the house is at a lower pressure to the outside, and cold air is leaking in rather than warm air leaking out. If the house is over-pressure and air is leaking out, it will be room-temperature air, whereas if it's under-pressure, the air leaving the house will have passed through the heat exchanger, and be at a lower temperature. But, this is going to make the heat exchanger less efficient. The heat exchanger can only exchange as much heat to one side as it takes from the other. If the air going in and out are at different speeds, they won't be able to exchange the same amount of heat.  My head starts hurting when I try to work this out, although that may just be because of the low pressure.

The morning after fixing these settings, the front door was still sucking in, and I went to see what the controls said. You can call up all the settings on the machine, so I saw it was expelling air from the house at 19 degrees and drawing in fresh air at minus 5. I also noticed that the flow rates were very different. It was expelling air as per the setting of 159 cubic metres per hour, but only bringing in air at 77 cubic metres per hour. 

This is a frost prevention technique. As the air leaving the house drops in temperature, it will reach saturation somewhere above freezing, then if it's cold outside it will hit the freezing point saturated, so it's going to start snowing in there, or icicles will start forming. This is a bigger problem with more efficient heat exchangers. The solution they use is to change the rates of flow going in and out, which makes the heat exchange less efficient and means that the air going out will not drop much below freezing. Now I understand how the frost prevention works, but I'm still not sure whether we're going to get back to atmospheric pressure!