When selecting water-to-water heat pumps for slab heating, consideration should be given to using a number of units rather than a single unit. Multiple units provide the opportunity of staging the running of the units, as well as some redundancy, as units in the two to five-ton range are easier and less costly to repair in the event of a failure and manufacturers offer longer and extended warranties on small compressors. However, pump and piping circuits have to be carefully designed when using multiple water-to-water heat pumps and ground source water. Pipe sizes and lengths depend on ground conditions, the designs of which are offered by various companies. The layout of heat and water pumps doesnt change much regardless of the number of units used or their size. The configuration of the system is generally the same for most applications. (See Fig. 1) The drawing shows five pumps and four five-ton heat pumps piped to a dual ground loop system. This arrangement provides 50 percent capacity reduction of pumps P2 and P3 and up to four stages of heat pump compressor operation. P1 provides a constant load side water flow through the loop differential bypass valve. The WRVs control the load side water flow to match the requirements of each heat pump for cold startups and source water temperature variation.
Constant discharge pressure
Head pressure water regulators (WRV) insure that the compressors run at a constant discharge pressure and temperature, regardless of the temperature of the ground source water. Ground source water in the system can go below freezing and requires antifreeze. Pumps P2 and P3 have to be sized for the pressure drop and gpm requirements of the ground source loop. Pump P1 has to be sized for the gpm and flow pressure required for the heat pump heat exchangers, water flow regulators and piping losses. The injection pump (P5) has to be sized for the same gpm as P1 however and doesnt require a lot of head pressure.
Staging controls, such as the Tekmar Boiler control D268, can be programmed to operate the heat pumps and pumps, as well to provide lead/lag operation.
Essential components
Fig. 2 shows other essential components such as expansion tanks, air separators with auto vent, hose bib valves for flushing, fill standpipe with side (clear vinyl hose) and general service valve requirements. These are best as hand filled systems, rather than pressurized by city water. Pressurizing only aggravates water flow if there is air in the system. Pressurizing the ground loop can be in the form of an antifreeze injector pump and pressurizer. Getting the initial air out of the system can be challenging as it will end up anywhere there is a trap in the line. A purge cart with a high head pump (1 1/2 to 3 hp) should be used with enough water flow to assure a four-foot-per-second flow rate through each circuit in the loop. This will ensure that all air is removed. By monitoring the flow through the purge drum you can visibly see when all air is removed. The purge cart is also used to introduce antifreeze to the loop and to do final loop pressurization prior to start up. Flow rates are generally four to six feet per minute in mains. This is high enough to force air through regular pipe, but when air passes through parallel heat exchangers the velocity will be slower and air can hang up and cause uneven fluid flow, especially when there are multiple units on a single pump. Regular air vents don't work without an air separator ahead of them the water velocity is usually too fast. Hose bib valves on the inlet and outlet of each heat pump provide the means of pushing air through heat exchangers. Air hang can be a problem in heat exchangers, especially with trapped coaxial heat exchangers...
