The most common pump in the hydronic market today is the wet rotor circulating pump. The pump also goes by the name circ pump. They are made by various manufacturers including Armstrong, Grundfos, Wilo, Taco, and Bell and Gossett. The pumps are common and fairly inexpensive. Even though they are relatively inexpensive having to return to the installation to replace one within a few years can drive up the cost of the pump far beyond the initial purchase price.
The basic design principle behind a wet rotor pump is that the rotor and shaft are enclosed in a stainless steel can. The fluid that is pumped has access to the inside of the can including the rotor, shaft, front and rear bearings. Different manufacturers accomplish this different ways but primarily through holes in the face of the can and by the clearance around the shaft. By allowing the pumped fluid access to this area the fluid acts as both a lubricant as well as a coolant for the heat generated while the pump spins (see Figure 1). The problem arises when fluid doesn’t displace all of the air that is initially occupying the empty space in the can. If air is left in the can then you end up running the pump with insufficient lubricant and insufficient coolant. The double whammy leads to premature wear on the bearings as well as superheated fluid/steam in the can. If the fluid that you are pumping has any minerals in it they will come out of solution under the extreme heat and end up precipitating on the bearing which in turn leads the bearing and shaft to lock up. This problem is exacerbated in solar heating applications because of the naturally high temperatures that are present. A drainback solar heating system would be the worst case scenario for this type of premature pump failure since you are frequently dealing with unpressurized loops. When a pump is installed and the fluid is under pressure the higher pressure of the fluid compresses the air in the can significantly during start up. This fluid being forced into the can helps to flush out the residual air in the can when the impeller starts to spin. If you don’t have any pressure (or very little pressure in a drainback situation) you don’t enjoy the benefit of forcing water into the pump.
There are several steps that can be taken during the original installation of the system to minimize or eliminate this problem. They are:
- Pressurize the pumps during the charging/priming of the system. This includes “non-pressurized” drainback systems. As the installer you should still prime the system to 15 psi of air pressure minimum to insure that you get as much water into the can as possible initially.
- Using demineralized water when you charge a drainback solar system. While you can use tap water, you want to make sure that you don’t have high mineral content in your water that can cause you problems.
- Once you have pressurized the system be sure to vent your pumps to flush out any residual air. This serves to insure that the pump can is fully primed.
- Keep the high limit temperature on your solar system lower than 150 degrees. Historically solar heating people have tried to get the tank as hot as possible. While this is good for putting BTUs into the tank, this approach puts the system at greater risk of failure from hard water in the pumps. The European solar differential controls come with a pre-set high limit of either 140 or 149. Except under extreme circumstances I don’t see a reason to make it any higher.
Next time you install a solar water heating system be sure to take these simple steps to give yourself and your customer the most robust system possible.