When trying to diagnose a system it is frequently important to determine whether the pumps are working in your solar heating system. The first (and least accurate) method most contractors go with is feeling the pump to see whether they are vibrating. This can be misleading because vibration from nearby mechanical equipment can cause you to assume the pump is spinning when it might not be. A very easy and accurate way to determine whether the pump is spinning is to remove the bleed screw from the top of the pump (see video). When you remove the bleed screw (not available on Taco pumps) you will see an inner slot on the end of the shaft. You can only see this when the pump is not spinning. Since the slot is directly connected to the shaft and impeller, if the slot is rotating you know the pump motor is spinning. Turn the power off and you should see the slot.
By removing the bleed screw and accessing the head of the pump you can also spin the shaft freely with a slotted screwdriver. You should be able to feel if the shaft is binding in any way.
This is the time of the year when the calls start rolling in. “I have a single tank solar water heating system and the water heater has stopped working.” This is not a service call you should need to run out on.
Using a standard electric water heater for both your solar storage as well as providing your back-up heating has many advantages including; reduced floor space, ability to use standard tanks, less standby losses from the tank, better heat recovery, and less cost. But now we come to the rub. Electric water heaters have a thermostatic breaker built into the top element. When the thermostat senses temperatures over 170° F it automatically trips this safety breaker. This breaker is designed as extra protection in case you have a run-away element in your tank to keep the system from getting too hot.
Frequently a solar water heating system will supply 100% of your hot water needs during May through September. If at any time during that period the top of your tank exceeded 170° F the tanks thermostatic breaker will trip. As the cooler and cloudier weather sets in the back up element is no longer able to kick in and you receive a service call. Fortunately, the fix for this problem is simple and within the reach of even the least proficient home-owner.
Simply remove the cover plate over the upper element, depress the red button with the eraser of a pencil and voila you are back and running again. (see picture)
Water Heater Reset Button
So next time you get this call don’t fret and think about the little red dot.
In the last blog I mentioned calibrating the customers expectations by providing them with a simulation showing what they can expect from their system. Occasionally, a customer will not realize the savings they should and they expect you to fix it. Under the right circumstances it is possible to install a solar heating system and see only a small fraction of the savings that should be delivered…
One such situation is when the customer uses a solar tank to pre-feed an indirect gas or oil fired tank that operates as the back-up to their system. When using a boiler fed tank as the back-up, it is possible to intermittently not achieve the desired temperatures in the indirect tank thus requiring the boiler to cycle on. If this is in the summer time when the boiler is only used to service hot water (and not heat the house) this means that all of the losses and waste associated with starting up and shutting the boiler down is still there. How can we eliminate (or at least minimize) the intermittent cycling of the boiler when the solar tank is already hot?
One way to reduce the boiler cycling is to tie the solar tank together with the indirect tank in such a way that any excess heat in the solar tank is shared with the back-up tank thereby reducing the demand on the boiler. This can be accomplished simply with an extra connection between the tanks, a pump, a check valve, and a differential control. When the solar heated tank exceeds the temperature in the back-up (indirect) tank the circulator turns on and causes the two tanks to be mixed together in such a way that the heated solar water moves into the back-up tank. By tying the tanks together in this manner you have accomplished several things including: reduced the firing of your boiler increased the overall efficiency of the solar collectors (by keeping the tanks at a lower overall temperature) significantly reduced the customers overall energy bill in the summer time. made the solar system more robust and resistant to overheating (glycol systems)
While this technique will not hurt in any way it’s greatest benefit will be seen during the summer when the solar system is capable of producing the extra energy. It is possible to totally eliminate the customer fuel oil usage in the summertime with this scenario.
With various types of plumbing systems whether solar heating systems, hydronic heating systems or just regular home plumbing it is common for gate type drain valves to leak (see picture 1).
