How Low Can You Go?
While such sounds may be a familiar — even reassuring — sound in some homes, they are very unwelcome in others, especially when loud enough to wake occupants in the middle of the night.
Noises from piping expansion, as well as swings in indoor temperature can be greatly reduced by using outdoor reset controls to regulate system water temperature. The basic idea is to provide constant water circulation through the distribution system while at the same time adjusting its temperature so heat output from the baseboards equals the building heating load. The graph in Figure 1 shows a representative reset line for such a system.
At design conditions, (which in this case are assumed to be -10 degrees F outside, and 70 degrees F inside), the water temperature supplied to the distribution system is 180 degrees F. As outdoor temperature increases, supply water temperature decreases along the line. Under minimal load conditions the water temperature supplied to the baseboards would (ideally) approach room temperature. The slope of the line in Figure 1 is called the “reset ratio” of the system. It’s determined by dividing the change in water temperature between two points on the line by the corresponding change in outdoor air temperature. For Figure 1 this would be:
Most reset controls have to be set for the reset ratio that’s appropriate for the type of heat emitters being used.
Limiting Factors: Systems that use a reset control to directly adjust the water temperature of a conventional gas- or oil-fired boiler should only lower water temperature part way down the reset line. The limiting factor is prevention of flue gas condensation inside the boiler and its flue. For a conventional gas-fired boiler this usually requires a minimum return water temperature in the range of 135–140 degrees F. If we assume the distribution system has a 10 degree F temperature drop under these conditions, the minimum supply temperature is somewhere around 150 degrees F. This corresponds to an outdoor temperature of about 12 degrees F for the reset line in Figure 1.
During milder weather the supply water temperature cannot be further lowered without risking condensation damage to the boiler. The distribution system must then resort to conventional on/off flow control to regulate heat output at the baseboards and thus prevent overheating. Under such conditions the boiler operates as if controlled by a simple high limit control set at 150 degrees F. Since outside temperatures above 12 degrees F represent a large portion of the heating season in many parts of the United States, the effectiveness of directly resetting boiler water temperature in such a system is limited.
The preferable way to use reset controls is for adjusting the supply water temperature in the distribution system, rather than the boiler water temperature. In some hydronic systems, these are the same thing. But take a look at the piping design shown in Figure 2.
Here the distribution loop and the boiler loop are separate piping circuits, each with its own circulator. The temperature of the distribution loop is controlled by regulating the flow of hot boiler water injected at point A. The greater the injection flow rate, the higher the temperature of the distribution loops. Connecting the boiler loop to the distribution loop is a variable speed injection system. Think of it as the “metering device” that determines the rate of heat transport between the two loops. The faster circulator P2 runs, the faster hot boiler water is pushed into the distribution loop and the greater the supply temperature to the baseboards becomes. The boiler loop serves to boost the return water temperature to the boiler and thus protects it from flue gas condensation and thermal shock.
Two piping details are critical in this type of system. First the tees connecting the injection mixing piping to both the boiler loop and distribution loop must be closely spaced to form “primary/secondary” connections. This helps prevent flow in the boiler loop, which may serve several other loads, from inducing hot water migration into the baseboard distribution loop when it doesn’t need heat.
The other critical detail is the vertical drop in the piping connecting the boiler loop and distribution loop. The minimum 1–foot drop creates a thermal trap that also guards against undesirable heat migration during no-load conditions. Also be sure to include some means of purging the distribution loop separately from the boiler loop. The primary/secondary connections make this necessary.
Making The Right Connections: There are two possible ways baseboards could be connected into the distribution loop — in series with each other, or in parallel.
Although series loops are the classic way of connecting baseboards to form piping circuits, a problem can develop when this approach is used in combination with reset controls. As water temperature decreases in response to rising outside temperature, the heat output capability of “downstream” baseboards drops significantly faster than the heating load itself.
This happens because the heat output from a baseboard is not strictly proportional to water temperature. The temperature drop along a series circuit in combination with reset control amplifies this effect. You can see this for yourself if you experiment with the baseboard sizing spreadsheet from last month’s column. For example, take a series circuit consisting of five baseboards that each supply a load of 8,000 Btu/hour. If you pick a design water temperature of 180 degrees F and a flow rate of 4.5 gpm for this circuit, the lengths of each successive baseboard are 16, 17, 18, 19 and 20 feet respectively. If you then experiment with decreasing water temperatures of 140 degrees F, 120 degrees F and even 100 degrees F you’ll see baseboard lengths increasing as expected. But the increase is not proportional for each baseboard. The farther downstream the baseboard is located in the circuit, the more “undersized” it becomes during part-load conditions. The bottom line: If you use series connected baseboards in combination with a “deep” reset of supply water temperature you may get complaints of underheated rooms near the end of the piping circuits during part-load conditions. The more baseboards the circuit contains, and the lower the reset control backs down supply water temperature, the more noticeable this effect can become.
Fortunately there is a cure for this problem: Pipe the baseboards in parallel as shown in Figure 3. In this case each baseboard gets the same supply water temperature from the supply manifold. The temperature drop effect created by the series piping configuration is no longer a factor. This piping approach also offers many design options. For example, the same type of manifold used in floor heating systems could be used to supply individual baseboards. The tubing runs to the baseboard could use PEX or aluminum/PEX tubing both of which are easily routed through the building’s structure. Such a system can also be zoned on a room-by-room basis by installing electrical valve operators on the manifold, or non-electric thermostatic valves at each baseboard.
If the distribution loop operates with continuous circulation, (which is preferable when elimination of piping expansion noise is important), the circulator must be protected against “dead heading.” If all baseboards are equipped with thermostatic control valves a differential pressure bypass valve should be installed across the circulator. Another alternative is to omit zone controls on at least one of the baseboards and thus ensure there’s always an open flow path through the distribution system.
By now you may be wondering if all these details are worth bothering with. Ask your customers these questions before you decide:
Do they expect a system that’s free of piping expansion noise? Do they expect a system that yields virtually no swing in indoor temperature? Does the house have rooms with the potential for wide variations in internal heat gains from sunlight, fireplaces, cooking, etc. If they answer yes, this is a system than can deliver the goods.