Multizone Systems: The Single Flow Problem
Over the past several posts, we’ve been hitting on the big deal with multi-zone HVAC Systems. You can attain amazing levels of control over how hot or cold any given room is. That is, except for one problem: you can only really push one “direction” at a time without making things incredibly complex.
One Temperature Change
Suppose most of your building is set around sixty eight degrees. Maybe one room wants seventy four and another sixty. The temperature outside is a warm but not unbearable eighty seven degrees. Our multi-zone system kicks in and sends exactly the right amount of cold air to exactly the right rooms as needed to achieve those desired temperatures. All sounds good, right?
What if someone wanted their room to be ninety eight degrees? Maybe they’re curing concrete or they’re homesick for some equatorial inferno. This shouldn’t be a problem for our amazing system, right? Just raise the thermostat and hot air will come pouring into the room?
We’ve been presenting the most straight forward, affordable, and common multi-zone system: one set of ducts, and one set of pipes. In order to heat that one room above the outdoor ambient temperature, the furnace and cooling system would be running at the same time. They would both be pushing hot or cold material into the building’s one distribution system. The air or water would mix and become the median temperature, something neither hot nor cold enough to make anyone happy.
There’s also secondary factors, like thermal stresses. Rapidly heating and cooling your pipes, having the chiller and furnace battling each other for dominance of the system, and everything else that follows, is going to lead to some very expensive repair bills.
In practical terms, you can’t have it both ways. Pick hot or cold, but never both.
For those who demand to have it both ways… There is a way. Near any problem can be solved by throwing money at it and waiting. To solve the conundrum of simultaneously running hot and cold, we add another division to the system. The furnace gets it’s own dedicated heating loops. The air conditioner or chiller gets its own dedicated heating loop.
With the two systems running separately a mix of hot and cold can be sent to both rooms. It does require you to basically double the amount of hardware involved, but it is a workable solution. There’s also some freedom in how you perform the division.
Independently controlled, electrical heaters can be supplied to individual rooms instead of a central heating system. Mini-split systems can be set up with heating elements instead of heat pumps. You can use a mix of central and independent heating and cooling solutions to gain better control, at the risk of losing some efficiency.
At the end of the day, the sky remains the limit, with enough money and creativity to solve the hurdles in your HVAC Design.
The Wrap Up
What did you think? Did we get something wrong? Got something for us to cover next time? Let us know in the comments below.