What's an ideal way to heat a 300-ft. by 500-ft. slab-on-grade industrial building with 20- to 30-ft. high ceilings, and a design load of 3 million Btu/hr.?

It's hard to imagine an avid reader of PM who wouldn't think of hydronic floor heating when presented with such a question. Tubing buried in the slab is well protected from industrial traffic above. Loading dock areas remain comfortable despite frequent opening of overhead doors. Because the air is not heated to high temperatures it doesn't stratify into the high ceiling bays. Worker comfort is maintained at lower air temperatures, which translates into substantial fuel savings. No other system could lay claim to such benefits.

The Building

Such a building is now nearing completion near Oriskany, N.Y. Soon it will serve as the new home of Bonide Products Inc., manufacturers of lawn and garden products.

Like many industrial buildings some floor space will be used to inventory both raw materials and finished products, other areas will be used for production lines, and still others for shipping and receiving. Several office and support facilities are also housed in this huge facility.

The building plans initially called for overhead unit heaters. But through the combined efforts of Jeff Haggas of New Hartford Plumbing Supply and Tom Kane of rep agency J. R. Baker Associates, the building designer was convinced that floor heating offered far more benefits.

Once the decision was made to go with floor heating, a number of options were considered. One involved installing between 16 and 20 manifolds along each of the 300-ft. end walls, with tubing circuits running about 250 feet straight out to the center of the building and then straight back to the manifolds. Although this would have made the tubing layout relatively easy, it didn't address the differing heating needs of the perimeter vs. core areas of the building.

Because of the location of the production equipment, significant internal heat gains were anticipated near the center of the building. By contrast, the perimeter areas, with their large ventilation louvers and overhead doors, would experience loading much more influenced by outside conditions. Such situations call for a system that can be zoned to react to the "prevailing" conditions in different parts of the building.

As a result, the plant floor area was broken up into 14 strategic areas, including perimeter alleys, inventory storage areas, production areas, and specialty areas including the maintenance shop and drum storage room. Tubing circuits were routed to a separate manifold location in each of these areas. The goal was to provide for separate zone control in each area as loads dictated.

Minitube System

: Once the manifold stations were located, attention turned to how best to distribute heat to them. Given their placement it quickly became evident that a standard two-pipe distribution system would involve thousands of feet of relatively large (from 2 inches up to 6 inches) piping.

One option was to connect each manifold as a secondary circuit with its own circulator to a crossover bridge between the supply and return main above each manifold. This approach would require extensive balancing, not to mention a large pump back in the mechanical room. Furthermore, it didn't allow for the various manifolds to operate at different water temperatures.

In the end, the building team opted for a minitube system. It's a variation of injection mixing using a variable speed pump. Mixing is done at each of the manifold stations rather than in the mechanical room. This approach "exploits" the large temperature differential available between hot boiler water (180 degrees F to 200 degrees F at design conditions), and the relatively cool water returning from the floor circuits (90 degrees F to 95 degrees F at design conditions). It also allows the possibility of operating each manifold station at its own water temperature. Figure 1 shows typical piping for a minitube zone.

To avoid a hodgepodge of thermostat settings, we decided to centralize all temperature settings for the plant floor areas to a control panel in the mechanical room. Each zone provides feedback to its mixing control via a thermistor-type indoor temperature sensor. If, for example, the air temperature in the loading dock area begins to drop, the injection control serving that manifold station can respond by increasing the flow rate of hot water to that manifold, or if necessary shifting the reset curve to counter the increased load.

Hot water is individually "metered" through each supply minitube by a small wet-rotor injection pump operated by a variable speed control. The largest injection pump in the system is 1/12 horsepower. The largest minitube is 1.25-inch copper. Most are 1 inch.

Each injection circuit is coupled to a crossover bridge between the boiler supply and return mains using a pair of closely spaced tees. The resulting piping assembly resembles a large horizontal ladder. An illustration and photo are shown in Figures 2a and 2b.

This piping arrangement ensures the following:

  • Each injection pump has water available at the same (high) temperature.

  • Thanks to the closely spaced tees, each injection pump is "hydraulically uncoupled" from the pressure distribution in the mains.
In effect, the mains and crossover piping become a "bus bar" for hot water. Each injection pump draws from it as needed without interference from any other pumps that may be operating.

The vertical drop in the supply pipe upstream of each injection pump, combined with the swing check on the return pipe minimize any migration of hot water into the minitubes when the associated injection pump is off.

The 6-inch supply mains were sized and located so that future expansion of the system is possible.

