Indirect Water Heater Repair in Mill Valley: A Boiler Owner's Guide
If your Mill Valley home has a boiler, your hot water probably comes from an indirect tank heated by it. Here's how those tanks fail, what's usually a simple fix, and when it's time to replace.
In Mill Valley, a lot of homes with a boiler don't have a separate water heater at all. The boiler heats the house and the hot water, through an indirect water heater: a storage tank with a coil inside that the boiler feeds. When showers start running lukewarm, the hot water runs out halfway through a bath, or the house cools off every time someone showers, the indirect side of the system is usually where to look. Call or text Local Plumbing at (415) 234-0370, or read more about our boiler replacement and repair work in Mill Valley.
How an Indirect Water Heater Works
An indirect tank has no burner of its own. Hot water from the boiler circulates through a coil inside the tank and heats the water you use at the taps. A sensor or aquastat in the tank tells the boiler when the tank needs heat, and a zone valve or a dedicated circulator pump sends boiler water to the coil.
Most boilers give the tank domestic hot water priority. When the tank calls, the boiler stops heating the house for a while and puts all its output into the tank. That's normal, and it's why a problem on the hot water side can show up as a heating problem.
A well-set-up system also has a thermostatic mixing valve at the tank outlet. The tank can be stored hot, and the mixing valve blends it down to a safe temperature at the faucets.
Signs Your Indirect Tank (or Its Controls) Need Attention
- Hot water runs out much faster than it used to
- Water is lukewarm even though the boiler is running
- The house gets cold during long showers or when the laundry and dishwasher run, and takes a long time to recover
- The relief valve on the tank drips or has discharged
- Water at the base of the tank or rust streaks down the jacket
- The boiler short-cycles every time the tank calls for heat
- Hot water works but the heat doesn't (or the other way around)
Most of these are control or valve problems, not a failed tank.
What Usually Goes Wrong
The tank sensor or aquastat
If the sensor has slipped out of its well or failed, the boiler doesn't know the tank is cold, or it thinks the tank is satisfied too early. The result is lukewarm water or hot water that runs out fast. This is usually a quick repair.
The zone valve or circulator feeding the coil
A stuck zone valve or a weak circulator means boiler water never reaches the coil, or reaches it too slowly. This is the most common reason hot water suddenly gets worse in the fall, after a summer of light use.
Boiler settings
Modern condensing boilers have separate settings for the hot water side: target temperature, priority time and how hard the boiler fires for the tank. After a control board replacement or a well-meaning adjustment, these can end up wrong. If the house gets cold every time someone showers, the priority settings are often the first thing we check.
The mixing valve
A sticking or worn thermostatic mixing valve causes temperature swings at the shower, or caps the hot water too low. They wear out, and they're simple to replace.
A leaking tank
This is the one that means replacement. When the inner tank itself leaks, there's no repair. Stainless steel indirect tanks tend to last a long time. Glass-lined tanks rely on an anode rod, and if the rod was never checked, the tank can rust through. A leaking coil (boiler water mixing with your hot water, or the boiler losing pressure for no obvious reason) also means a new tank.
Triangle Tube Smart Tanks in Mill Valley
We still see a lot of Triangle Tube Prestige boilers paired with Triangle Tube Smart indirect tanks in Mill Valley mechanical rooms. Triangle Tube ceased operations in November 2025, so there's no factory parts or warranty support behind those systems anymore. (Our Triangle Tube shutdown guide covers what that means for the boiler.)
A Smart tank that isn't leaking can often stay in service. If we're replacing the Prestige, we look at the tank too. If it's sound, we may keep it and connect it to the new boiler. If it's old or showing rust, replacing both at once saves a second visit, a second drain-down and a second set of piping changes.
Why Mill Valley Homes Are a Little Different
- Hillside mechanical rooms. In Cascade, Blithedale Canyon, Warner Canyon and the hills above downtown, boilers often sit in a lower-level room or under the house. Many homes are reached by stairs or a lane, which matters when a full tank has to come out and a new one has to go in. A wall-hung boiler with a compact indirect tank is often the best fit for tight spaces.
- Older homes with upgraded heat. Many older Mill Valley homes have had their heat replaced or added on more than once. We regularly find an indirect tank piped to a boiler it was never matched with, or controls from two different installers.
- Soft water. Marin Water's water is soft (about 3.5 grains per gallon from the local reservoirs, according to its 2025 water quality report), so scale on the coil is less of a problem here than in hard-water areas. Tank life is more about the tank's construction and, for glass-lined tanks, the anode rod.
- Permits. The City of Mill Valley requires a permit for water heater replacements. Homes in Tamalpais Valley, Homestead Valley, Almonte, Alto and Strawberry have Mill Valley addresses but are permitted through the County of Marin. We pull the permit either way.
Repair, Replace or Rethink?
Repair when the tank is sound and the problem is a sensor, valve, pump, mixing valve or boiler setting. That's most calls.
Replace the tank when it leaks, the coil leaks, or it's simply too small for how the household uses hot water now. We size the new tank for your actual use (number of bathrooms, big soaking tubs, guests) and for the boiler's output. Stainless indirect tanks from NTI, HTP and Lochinvar are what we install most.
Rethink the system when the boiler is also near the end of its life. In smaller homes, a combi boiler that makes heat and on-demand hot water in one unit can be a good fit. In larger homes and homes with heavy hot water use, a modulating condensing boiler with an indirect tank usually works better, because the tank stores a reserve of hot water.
What about the Bay Area gas water heater rule?
An indirect tank has no burner, so it isn't a gas water heater. The boiler that heats it falls under the Bay Area Air District's boiler timeline: home-size gas boilers made after January 1, 2031 must be zero-NOx under the current rule (a July 2026 draft would move that to 2033). The separate rule for gas tank water heaters starts January 1, 2027 (a proposed amendment could push it to 2028). Neither rule requires anyone to remove working equipment.
What Our Service Visit Includes
- Boiler check and combustion analysis. We do combustion analysis on every boiler service.
- Hot water controls. Tank sensor or aquastat, zone valve or circulator, and the boiler's hot water settings and priority.
- Tank condition. Relief valve, signs of leaks or rust, and the anode on glass-lined tanks.
- Mixing valve and safety. Delivered temperature at the taps and expansion control on the hot water side.
Mill Valley Indirect Water Heater FAQs
Why does my house get cold when someone takes a long shower?
Your boiler is giving the hot water tank priority, which is normal for a short time. If the house cools noticeably, the tank may be undersized, the coil may not be getting enough flow, or the boiler's priority settings need adjusting.
Can I keep my indirect tank when I replace my boiler?
Often, yes, if the tank isn't leaking and is the right size. We inspect it before recommending anything. If it's near the end of its life, replacing both at once is usually cheaper than two separate jobs.
Is an indirect water heater better than a tankless?
For homes that already have a boiler, an indirect is usually the most efficient way to make hot water, because the boiler is already there. It also stores hot water, so several showers can run at once. A tankless or combi makes more sense where space is very tight and hot water use is modest.
Boiler and Hot Water Service in Mill Valley, Tam Valley and Strawberry
Local Plumbing specializes in boilers and radiant heat across Marin and Sonoma counties, from our warehouse and training facility in San Rafael. We're a licensed C-16 and C-36 contractor (Lic #1091739). Learn more about boiler replacement and repair in Mill Valley.
Hot water not keeping up this fall? Call or text (415) 234-0370, or contact us to book a visit.
