Load diversity allows a heat source to be reduced with respect to peak load.

It seems creative hydronicians are always coming up with new things to heat. Some of the "unique" heating loads I've come across lately include:

  • Melting snow off roofs and sun decks.

  • Warming the contents of "18-wheeler" tank trailers when stationary.

  • Heating deck areas around swimming pools.

  • Soil heating in greenhouses to get a jump on the growing season.

uch unconventional loads are often served by the same boiler that provides space heating and domestic hot water to the associated building.

Calculated Vs. Colossal

One approach to sizing the boiler for a multiload system is to add up the Btu/hr. requirement of all loads (assuming they'll all operate simultaneously), add a generous safety factor to that number and then pick the next bigger boiler from the catalog. This approach appeals to those who would purchase a turbo-charged V8 4x4 pick up so they can proudly haul some 2x4s home when it's time to build a doghouse.

A more refined approach would examine the probable operating schedule of each load, as well as the absolute necessity of its operation at any hour of the day. With the possible exception of recovering a building from a deep temperature setback, it's rare that all heat loads would be on at the same time. This concept is called "load diversity." When recognized and accommodated, it allows the size of the heat source to be reduced with respect to peak load. It also increases the seasonal efficiency of boilers. The comfort provided by a properly planned and controlled system with load management seldom deviates noticeably from that provided by one with a grossly oversized boiler. Its owners will save both in installation cost and reduced fuel usage over the life of the system.

Figure 1 shows a representative "load profile" for a residential system supplying space heating, garage heating and domestic hot water. This graph shows the expected rate of heat delivery (in Btu/hr.) to each load for each hour of a design day (maximum space heating demand). Since the units on the vertical axis are Btu/hr., and the units on the horizontal axis are hours, the shaded area under the green curve will have units of Btu/hr. times hr., or simply Btu. Specifically the total Btus required by all loads over the 24-hour period.


    1.Space heating load = 80,000 Btu/hr. at 70 degrees F inside and -10 degrees F outside.

    2.Garage heating load = 18,000 Btu/hr. at 60 degrees F inside & -10 degrees F outside.

    3.Design heating day has minimum outside temperature of -10 degrees F with 20 degrees F swing. Minimum outside temperature occurs at 5 a.m.

    4.Domestic hot water based on ASHRAE high-morning user profile with a total of 120 gallons per day of water heated from 50 degrees F to 125 degrees F delivery temperature.

The peaks and valleys in the load profile result from variations in each load over the 24-hour period. The average demand on the heat source is found by calculating the area between the green curve and the bottom of the graph and then dividing it by 24 hours. In this case it's about 88,500 Btu/hr. as shown by the horizontal red line. This means that a boiler producing 88,500 Btu/hr. running continuously over the 24-hour period shown, would yield the same total heat output as required by all the loads over that same period. The average load of 88,500 Btu/hr. is about 17 percent less than the total peak demand, and that's not including any oversizing factor tacked on for good measure or through sheer guestimating.

Robin Hood Hydronics

Shifting loads to match available boiler capacity requires assigning priorities and then enforcing those priorities with system controls. Surplus boiler capacity is used to make up for heat deficit periods as shown, for the design loads assumed, in Figure 1. If the shed load has a high thermal mass (like a heated floor slab), interrupting heat input - even for three or four hours - may not even be noticeable.

I've listed loads I believe can be assigned lower priority in a multiload system. They are not listed in any particular order. Read the explanations and think about some of your own projects that included similar loads. You may feel differently about the priority of satisfying these loads. That's fine. The point is that they all represent potential "sheddable" load when the system's heat source can't quite keep up. The conditions and sequence under which one or more loads drop off-line would be determined by discussing expectations with the owner. Controls can take it from there:

  • Swimming pools: A 20,000-gallon pool can accept 166,000 Btus of heat with only a 1 degree F rise in temperature. That is roughly equivalent to the thermal mass of a 4-inch concrete slab 17,000 sq. ft. in area. Because they have tremendous thermal mass, swimming pools can accept widely spaced "pulses" of heat when it's available during excess capacity periods with very minor changes in temperature. Swimmers will also tolerate minor variations in water temperature (for example from 75 to 80 degrees F).

  • Garages: I've yet to hear a car complain when the garage temperature drops a few degrees. If slab-on-grade radiant floor heating is used its large thermal mass allows the garage to "coast" for a few hours of heat interruption with only a minor drop in temperature. If the garage is routinely occupied - as a workshop for example - its priority may have to be higher.