Small But Powerful

The boiler plant consists of four, gas-fired copper tube boilers, rated at 750,000 Btu/hr. each, yet having a footprint of only 3 ft. by 3 ft. The boilers are designed for sealed combustion and lined up and directly vented through the exterior wall of the mechanical room. They're connected to the primary loop as individual secondary circuits. When a boiler is fired it's circulator is turned on bringing it online to the 6-inch primary loop. Boilers that are not fired remain off-line to minimize heat loss.

The boilers are controlled by a four-stage multiple boiler control that adjusts (e.g. resets) the primary loop temperature based on outside temperature. The control also rotates the firing order of the boilers to provide approximately equal firing time.

The building's heated floor slab contains about 2,900 cubic yards of concrete. Warming that slab by 30 degrees F takes as much heat as an average 2,000-sq. ft. house in a cold climate uses during an entire winter!

When the slab is cool the water temperature returning to the manifolds - and eventually to the boilers - is well below the dewpoint of the exhaust gases. Protecting the boilers against sustained flue gas condensation is a must. It's accomplished through the individual injection controls. Each has a return temperature sensor and is programmed to slow its associated injection pump whenever boiler return temperature drops into the range of 130 degrees F. This prevents the cool slab from drinking up heat faster than the boiler plant can produce it.


In total the Bonide building contains 237 circuits of 3/4-inch barrier PEX tubing totalling almost exactly 21 miles in length. Each circuit was drawn and measured on a CAD system. (I'd show you the tubing layout drawing except it would look like a solid blob of ink if scaled to fit in the magazine.)

Each circuit was then assigned a letter based on its manifold station as well as a circuit number. After all lengths were determined, a spreadsheet program was used to assemble combinations of length that totaled as close as possible to 1,000 feet (a stock coil length). Once these combinations were established, it was essential they stay organized. Each coil was numbered and tagged with the circuits that would be cut from it. This tag stayed with the coil until it was used up.

Twelve-inch lengths of thin-wall corrugated polyethylene sleeving were installed over the tubing at all locations of sawn control joints. All tubing was secured to the welded wire reinforcing using nylon pull ties.

Needless to say there were several "person-weeks" expended installing the tubing. The drawings and cutting list saved what could have been a lot more time in trial and error routing and avoided what could have been costly on-site errors. In all likelihood, it prevented the creation of thousands of feet of leftover tubing.

Manifold Stations: Each minitube manifold station provides several functions including:

  • Floor circulation
  • Mixing
  • Circuit isolation and balancing
  • Heat flow monitoring
  • Electrical servicing
A diagram of the concept used is shown in Figure 3a. A photo of a partially completed manifold station is shown in Figure 3b.

Each manifold station uses pre-built 2-inch copper headers with 3/4-inch copper take-offs spaced 3 inches o.c. The length of the headers varies to match the number of circuits served. The smallest manifold serves eight circuits, the largest 24 circuits.

Each take-off is equipped with a mini-ball valve that in turn connects directly to the 3/4-inch PEX tubing. The supply end of each circuit is equipped with a standard mini-ball valve. The return side uses a combination ball/balancing valve. These valves allow any circuit to be isolated if punctured in the future. They also allow for individual circuit balancing if needed. However, variations in circuit length were held to a minimum during design, and individual circuit balancing is not expected to be much of a concern.

Heat input to each manifold station can be monitored by measuring the _T between the supply and return minitubes, and the flow rate across the pressure-tapped balancing valve in the return minitube.

Purging can be done in a number of ways: Make-up water can be fed to the manifold station through the supply minitube (flow in the return minitube is blocked by a swing check). Such flow should be sufficient to purge the floor circuits one at a time. If not, the manifold stations are equipped with valves that allow a high capacity purge pump to be directly attached.

Fused disconnects at each manifold station also allow each circulator to be isolated for servicing if necessary.

Eventually each manifold station will be enclosed by a housing protecting it from routine plant traffic and tampering.

Office Areas

The Bonide building also houses a number of offices, a cafeteria and locker rooms. The offices are heated by individual tubing circuits that can be individually balanced as needed. The same 3/4-inch PEX was used to keep scrap and hardware variations to a minimum.

Several offices along the exposed perimeter were grouped together as a single zone. They all have similar load influences from solar gains and envelope losses. Likewise several of the interior offices are grouped together as a single zone. Such grouping prevents conflicts between heating in perimeter areas and cooling in interior areas during swing seasons. Using individual tubing circuits to each office also allows the possibility of installing individual room thermostats and associated valve actuators if desired in the future.

As this article is going to press, water is being added to the system. Considering there's about 3,000 gallons in total even filling and purging will take some time. I expect it will take a bit more time to verify proper operation of all parts of the system. But when the system is fully commissioned, and Bonide employees finally arrive, they'll be greeted by comfort far superior to that of the average industrial environment.