Zoning: Why, why, why?

Heating zones have been touted as energy saving measures, and many customers have shown us their multi-zoned systems, proud of the number of zones they have. But when we look at how the house’s space is divided and used, the “zoning” is often nonsensical. Sometimes the installer has “zoned” in the easiest way the piping will allow. Old gravity heating mains, for instance, were often piped left and right side, or front and back through all the floors in the house, to shorten the distance the water had to travel to and from the boiler. An installer may say he can zone using the existing piping, but it makes no sense to have a left and right division. Customers have been sold zoning for an upstairs and downstairs scheme, but once their children can go up and down stairs themselves, the temperature in the living room and the bedrooms will stay the same as people move back and forth while they are awake, and then the whole house temperature will be lowered when the family goes to sleep. We have customers in whose homes there may be seven programmable thermostats on seven zones and all of them are set to come on and turn off at the same times and temperatures. That is actually one zone with seven expensive control systems stuck onto it.   What is the point of a zone?  Zones need to make sense with the way the house is used, but they have to obey the laws of thermodynamics as well. If the zoned rooms can’t be thermally isolated from one another, there is no point in pretending the btu’s will stay in their respective areas. Heat goes to cold no matter what. You may turn the thermostat down in the dining room, but if there is an open archway between it and the living room, the living room zone will attempt to heat the dining room as well. No savings -in fact, lost money in the equipment wasted in setting up the “zones”, and plenty of discomfort in the drafts caused by wandering btus. A third floor guest room with a door that stays closed, or a basement family room isolated by the cellar door can be zoned successfully- the temperature can be set lower than the occupied part of the house and it will stay that way. Another reason to set up a separate zone is to accommodate heating units with different heating qualities which would not work well together on the same thermostat- for instance, a main house with cast iron radiators and a playroom with fin-tube baseboard. The cast iron will heat more slowly and hold its heat much longer than the fin-tube. If the thermostat is in the room with the radiators, the baseboards will be cold as soon as the thermostat turns off. Put the thermostat in the playroom and the radiator rooms will overheat.   The final factor in deciding whether and how to zone a heating system is the effect of the zones on the boiler’s efficiency. The most efficient way for any boiler to operate is in long runs where the boiler can reach its steady state. Think of a car’s mileage when it is cruising on the highway, vs. stuck in stop and go traffic. Having a system with many small zones can cause a boiler to stop and go, or “short cycle”. If one little office or bathroom is asking for heat, the boiler (sized for the whole house) will quickly make that heat and have to shut down, and the process will repeat itself until a larger space asks for heat. Even a modulating boiler, which is designed to drop its firing rate when less output is required of it will short cycle; there is a limit to how far the boiler can turn itself down. Combining unnecessarily chopped up zones into a larger load can remedy a short cycling problem. On a new installation, fewer zones will have a lower upfront cost and will operate more efficiently over the lifetime of the system than multiple small zones.   See discussion of outdoor reset controls, air sealing and insulation and alternative ways to make domestic hot water for other tools to save energy.