Why Can’t We Crank the Heat AND the AC?
We’re entering the awful, awful season where mother nature will swing between very warm and very cold. Just last week we had a good four inches of snow, a high of forty degrees, and a low below freezing, only for a high of over fifty the very next day. This weather calls for both heating and cooling in some circumstances, but we can’t always give you both.
Fond, Freezing Memories
The schools I went to growing up had a very annoying problem: they could heat or they could cool, but they could not do both. This was always problematic and stupid to kid-me sitting at his desk contemplating a textbook bonfire for warmth. Of course, it’s wasteful to run the heating and cooling at the same time, but surely we can just turn off the furnace and crank up the AC? We’ve done it in the car all the time, you just twist a knob and you go from freezing to roasting and vice versa.
At small scales, climate control is pretty easy to accomodate. You use air ducts and mix hot and cold air to get the net desired temperature. Facilities with forced air heating and cooling can pretty readily fling themselves from one temperature to another like your car. Other facilities have completely separate systems for heating and cooling. There might be a little split AC unit in every room and a master hot water heating system.
These configurations don’t always scale well. Sometimes the cost just doesn’t fit or there are concerns about long-term efficiency. When that happens, you could be left choosing one discomfort over another, but why?
Single Pipe Central Heating & Cooling
The issue with my early schooling came down to efficiency. The buildings were equipped with huge heaters and massive pipes that carried fresh, boiling water to a radiator in every room. That worked amazingly in the winter. No matter how freezing it was outside (very), we could stand to focus on our books and math without wearing seven layers of clothes. It didn’t work for those transitioning times.
With a single pipe HVAC system, you run into problems with the laws of physics. At the outset, what happens when you change the temperature of metal rapidly and frequently? The metal expands and contracts, slowly weakening. At some point, this miniscule amount of change creates stress and boom, ruptured pipes. Whatever changes we make must be gradual. How gradual? It was said that my schools needed up to three days to heat up from a cold start and a good day to return to warmth after being turned back over a weekend. The fact of the matter is, we’re using pipes designed to retain as much heat as possible, except around radiators. We might have over a hundred miles of pipe, but only ten miles of all that piping can actually cool things down. On top of that, this is a massive amount of energy. Heating an entire facility, anything bigger than a home, requires fuel by the barrel. It takes even MORE power to put on the brakes and forcibly cool down such a setup.
Now, the question might be: what kind of fool builds half a system? Well, an efficient one would. When you build something like a high school, you want to make everything efficient. Individual heaters per-room are expensive and inefficient. Forced air systems might have concerns of cost efficiency and effectiveness over a large building with many small rooms. This sort of situation calls for a water based heating system, which can efficiently use huge boilers and chillers to handle the whole building, with minimal thermal losses, leaking, and maintenance.
Add Another Pipe, Cheap-o!
The next option is to add a hot pipe and a cold pipe. So long as the pipes aren’t touching each other, there’s not a lot of issue for thermal stresses. You blow air over both pipes, mix it, and exhaust that to the room. The mixed air will settle at the net temperature between both pipes. Boom, AC and Heating at once, but it’s not quite perfect.
Any scenario requiring heating and cool at once, will inherently be inefficient. You’re spending twice the energy to achieve a particular result. Imagine driving with your foot hard down on the gas so you can drive at highway speeds, but then having to slam on the brakes around town so you don’t go careening off the streets. You’re burning gas AND you’re going to need new brakes every other day.
There’s also the logistical issue. You’re now running heating and cooling at once, and instead of 100 miles of piping, you’ve now had to double that. There’s 200 miles of pipe so every room gets the best of hot and cold. And it’ll still fall short. The heating and cooling systems need time to ramp up and do their thing. There’ll still be sucky days.
At industrial scales, you have a choice to make: spend a fortune and be comfortable, or be efficient and get near enough to comfort. Unfortunately, there’s little middle ground so far. Remember, sometimes that uncomfortable environment is the best your HVAC guys can do. Thank them for doing their best and bundle up.