Part IV: Solar Water HeatingChapter 7. Active Domestic Hot-Water Systems

Heat Exchangers and Storage Tanks for DHW

How to move collector heat into potable water without mixing the two streams.

Difficulty: Intermediate
Cost: Varies

published

The collector loop and the water you shower in are different fluids on purpose. The loop may be plain water that has been through a roof, or it may be propylene glycol. Either way, a leak should not put that fluid into a glass of water. The heat exchanger is the wall. The tank is the time shift, because the sun shows up at noon and the showers show up in the morning.

Code often wants a double-wall exchanger when the collector fluid is toxic or is not potable, with a path so a leak vents to the room instead of into the drinking water. Some places allow a single wall for a drainback that uses water, with backflow protection. Do not guess from this page. Ask the inspector before you buy the tank. A brazed-plate exchanger is efficient and usually single-wall. Efficiency does not outrank the code.

A planning band for storage is about 1.5 to 2 gallons per square foot of collector. Two 4 by 8 collectors are 64 square feet, which points at roughly 100 to 130 gallons. That is a sketch so you do not pair a large array with a 40 gallon tank that is hot by 11 in the morning and stagnating after lunch. It is not a code minimum.

Difficulty

Level: Intermediate
Time: An evening to choose, and a plumber if the potable side is new to you
Skills: Reading a code answer and respecting stratification

Cost estimate

Varies

A solar storage tank with a coil is often several hundred dollars to a couple of thousand, depending on size and whether the exchanger is built in. A separate plate exchanger can look cheap and still be the wrong wall for glycol. The mixing valve and the relief valve are not optional line items.

Piece	Typical cost
Solar storage tank with a coil	Several hundred dollars and up
External exchanger, if used	Less than the tank, and only if code allows it
Mixing valve, relief valve, expansion tank	$80–200 together, and required

Materials

Item	Qty	Notes
Storage tank	About 1.5 to 2 gallons per square foot of collector	Glass-lined potable tank with an anode. Not an open stock tank. Not a drum.
Heat exchanger	1	Internal coil or external unit. Double wall with a visible leak path when glycol or a non-potable loop is on the other side, unless your inspector says otherwise in writing.
Mixing valve	1	On the outlet to the fixtures, set near 120 degrees F. Test it with a thermometer.
Temperature and pressure relief valve	1	On the potable tank, rated for it, discharge full size and unvalved to a safe place.
Expansion tank	1 on a closed potable side	Needed when a check valve or a pressure-reducing valve means the water cannot push back to the street.
Backup heat	In series or in the top of the tank	Solar preheats. The backup finishes the job. See the backup guide.

How the two streams should meet

Collector fluid enters the exchanger hot and leaves cooler. Potable water takes that heat and rises. In a tank with a coil, put the coil low, where the water is coldest, so the exchanger still has work to do after the top of the tank is hot. Draw household hot water from the top. Return the collector's cold side in a way that does not stir the whole tank into one lukewarm average. A high-velocity inlet aimed at the top is how you ruin stratification.

If the collector is hot and the bottom of the tank never warms, the exchanger is too small, the coil is fouled, or the flow is wrong. A bigger collector will not fix a coil that cannot move the heat. Measure both sides before you buy more glass.

Glycol belongs in a closed loop with the metals and the pump the fluid was mixed for. Use a propylene glycol sold for solar or hydronic service, premixed or mixed with the water the label allows. Do not use automotive ethylene glycol. It is more toxic, and the inhibitor package is for an engine. A leak of either fluid is a cleanup, which is why the double wall and the vented gap exist.

Potable water stays in the tank. Collector fluid stays in the coil. A double wall gives a leak somewhere you can see it.

Placing and protecting the tank

An 80 gallon tank of water weighs about 700 pounds before the steel. The floor under it has to be a real floor. A platform of loose blocks is not a plan. Set it where a leak can reach a drain, not where it can soak a ceiling below for a week before anyone notices.

The backup heater belongs in series, solar first, or as an element in the top of a solar tank. The backup page explains why you should not use the boiler to heat the entire solar store. The bottom can stay cooler. The top needs to reach a temperature that finishes the heating so you are not storing a whole tank of lukewarm water as the only supply. Lukewarm storage is a bacteria risk. People with reduced immunity should not experiment with that.

The mixing valve is what keeps a successful sunny day from becoming a scald at the tap. Solar tanks regularly exceed 140 degrees F. Set the valve, then measure the outlet. Relief-valve discharge is the tank's last defense, full pipe size, no valve, aimed where it will not hit a person or freeze shut.

Anodes wear out. Without a working anode, a glass-lined tank starts to rust through. Check it on the schedule the maker gives you.

Ask what wall the code wants
Double wall for glycol unless you have a different answer from the authority that inspects your house. Buy the tank after that answer, not before.
Size the store to the collector
Use about 1.5 to 2 gallons per square foot as a start. Put the tank where the floor can carry it and a leak can drain.
Keep the coil low and the draw high
Do not stir the tank with a fire-hose return. If heat does not show up in the tank, fix the exchanger before you add collectors.
Fit relief, expansion, mixing, and backup
No valve on the relief line. Mixing valve tested. Backup in series so solar preheats and the backup finishes.
Insulate the tank and the pipes you can reach
The first insulation is the one that matters. Leave the relief valve, the anode, and the controls accessible.

Difficulty

Cost estimate

Materials

How the two streams should meet

Placing and protecting the tank

Ask what wall the code wants

Size the store to the collector

Keep the coil low and the draw high

Fit relief, expansion, mixing, and backup

Insulate the tank and the pipes you can reach

Further reading