  • Second floor bedrooms: One thing I've noticed over many projects is that air temperatures in (non-heated) second floor rooms are often only about 5 degrees cooler than on the heated first floor. Buildings with open floor plans, loft-type second floors, and well-insulated thermal envelopes minimize internal temperature variations. Combine this with the fact that many people don't often want their bedrooms kept warm at all times, especially during sleeping hours, and you've got another load that be temporarily shed if necessary to shuttle heat elsewhere.

  • Selected areas of snowmelting: Snowmelting in parking areas is usually less critical than in traffic lanes. If these areas are separately circuited the parking area load could be temporarily shed during peak demand periods. Melting snow from a roof for purely aesthetic reasons is also a pretty low priority in my book.

  • Infrequently occupied basement areas: If slab-type floor heating is used, these areas can be shed when total load demands it. Changes in temperature will be minimal if indeed anyone is even present to sense them.

Don't Mess With Me!

Ask most Americans if they would rather occasionally run low on domestic hot water or undergo a root canal at the dentist, and you might be surprised at the answers. Most of us put a high priority on readily available domestic hot water and quickly lose patience when our mechanical system can't deliver it. Someone who just spent $30,000 on a luxurious bathroom isn't amused to discover their new faucet can fill the family-size tub in 10 minutes - unless of course they want it filled with hot water - which takes about an hour.

As hydronic professionals, we must recognize such situations both from a technical and marketing standpoint. It's a great opportunity to employ a key hydronic strategy called priority domestic water heating.

The technical concept behind priority DHW is simple: To provide the fastest DHW recovery, all other loads are temporarily turned off whenever the DHW tank calls for heat. Several commercially available multiload relay centers and microprocessor-based controllers have DHW priority capability. It also can be achieved using a field-wired relay. The basic wiring concept is shown in Figure 2.

A potential drawback of priority DHW is that a control failure could lock the system into a perpetual domestic water heating mode. Suppose, for example, that a strap-on temperature sensor was used to provide feedback to the DHW setpoint control, and that the sensor either fell off or was accidentally pulled from its mounting location. The control would have no way of knowing when to terminate the DHW heating mode. As the P&T valve is filling the mechanical room with steam, water pipes in other parts of the building could literally be freezing from the lack of space heating. A hot water faucet accidentally left open might also create an insatiable DHW load - again preventing operation of space heating loads.

The most common approach to circumvent this problem is to limit the time period during which priority domestic water heating can take place. A time delay relay is the key component. The wiring of a typical 120 VAC time delay relay and its 24 VAC counterpart are shown in Figure 3.

The timing circuit in the time delay relay is powered up at the beginning of each DHW heating cycle. Its normally open contacts will not close, however, until after its set time delay period has expired. I usually set the time delay for one hour. If the DHW load is still calling for heat after this period has elapsed, the normally open contact (TD-1 in Figure 3) closes to re-enable space heating in combination with the DHW load. A repeating cycle consisting of one hour of exclusive domestic water heating followed by an hour of combined load operation can also be programmed into some time delay relays.

Dumping Decisions

If the temperature of the system's primary loop is dropping while one or more loads are operating, those loads are removing heat faster than the heat source is replacing it. Load shedding logic can be based solely on primary loop temperature. For example, if a setpoint control was used to monitor primary loop temperature, it could disable a specific secondary circuit whenever primary loop temperature is below 150 degrees F. (Its normally closed contacts would open at 150 degrees F and close again at, say, 160 degrees F.) Other setpoints and differentials are also possible. This control action would accomplish the following:

  • Prevent the secondary load from operating until the primary loop temperature (at the sensor location) is at or above the setpoint plus differential (160 degrees F in this example). This allows faster boiler warm up following a cold start minimizing the duration of flue gas condensation.

  • Either decrease the rate of temperature drop or allow the primary loop temperature to start climbing depending on the Btu/hr. demand of the shed load.

If primary loop temperature continues to drop, additional loads could be shed at specific temperatures. You decide at what temperatures and in what sequence these loads drop off-line. Keep in mind, however, that a conventional gas- or oil-fired boiler should not be held in sustained condensing mode operation (return water temperatures below approximately 130 degrees F). This is likely if a system allows simultaneous start-up of several high-mass loads from "cold-start" conditions. Use of the staggered setpoint temperature priority scheme described above can prevent this.

In July I'll continue this discussion of priority control. In the meantime, remember that even a properly sized boiler seldom operates continuously - even on design days. Those off-periods represent opportunities to shuttle heat to additional loads without affecting comfort. It's often a win/win strategy to "manage" rather than "man-handle" (oops ... I mean person-handle) those Btus.