High-Altitude Seasonal Lodge: Choosing a Freeze-Safe, Scale-Tolerant System
A hunting lodge at 2,450 m elevation is occupied only on weekends from October to April and may sit empty for six weeks in January. Design low is -28 °C. Well water tests 320 mg/L total hardness as CaCO3. Grid power is reliable but fails several times each winter. Occupied-day draw is 250 L at 55 °C; on vacant days it is near zero. The owner is comparing three quotes. (A) A direct thermosyphon system with roof-mounted evacuated tubes, well water circulating through the collectors. (B) A flat-plate array with an inhibited propylene glycol loop, a load-side heat exchanger, and an AC pump. (C) A flat-plate drainback array with a PV-powered DC pump and a 500 L unpressurized drainback tank. The owner wants to know which system will survive the vacancy periods, protect the well water from scale, and still work during outages.
1. Choose between A, B and C and justify the choice against freeze, stagnation, hardness and power availability.
Read the worked answer
A is out. Well water in roof-mounted tubes at -28 °C with no pump and no drain path will freeze and split the absorbers or the manifold. It also exposes the hardest water in the building directly to the hottest surfaces, and it will stagnate to boiling during a vacant January.
B works but carries avoidable risk. Glycol is a valid freeze protection, but a system that stagnates for six weeks will cook the inhibitor package; glycol oxidizes to acid at high temperature, which is precisely the failure mode seen later as low pH and sludge. The AC pump also stops during outages, though a glycol loop tolerates that.
C is the best fit. Drainback collectors drain by gravity whenever the pump stops, so freeze protection is passive and there is no glycol to test or replace, and dry stagnation is something the collectors are designed for. The PV-driven DC pump keeps circulating during grid outages. And because the loop is indirect, the hard well water never enters the collectors; it only sees the load-side heat exchanger, which can be descaled or replaced.
Conditions to verify before signing: the drainback tank must sit below the collectors with continuous slope, the pump must have enough head for the vertical lift, the pipe runs must not create a trap, and the tank must be drained if the building is winterized.
2. What design and maintenance measures would you specify for the 320 mg/L hardness?
Read the worked answer
At 320 mg/L, scale forms readily on any surface much above 60 °C, so the strategy has three parts: keep the water out of the collectors, keep temperatures moderate, and make the scale-prone component serviceable.
The collector loop is already separated in options B and C. Specify a generously sized stainless brazed-plate heat exchanger, with double-wall construction if the local code requires protection of potable water. A larger exchanger gives a lower approach temperature at design and tolerates a thin scale layer before performance falls off.
Hold the tank at 55–60 °C rather than 70 °C; the scaling rate climbs steeply above 60 °C, and a thermostatic mixing valve still delivers 45–50 °C at the tap. If a softener can realistically be maintained at a seasonal lodge, add one; if not, plan annual descaling of the exchanger with a food-grade acid such as citric or vinegar.
Instrument the decision: log the exchanger approach temperature at a known flow and inlet condition. A drift from about 2 °C toward 6 °C means fouling and triggers cleaning. Test hardness annually, and note that the higher tank temperature makes a backflow device on the potable connection even more important.
3. How should the controller be set up for the vacant periods, and what would you tell the owner not to do?
Read the worked answer
With drainback, freeze protection is by default: when the collector sensor is not above the tank by the on-differential, the pump stops and the collectors drain by gravity. Set the on-differential around 8 °C and the off-differential around 2–3 °C, and add a high-limit that stops the pump near 95 °C.
The common mistake is to enable a freeze-recirculation function copied from a glycol system. On a drainback loop that is exactly wrong: running the pump at night floods the collectors with water that then has to drain again, wasting energy and creating freeze risk if the pump stops mid-cycle. Tell the owner that no antifreeze is needed and no winter pump exercise is wanted.
The real vacancy issue is stagnation. With no draw, the tank reaches setpoint and the pump sits off, so the collectors stagnate dry each day. Flat plates rated for stagnation can take this, but verify the absorber coating, gaskets and drainback tank headspace are rated for steam, and keep the tank in a freeze-protected room. If the building is winterized, drain the drainback tank and the load-side exchanger as well as the potable plumbing, and tag the valves so the next person knows what was done.