Part IV: Solar Water HeatingChapter 7. Active Domestic Hot-Water Systems
Commissioning and Measuring Performance
A first-week checklist: flow, temperatures, leaks, and a simple way to estimate the solar fraction.
- Difficulty
- Intermediate
- Cost
- Free
Commissioning is the day you find out whether the loop you built is the loop you drew. You are looking for leaks, backward flow, a sensor on the wrong pipe, a drain that does not drain, and a mixing valve that does not mix. The performance number comes after those are true.
Heat in a water stream is about 500 times the flow in gallons per minute, times the temperature rise in degrees F, per hour. That 500 is just the weight of a gallon and the minutes in an hour, rounded. Measure flow and two temperatures and you can say whether the collector did anything. One sunny afternoon is a snapshot. The year's solar share is lower, because winter and clouds are part of the year. Do not advertise the snapshot as the annual fraction.
Difficulty
- Level
- Intermediate
- Time
- A careful half day, then a week of notes
- Skills
- Reading thermometers and believing a small number
Cost estimate
Free
A couple of thermometers and a way to read flow. A hose-bibb meter or the flow meter you already installed is enough. The calculator runs in the browser.
Materials
| Item | Qty | Notes |
|---|---|---|
| Two thermometers | 1 pair | Clamp-on or wells. An infrared gun on a shiny pipe lies. If you use one, put a piece of tape on the pipe and shoot the tape. |
| A flow reading | 1 | A meter in the loop, or the pump curve at a head you have actually calculated. A bucket test only if you can divert without scalding anyone. |
| A notebook | 1 | Time, sun, flow, inlet, outlet, tank temperature. The memory of a good day will inflate. |
The checks before the number
Walk the collector loop with the pump off. On a drainback, confirm the collector is empty, using the test on the drainback page. On a closed loop, confirm pressure is where the fill procedure left it and that the relief valve is not dripping. A relief valve that weeps every sunny day means the potable side has nowhere to expand.
Start the pump. Feel for flow in the right direction. A pump that runs backward or that is air-locked will spin and move little heat. Bleed air at the points the design gave you. A drainback should fill and then run quietly. A hammering loop is a slope or a velocity problem. See pipe sizing.
Read the controller sensors and a thermometer on the same pipes. They should agree within a few degrees. If they do not, fix the sensor contact before you change setpoints. The controller page covers where those sensors belong. Swap nothing "just to try." A swapped pair runs the pump when the collector is cold and cools the tank.
Open a hot tap and measure before and after the mixing valve. The upstream side may be very hot. The downstream side should sit near the valve's setting. If they match, the valve is not mixing, and on a sunny day the taps can deliver scalding water.
From three readings to a snapshot
Write down flow, the temperature entering the collector, and the temperature leaving it, after the system has been stable for a while. Rise is outlet minus inlet. Multiply. The sketch below turns that into BTU and compares it with the heat in the hot water you actually drew.
Use a draw you measured, not a national average you like better. Inlet temperature is the cold water, not the tank. Delivery temperature is what you are trying to provide, often near 120 degrees F. If the snapshot covers the whole draw, good. The annual share will still be lower. If the snapshot is a small slice of the draw, the loop may be undersized, shaded, dirty, or just reflecting a short day. Check shade and flow before you buy another collector.
Tank losses are not in the sketch. A tank in a cold garage can give back a lot of what the collector delivered. Insulation is part of performance. So is a backup that is not fighting the solar tank. Read the backup page if the boiler runs all afternoon and the solar tank stays cold. You may be heating the store backward.
Leak check and drain check, pump off
Every joint dry. Drainback collector empty. Closed-loop pressure steady. Relief valve silent.
Pump on, flow in the right direction
Bleed air. Confirm the controller sensors match thermometers. Confirm a failed sensor leaves the pump off.
Mixing valve test
Hot upstream, tempered downstream, near 120 degrees F. If they are the same, stop and fix the valve before anyone showers.
Record one stable sunny hour
Flow, inlet, outlet. Run the sketch. Write the sun and the time next to the result so you do not compare July to December later.
Keep a week of tank temperatures
Morning and evening. A system that is hot at dusk and cold at dawn may be losing the heat overnight, not failing to collect it.
Further reading
These links go to outside sources. This book cites them by link and does not copy their words or pictures.
- https://www.builditsolar.com/Experimental/OffShelfDHW/OffShelfSolarDHWV2.htm
- https://www.builditsolar.com/Experimental/PEXColDHW/Overview.htm
- https://www.builditsolar.com/Experimental/CopperAlumCollector/Construction.htm
- https://www.builditsolar.com/Projects/WaterHeating/Fins/Fins.htm
- https://www.builditsolar.com/Projects/WaterHeating/LargeDB/LargeDrainBack.htm
- https://www.builditsolar.com/Projects/WaterHeating/ClosedLoop1K/ClosedLoop1K.htm
- https://www.builditsolar.com/References/Calculators/FinEficCalc/FinEficCalc.htm
- https://www.builditsolar.com/References/SHWKits/SHWKits.htm
- https://www.builditsolar.com/References/Ratings/SRCCRating.htm
General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.