ASHRAE Journal - February 2012 - 40

for heat in the system. It knows that the primary loop is hotter than the DHW tank. So why not run the primary loop pump and the DHW heat exchanger pump to pull those 2,000 Btus (2.11 MJ) out of the primary loop and put them into the DHW tank? Figure 3b shows just that: over a period of 10 minutes, the primary loop is cooled and the DHW tank temperature is raised by 5°F (2.8°C). To do this type of integrated, multi-level logic with differential controllers and relays would be nigh impossible. We estimate that just this one algorithm can save a homeowner $50 to $100 per year in fuel costs for a home using fuel oil or propane for backup heating.

180

4

DHW Top Temperature 3 Temperature (°F) 120 ac Current (A)

2 Primary Loop Temperature Boiler on for DHW DHW Pump is On 0 8:04 p.m. 10:04 p.m. 12:03 a.m. 0 2:03 a.m. Boiler Quits at 110°F 1

60

Failure Diagnostics and Alerts

With signiﬁcant amounts of data from around the system coming into the control computer, it is easy Figure 4: A boiler failure (safety lockout) is the cause of a lack of DHW. to set up alerts and diagnose a problem when something goes wrong. Over the cumulative system time 200 of eight years, our systems have experienced valve 180 failures, pump failures and a boiler lockout from waSolar Collectors 160 ter leaking onto the burner. Valve failures are identiﬁed by knowing when a 140 valve is asked to open and then knowing that an end DHW Tank 120 switch signal has not been received within a ﬁxed time. There are two possibilities: either the valve is not 100 responding or the end switch is not working. Because 80 we can control every component in the system over the Internet, when we suspect that a valve is malfunction60 Pump On ing, we log on to the system remotely, ask the valve to 40 open and send heat to that zone by turning on the appropriate pumps and the boiler. This last aspect allows 20 Pump Off DHW Heat Exchanger Pump us to test which of the two possible failures we are 0 dealing with. If the valve is indeed opening but the end 12:02 a.m. 6:02 a.m. 12:02 p.m. 6:02 p.m. 12:02 a.m. 6:02 a.m. switch is not reacting, then we will see a temperature rise in that manifold even though the end switch is tell- Figure 5: In this home, the DHW recirculator is on continuously and maning us that the valve is closed. ages to waste all of the solar heat stored in the DHW tank within six Pump failures are identiﬁed in a similar fashion. hours after sunset. One opens a zone valve of choice and turns on the pump in question along with the boiler and looks for a tem- temperature relief valve had leaked water into the boiler perature change on an appropriate thermistor. and caused a boiler safety lockout. In this case, no service We were alerted to another failure by the homeowner that call was required. The owner was able to replace the faulty although the system was clearly calling for the boiler to be P&T valve himself and powering the boiler on and off reset making DHW, in fact, the water temperature in the DHW the lockout. tank was not going up. Figure 4 from the log ﬁles shows clearly what happened. Shortly after midnight the boiler, Performance Analysis the primary loop pump and the DHW heat exchanger pump Having a history of more than 250 data values taken evwere turned on for making DHW. The primary loop tem- ery ﬁve minutes allows for a performance review of the perature goes up to 110°F (43°C) indicating that every- system on a routine basis. Figure 5 shows how this analything is normal but then plateaus at 110°F (43°C). After sis informed a user decision and saved signiﬁcant money on that, the DHW temperature is going down slowly as the pri- a system near Santa Fe.3 The chart shows one day’s cycle mary loop temperature is going up slowly; they are in fact of solar heat into the DHW tank and what happens to that coming to equilibrium because the boiler is no longer on. heat overnight. During the day (summertime) solar heat is When the homeowner went to the mechanical room after put into the DHW tank, heating it up to 155°F (68°C) by the our over-the-Internet analysis, he found that a pressure and end of the afternoon; The DHW heat exchanger pump is on
40 ASHRAE Journal
Temperature (°F)

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February 2012
ASHRAE Journal - February 2012

Table of Contents for the Digital Edition of ASHRAE Journal - February 2012
