The Dual Nightmare

There is an ongoing heated discussion among homeowners and HVAC professionals about what solution to the problem of properly balancing the house is better - zoning the house, or having multiple units installed. There are all kinds of arguments on both sides.

The proponents of zoning are usually saying that it would be silly to use one switch to turn on or off all the lights in the whole house, whereas the opponents are pointing out that it is silly to pump the amount of air intended for all ducts together through one.

The opponents of zoning are usually saying that it is more reliable to have two simple HVAC units than it is to have one complicated HVAC unit and one complicated zoning system, whereas the proponents are pointing out that maybe HVAC professionals may be simply out of their depth and are trying to cover that up by silly arguments.

The opponents of zoning are saying that having two units is an adequate solution to the balancing problem and point out to the fact that existing commercial zoning systems are rarely extending "affordable" options beyond four zones, to which I say they are completely nuts and give the room (read: zone) count for three quite average houses that I've lived in: roughly 11 zones in Iowa farmhouse (2 levels, 4 bedrooms, full basement), 14 zones in one house in Arizona (3 levels, 4 bedrooms, game room, library) and 13 zones in the house I live in now (2 levels, 3 bedrooms). Oh, I do include bathrooms, too.

But I digress.

Sometimes you just have two units - for example, when you simply buy the house with them installed - it would be silly to rip them out, wouldn't it? So one just has to deal with it...

Whereas having two units is certainly nice, it is by no means a guarantee of any improvement whatsoever. Like it was pointed out before, all your luck depends on having the job done right.

Sometimes your luck is improved by the fact that the house was designed with heat exchange physics in mind (little heat transfer between floors, narrow staircases).

Sometimes it is degraded by the fact that aesthetic considerations took priority over common sense - wide open floor plans with walls being mostly glass, rooms spanning several floors vertically, and different floor count in different parts of the house.

Which is exactly the situation I have to deal with right now. The north side of the house (1 floor, 2 rooms + 2 bathrooms) is served by a nice powerful variable speed high SEER Lennox split unit. The south side (2 floors, 4 rooms + bathroom + kitchen + laundry room) is served by a puny rooftop Trane and a set of drinking straws they called ducts.

So let me skip the rant about how unthoughtful that was, and get directly to describing various interactions between two units that may happen. Let's assume for simplicity that we're talking about cooling season behavior.

Unhappy Load Meter

If both units kick in at the same time, and you are on a load controlled electrical billing plan, you're not gonna be happy. Neither is your electrical company, but that's a completely different story for a different time.

Cold Spots

It may happen that both units serve the same room[s], directly or indirectly, and the room has enough or more than enough supply and return capacity, and neither of the thermostats is in it. If both units kick in at the same time, the room will get uncomfortably cold.

Hot Spots

It may happen that the cold air supplied by unit A drags down the temperature at the thermostat controlling unit B. In this case, it will not get unhappy and cause unit B to switch on until unit A is off. Which will cause the rooms normally served by unit B to become uncomfortably hot.

The Skew

To add insult to injury, it is quite possible that the layout described above also makes unit A serve, at least partially, the zone that is supposed to be served by unit B - and in this case, unit A will keep running and running, possibly not even catching up with demand - and it will not get any help from unit B because the effect described above robbed unit B of necessary control input and is preventing it from switching on. You end up with one unit trying to do the work of two, for which it is clearly undersized. To further aggravate the situation, it is possible that the overworked unit will go beyond its normal design range, and then you have a dead unit (ask me how I know).

For more than one floor, The Skew is rather a rule than an exception.

Ask yourself a question, don't you always have to set the thermostat downstairs a couple of degrees lower than upstairs, otherwise it feels hot and stuffy? The scenario above explains it vividly. All there is is a leak of cold air from upstairs through the staircase (or a tall room) that causes the thermostat downstairs to make the unit stay off.

Catch-22

To prevent The Skew (further defined as one of the units working significantly longer than the other), you need to carefully balance the thermostats in such a way that the race never starts. This will usually mean that you will have one of the thermostat set below the comfort level, and the other above. Also, this is going to cost you, because the difference between "comfortable" and "balanced" positions of thermostats is going to make a significant contribution to your electric bill. Also, it robs you of comfort. Also, it will make you fiddle with controls all the time, unless you have monitoring set up, in which case you'll just spend half of the time fiddling.

It would be interesting to know what side of argument described at the beginning of the article you are now...