Picture 1
The valve will often leak when it gets old or if something gets caught on the seal. Most people will just leave a small drip and not worry about it. If you have a machined cap for the drain you can install that and be done. Although getting a metal cap can sometimes be difficult and they are never right at hand. An easy solution if you have access to a plastic bottle of soda is to take the cap from the soda bottle (I know pepsi works) and screw that onto the drain. Voila a quick and easy solution to that leaking boiler drain, hose bibb, washing machine connection or drain on your solar heating system. (see picture 2)
A question we will get from time to time is “how do I know that my solar pump is running?” There are several answers to this question so we will cover one way to know if the pump is operating properly. When you look at it all a pump does is create a pressure differential across itself. The pump (if working properly) will have lower pressure before the pump and higher pressure after the pump. This pressure differential causes fluid to flow away from the higher pressure region towards the lower pressure region. On a closed loop glycol system (or other hydronic systems) you should have the following components in addition to the pump: an expansion tank and a pressure gauge. A properly designed system will place the expansion tank immediately prior to the inlet to the pump. The expansion tank serves as the zero pressure change point of the hydraulic loop. Since the expansion tank doesn’t see pressure change as flow is generated the only way for the pump to do its job (creating a pressure differential) is for the pump to create an increase in pressure on the outlet side of the pump.
This pressure increase on the outlet side of the pump can be easily observed by watching the pressure gauge when the pump is turned on and off. Watch this video to get a better sense of what you are looking for. The higher the head of the pump the larger the pressure spike you will see when the pump is turned on.
Problem: A customer has a drainback system that turns on properly when the differential is achieved. Once the system turns on the pump starts pumping and then shortly thereafter the flow can be heard dropping into the drainback tank. Everything is working according to design. A short while later (5 to 10 minutes) while the pump is still running the system ceases pumping over. When the system originally started the site glass was at the top of the site glass. After the water started falling back into the drainback tank the water level in the site glass was down about 6 inches from the full level. After the system ceased pumping around the water level in the site glass was now 3 inches below the full level. This situation repeated any time the system turned on.
The installer, thinking there was a problem with the pump, replaced the pump. No change. The installer then added another pump in series to address the problem. The system operated identically except it “lost prime” faster than with a single pump. What was the problem?
Answer: The particular drainback tank that the installer was using had the return from the collectors coming straight into the top of the tank immediately above the line leaving the drainback tank going to the heat exchanger and then back to the collector. After the pump started running the fluid coming from the collectors picked up air as it splashed in the drainback tank. Enough of this air flowed out of the drainback tank and ultimately collected in the pump. With a small amount of air in the pump body the pump was no longer able to generate enough lift to get the water past the highest point in the system and the prime was broken. The solution to the problem was to reduce the flow out of the pump by partially closing the ball valve on the exit side of the pump. By doing this the volume of flow going through the drainback tank was reduced. This allowed enough of the air to come out of solution to prevent the pump from air locking.
Contractor leaves the job site satisfied that they have done a good job installing a solar hot water system only to be called back 2-4 weeks later to be told that the system doesn’t seem to be heating the customer’s tank. The homeowner has been keeping a detailed log of the temperatures in the tank as well as on the roof and shows you a log that never shows the tank
sensor getting hotter than 105 degrees. They say the system has been running and they haven’t noticed anything else unusual but clearly the system isn’t working properly with the solar only getting the tank this hot. What went wrong?
A modern differential control operates by measuring the temperature (via a temperature probe) in the bottom of the tank (T2) and the solar collector temperature at the outlet of the collector array (T1). Hot water tanks are constructed so they introduce any cold water into the bottom of the tank primarily via a dip tube that carries the cold water from the connection on
the top of the tank to the bottom of the tank. The Steca 0301U differential control (which represents greater than 50% of the control sales in the U.S.) only comes equipped with two sensors although it can take a third. In this case what the homeowner is seeing as the tank temperature is actually the temperature at the bottom of the tank (T2) where any cold water entering the tank is mixed with the solar heated water in the tank. The stratification in the tank allows the homeowner to have access to hot water (120 degrees +) at the top of the tank while only seeing “cold” water temperatures on the readout for the bottom of the tank.
solution: Supply the homeowner with a third temperature sensor that would then be mounted to the top of the tank showing what temperature the tank is actually delivering.
