Zoning With Constant Circulation
It doesn't take long for someone new to the hydronic heating trade to discover the versatility and benefits of zoning. If that person came from the forced-air side of the fence, they'll likely be amazed at how easy and relatively inexpensive it is to create a residential heating system with six or more zones. When the system is properly designed and installed, they'll also find it operates in a very “well-behaved” manner. No whistling registers or “dump zones” to be found.
Hydronic heating lends itself to several methods of zoning. The two most common approaches in North America use two-way zone valves or zone circulators. Both have been around for several decades, and both are predicated on the idea that stopping heat flow to a zone requires stopping flow through that zone.
While it's true that this is one way to accomplish zoning, it's also true that approach is not necessarily the only approach or, for that matter, the best approach.
Stop & GoFlow interruption through heat emitters can lead to several undesirable effects. First, it greatly increases the likelihood of noticeable expansion noises when a zone circuit turns on, especially if the circuit has been off for several hours. Under such conditions, the cool tubing and heat emitters receive a transfusion of hot water within a period of only a few seconds. The resulting rapid expansion is much more likely to create noticeable sounds compared to the same expansion taking place over hours or days.
Turning the flow on and off also allows portions of the zone to cool at different rates. For example, a heated floor slab covered with ceramic tile will cool noticeably faster than the same slab covered by carpet. The floor area adjacent to a large overhead door can cool a lot faster than the floor area several feet in from the door. The latter scenario has lead to frozen floor circuits adjacent to the doors even when the air temperature in the space is around 60 degrees F.
Another undesirable consequence of zoning with on/off zone valves is an increase in differential pressure across the circulator as additional zone circuits close off. This can lead to excess flow velocity in operating zones, stem lift in zone valves that are supposed to be closed, increased circulator wear and, in extreme cases, erosion corrosion. When this method of zone control is combined with a “flow-sensitive” low-mass boiler, further complications can arise. When only one zone is operating, the boiler may have insufficient flow for the rate of heat input from the burner. I actually heard someone describe the resulting sounds as “the boiler sounded like it wanted to get up and walk across the room.”
The idea of turning flow on and off to provide zone control stems from the long-standing assumption that boilers should deliver a fixed water temperature sufficient to heat the building on the coldest day of the year. This approach had been the norm in the North American hydronics market since the 1940s. However, the “new hydronics” era that began during the 1980s refocused attention on an alternate method of heat output control: outdoor reset. Simply put, it's the concept of heating water just enough to meet the current heating load based on the outdoor temperature.
Now think about this. If the water supplied to the distribution system is regulated by an outdoor reset control, should the circulator have to turn off to prevent overheating? The answer depends on two other questions:
- 1. How is the outdoor reset control set?
2. Does the space being heated experience internal heat gain?
Internal heat gains, which are almost always present in modern buildings, also create situations in which the heat emitters do not require water as warm as that supplied by the reset controller. Heat input to the zone must again be stopped.
In all of the above cases it was necessary to stop heat input to the zone circuit to prevent overheating. This doesn't necessarily mean that flow through the zone circuit must be stopped.
Stop The Heat, Not The FlowWhy throw away the benefits of continuous flow simply because you need to zone the system?
The schematic in Figure 1 shows one way to provide constant circulation and independent zoning. The essential component is a three-way zone valve, which acts as a fork in the road for the water returning from the zone circuit. If the zone circuit needs no heat input, all return flow is routed right back to the supply side of the circuit. When heat input is needed, return flow is routed back to the heat source, which allows heated water to flow to the supply side of the zone. It's really a pretty simple idea.
This concept can be applied with or without reset control. When the latter is present and properly set, the three-way zone valve should direct flow back to the boiler most of the time, bypassing water back to the supply side of the zone only when internal heat gains are present. In such systems it's helpful to think of the zone thermostat and three-way zone valve as an occasionally active “over-temperature protection subsystem,” rather than a constantly cycling heat input subsystem.
Condensed ThinkingFigure 2 shows another approach that achieves zoning control and constant circulation. In this case the system uses a condensing boiler to supply the relatively low-temperature radiant panel load while operating at high efficiency. Each manifold is equipped with a small circulator that maintains flow through the radiant panel circuits whenever the outdoor temperature is below some preset “cut-in” value, such as 60 degrees F.
Zone control can be provided by (a) zone circulators or (b) zone valves that can interrupt the transport of heated water to the closely spaced tees that connect to the zone circuits. Stopping the flow in the crossover bridges simply denies heated water to the zone circuits without affecting the flow through them.
If the outdoor reset control that operates the boiler is properly adjusted, the water temperature supplied to the zones should be just hot enough to meet the current load. Therefore, the zone circulator or zone valves should be on or open most of the time. They will only need to stop when internal heat gains are present in the zone.
The condensing boiler does not require protection from flue gas condensation; thus the system can operate over the full range of outdoor reset.
When the indirect water heater calls for heat, circulator (P2) is turned off and circulator (P1) is turned on to provide “priority” domestic water heating. During this mode, the boiler temperature is regulated by its own internal limit control rather than outdoor reset. This allows the boiler to climb to a higher temperature (say 180 degrees F) to provide good heat transfer through the heat exchanger of the DHW tank. As soon as the tank is satisfied (or a preset period of time has elapsed), the system reverts back to space heating and the boiler temperature again is regulated based on outdoor reset.
Say It Isn't SoI once overheard a fellow hydronic heating pro describe the befuddlement of European heating designers when they first discovered how many of their North American counterparts choose to start and stop water flow to provide hydronic zoning. Most find it just short of unbelievable that someone would choose this method of control. Perhaps they see it as akin to driving a car using a repetitive cycle of “peddle-to-the-metal” bursts of energy input followed by periods of coasting.
It doesn't have to be that way. As our European equivalents have long understood, constant circulation and zoning are not mutually exclusive. If you've never bothered to try constant circulation with zoning, I highly recommend you do so. Be sure to include outdoor reset control as you plan the system. Set the resulting system in motion and watch as heat is delivered smoothly, silently and precisely where it's needed. Just what you would expect from modern hydronics technology.