Solar Hot Water Preheat Kept Gas Tankless on a California ADU

A builder on a California ADU did not want a heat pump water heater. His reason was blunt: "They're slow to recover and expensive." He wanted gas tankless. California's energy code is not friendly to anything except a heat pump water heater, and his first choice failed the model by a lot. What got it to pass was one small solar hot water panel on the roof, about 4 ft by 4 ft, plus a 40 gallon tank. It took the ADU from a big fail to an easy pass, and the approach should work pretty much anywhere in the country.

The Real Project

The project was an ADU in California. The builder had a clear preference and a clear reason for it. Heat pump water heaters, in his words, are slow to recover and expensive. He wanted gas tankless because it gives fast hot water on demand.

The energy code had a different preference. So we started where most teams start. We kept the gas tankless and upgraded everything around it. We modeled a high-end gas tankless. We added better windows, more insulation and a better roof. We put in a high-efficiency mini-split for heating and cooling.

It still failed, by a lot. This was not a near miss that one more measure could close. It was a real gap, and the envelope and HVAC upgrades we stacked on top were not enough to close it.

Then we tried the two moves that usually come up next. A battery barely helped. More solar panels didn't count. Every lever the team reached for was either too small or not counted where the project needed it.

What worked was simple. One small solar hot water panel on the roof, about 4 ft by 4 ft, plus a 40 gallon tank. The sun heats up the water. That water then travels through the gas tankless water heater. If it needs more heat, the tankless turns on. If not, nothing happens. That took the ADU from a big fail to an easy pass, and the gas tankless stayed in the design.

The Water Heater Penalty

California's performance path does not grade a house on its own. The compliance software builds a reference version of the same house using the code's preferred equipment, then scores your design against it. For water heating, that preferred equipment is typically a heat pump water heater. A gas tankless starts the run already behind, before a single window or wall is counted.

The reason is efficiency. A heat pump water heater moves heat out of the surrounding air instead of making it, so it delivers hot water for a fraction of the energy. A gas tankless, even a high-end one, burns fuel to make every degree of temperature rise. The model sees that difference on one of the largest loads in the dwelling, and it charges for it.

Small dwellings make the penalty harder to buy back. An ADU has a small envelope and a small heating and cooling load to begin with. Better windows, more insulation and a better roof all cut that load, but they are cutting from a small number. A high-efficiency mini-split does the same thing. Each upgrade is real, and each one is aimed at a load that was never the problem.

Water heating does not shrink with the floor plan the way heating and cooling do. People still shower, run the dishwasher and do laundry in a small unit. So in an ADU, water heating is a large share of what the model counts. When that share is penalized, envelope dollars cannot catch up. That is why the high-end package still failed by a lot.

The battery and the extra panels ran into a different wall. California scores efficiency separately from generation. Solar panels make electricity, but they do not make the water heater any more efficient, so more panels didn't count toward the shortfall. A battery shifts when power gets used. It earns a limited credit, and it does nothing to the energy the water heater burns. Both moves improved the house in some way. Neither touched the line item that was failing.

The Solar Preheat Math

The solar hot water panel worked because it went straight at the load that was failing. The sun heats water in the panel, and the 40 gallon tank stores it. That preheated water then feeds the gas tankless. The tankless only has to make up the difference between the water coming in and the temperature the house needs.

On a sunny day, that difference can be small or nothing at all. If the water needs more heat, the tankless turns on. If not, nothing happens. Every unit of heat the sun supplies is one the gas burner does not have to make. The compliance model credits the share of the water heating load the sun covers, and that credit lands exactly where the penalty lived.

That is why one panel about 4 ft by 4 ft outperformed a list of envelope upgrades, a battery and more solar panels. Size was never the issue. Aim was. The small panel cut water heating energy directly. Everything else cut something the model was not short on.

The pass came with margin, and margin is something you can spend. We were even able to VE out the ERV and drop the efficiency on the gas water heater to help offset the cost of the solar water heating system. The high-end tankless had been carrying compliance weight. Once the sun took over that job, a less expensive unit could do the rest. The ERV was a cost the house no longer needed to pass.

The result also answers the builder's original objection. Recovery was his complaint about the heat pump water heater. With the gas tankless still in the design, the house keeps reliable, fast hot water. The system lowers your power bill by using the power of the sun, and the gas tankless covers whatever the sun does not.

When Solar Preheat Still Works

The builder will not take a heat pump water heater. This is the case this ADU proved. If the team has a firm reason to keep gas tankless, a small solar preheat can close the water heating gap where envelope upgrades cannot. It lets the builder keep the equipment he trusts without fighting the model.

The dwelling is small. ADUs, single family homes and townhomes have small envelopes and small heating and cooling loads. That leaves water heating as a big share of what the model counts. A preheat that cuts water heating directly moves those scores more than another envelope upgrade will.

The roof has a spot for one panel. About 4 ft by 4 ft is a small footprint, but it still needs a sunny, unshaded piece of roof. The 40 gallon tank needs a place to live inside as well. When both fit without a redesign, the system drops in cleanly.

There are measures to trade back. The real value showed up in what the team could remove. On this ADU, the margin let us VE out the ERV and drop the gas water heater efficiency. If your design carries measures that exist only to earn compliance, solar preheat can free them up to offset its own cost.

The project is outside California. The approach should work pretty much anywhere in the country. Each state scores water heating its own way, so the size of the credit and the measures you can trade back will change. Run it in the code path your project actually uses before you count on it.

The Decision Framework

Start with what the team will not give up. On this ADU, it was gas tankless. Once that is fixed, find where the penalty actually lives before buying upgrades. Here, better windows, more insulation, a better roof, a high-efficiency mini-split, a battery and more solar panels all aimed somewhere else. The fix was the one measure aimed at water heating.

Price the whole package, not the panel alone. The solar water heating system added a panel and a tank. The margin it created let the team VE out the ERV and drop the efficiency on the gas water heater, and those cuts help offset what the solar side cost. Compare that net package against a heat pump water heater design, not just the equipment line against the equipment line.

Treat this California ADU as one building's answer. A different roof, climate, occupancy or code path can change how much credit the sun earns and what you can trade away. No two projects carry the same constraints, and there is no single water heater that wins on every job. Sometimes the heat pump water heater is the cheapest pass. Sometimes a small solar preheat keeps the gas tankless and passes easily. The way to know which one fits yours is to model both before the plumbing gets drawn.