When we have days like these, with highs in the low single digits or even negative, you really have to appreciate a heating system that keeps your home or workspace in the 60s or 70s. It’s these frigid days that test the mettle of our furnaces, boilers, or heaters (and/or sends us fleeing to space heaters and fireplaces). But as I was shaking off the cold and looking forward to hunkering down for a long winter’s night at home, it got me thinking about how it’s these days of extreme cold that make variable speed drives like the Phoenix work so well for saving energy (and I know you are all going to be thinking more about energy savings when the heating bills for December and January start rolling in).
Size is the Thing
It probably seems obvious that the heating system in an office building or a school or a shopping mall is bigger than the one in your house. The boiler in my basement is a box about the size of a two drawer filing cabinet. The boiler in a school or municipal building can occupy the better part of a room larger than my living room. It’s fairly intuitive that a large building needs more heat to keep it warm than a small building and, all things being equal, you will find larger (in terms of heat capacity) heating systems in larger spaces, until you get to heat plants that are buildings unto themselves (and generally serve a campus of buildings).
But what may be less intuitive is the fact that the size (again, in terms of heat capacity) of your heating system has to be correct for the coldest possible weather conditions you want it to work through. I imagine everyone who has lived or worked in the northeast can remember a building that just couldn’t keep up with really cold days like today.
In heating and cooling lingo, HVAC contractors talk about correctly “sizing” your system to your space. Most systems have a heat rating that can be roughly converted into the square footage it is meant to serve based on the climate you live in (and the coldest likely days and nights there). There is a lot that goes into properly sizing a heating system for a space, including ceiling heights, insulation, what other heat sources there are (like people, equipment, solar gains,), etc. I will leave proper sizing of heating systems to the pros.
The bottom line is that your heating system is probably designed to heat your space when it is -15 degrees Fahrenheit (or even lower) outside.
So, what’s all of that heating capacity doing on all of the other days, when it’s only 10 or 30 or 50 degrees out?
Turning Off and On
The maximum heating of your system is what you get when it runs full out without a break. You are probably used to the idea that heating systems turn on and off when they are needed, generally governed by a thermostat. The thermostat looks at the temperature in the room and the temperature you want (the setpoint). If the room is too cold, it tells the heating system to turn on. When it notices that the room has hit the desired temperature, it turns back off. The actual heating elements in your furnace or boiler or even electric heater are basically on/off.
But what about the fans?
Fans are for distributing the heat from the place where it’s made (or piped to in water or steam systems) into the actual space you want heated, typically a room. Old school fans are on/off and controlled manually or, more likely, linked to the call for heat (auto mode). When the heat source turns on, the fan turns on. When the heat source turns off, the fan turns off, maybe with delays built in to improve efficiency.
Adaptive Variable Speed Gets the Most Out of the On Time
Having the fan turn on and off with the heating element seems to make sense, and it’s certainly better than just running the fan all of the time. But what you really want to do is maximize your use of the heat that is still kicking around in the off time to get the most out of your on time.
By reducing the fan speed, rather than just turning it off, you can get a double benefit. First, your fan should use less power at lower speeds (though that isn’t always the case with some fan speed controls). Second, slower air provides more efficient heat transfer while making sure it still gets into the room and is not trapped in ducts, walls, or cabinets (many of which lose heat faster than the rooms themselves and in ways that don’t help your comfort at all).
As it turns out, the best bet is to match the fan speed to the heat that is currently available at the individual fan. This is where variable speed makes a big difference. Variable speed fans are an expectation on large and high-end systems. They provide massive savings to the degree that some regulators are making them mandatory on larger fan systems.
Most variable speed drives, particularly those based on digital signal processing, like VFDs, offer a range of fixed speeds (perhaps 5 or 10) and, really, the more the better when it comes to adapting to the available heat. The Phoenix goes farther and is continuously adaptive, without stepped speeds.
So, it’s these super cold days that mean our heating systems have to be able to turn off and on to deliver the right amount of heat on the right days. And adaptive variable speed drives take things a step farther to allow the fan to use the off time and the on time to get the most out of the money you are spending on oil, gas, electric, or other fuel for your system.