Quick Answer: An indirect water heater is an insulated storage tank heated by your home’s existing boiler instead of its own burner — a different fix than the standalone tankless units this site otherwise covers. The HTP SuperStor Ultra SSU-45 (45 gallons, 316L stainless, limited lifetime tank warranty, about $1,412) is the best overall pick if you’re keeping the boiler long-term. The Amtrol BoilerMate CH-41Z (41 gallons, HDPE-lined, no anode rod, about $1,845) skips anode-rod maintenance entirely. On a budget, the HTP SuperStor Contender SSC-35 (35 gallons, glass-lined, 7-year warranty, about $927) costs less up front but needs anode-rod replacement every 2-4 years to hit its rated life.
If you landed here from a tankless-coil boiler setup, this is very likely the upgrade you’re looking for: an indirect tank fixes the summer efficiency problem a boiler-coil setup has by heating a full tank in one longer cycle instead of firing the boiler on every single draw. See our tankless coil water heater guide for the full explanation of why that switch pays off. Everything else on this site — Rinnai, Navien, Rheem, and the rest — is a standalone tankless unit with no boiler dependency at all; an indirect tank is a genuinely different product built for a genuinely different situation: you already have (or are installing) a boiler and want it to also make your domestic hot water.
Our top picks at a glance
| Model | Best for | Tank Material | Capacity | Warranty | Price |
|---|---|---|---|---|---|
| HTP SuperStor Ultra SSU-45 | Best overall | 316L stainless steel | 45 gal | Limited lifetime (tank) | ~$1,412 |
| Amtrol BoilerMate CH-41Z | Best (no anode rod) | HDPE-lined steel | 41 gal | 6-year factory | ~$1,845 |
| HTP SuperStor Contender SSC-35 | Best budget | Enamel/glass-lined steel | 35 gal | 7-year | ~$927 |
By the numbers
- AHRI standby-loss ratings run roughly 0.4-1.0+ for indirect tanks, on a scale where lower is better — aim for 0.8 or under. This is a different metric than the DOE’s UEF rating used for tankless units, so the two efficiency numbers can’t be compared side by side. — AHRI / DOE
- HTP rates the SuperStor Ultra’s insulation at under 0.5°F of heat loss per hour, thanks to 2 inches of foam insulation around the 316L stainless tank — one reason stainless indirect tanks hold their stored heat longer between boiler-reheat cycles than older glass-lined designs. — HTP product specifications
- Stainless steel indirect tanks typically last 20+ years versus roughly 10 years for glass-lined tanks, per field reports from licensed heating contractors — but only if a glass-lined tank’s anode rod gets replaced every 2-4 years; skipping that maintenance shortens the lining’s life well before the tank warranty runs out.
- HTP backs the SuperStor Ultra’s stainless tank with a limited lifetime warranty, versus 7 years on the enamel-lined SuperStor Contender — the same manufacturer prices the maintenance-free option into a longer warranty term.
1. HTP SuperStor Ultra SSU-45 — Best Overall
HTP SuperStor Ultra SSU-45 Indirect Water Heater
- 316L stainless steel tank resists corrosion without an anode rod to maintain.
- 2 inches of foam insulation rated under 0.5°F heat loss per hour, per HTP's own spec.
- Delivers 3-5x the recovery rate of a conventional gas-fired storage water heater, per HTP.
- Limited lifetime warranty on the tank — the longest term of any option here.
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The SSU-45 is the tank we’d point most homeowners keeping their boiler toward. Stainless steel tanks skip the anode-rod maintenance that limits a glass-lined tank’s life, and HTP backs that up with a limited lifetime tank warranty — the strongest term on this list by a wide margin. The tradeoff is price: it costs roughly 50% more than the glass-lined Contender at the same rough capacity. For a home that plans to keep the boiler for the long haul, the stainless tank’s 20+ year field-reported lifespan makes the extra cost a one-time decision instead of a recurring anode-rod chore.
2. Amtrol BoilerMate CH-41Z — Best for Skipping Anode-Rod Maintenance
Amtrol BoilerMate CH-41Z Indirect-Fired Water Heater
- High-density polyethylene (HDPE) tank liner rated to 180°F — no anode rod at all.
- Available in the Champion (CH) and Classic (WH) series, both sized at 41 gallons here.
- 6-year factory warranty on the tank and heat exchanger.
- Sold through the same big-box and plumbing-supply channels as the HTP line, including Amazon.
The BoilerMate CH-41Z solves the anode-rod question a different way than stainless does: its HDPE liner simply doesn’t corrode the way a bare steel tank would, so there’s no sacrificial rod to inspect or replace on a schedule. That makes it a genuinely maintenance-light option, though it’s currently the most expensive tank on this list and its warranty term (6 years factory, with lifetime options depending on registration) runs shorter than the SuperStor Ultra’s stainless tank. It’s the pick if you specifically want to never think about anode-rod service, even at a premium.
3. HTP SuperStor Contender SSC-35 — Best Budget
HTP SuperStor Contender SSC-35 Indirect Water Heater
- Enamel-lined steel tank with 2-inch foam insulation, priced well under the stainless SuperStor Ultra.
- 7-year warranty, versus the Ultra's limited lifetime tank coverage.
- Needs the anode rod replaced roughly every 2-4 years to reach its ~10-year typical service life.
- Also sold in 50, 80, and 119-gallon sizes for larger households.
The Contender is the same HTP engineering at a lower price point, with the tradeoff built into the tank material: glass/enamel lining instead of stainless. Budget the anode-rod swap every 2-4 years into the ownership cost — skip it, and the roughly 10-year typical lifespan drops. For a household that isn’t sure it’s keeping the current boiler for the next 20 years, the lower upfront cost here can make more sense than paying for stainless longevity you may not use.
How to choose an indirect water heater
- Keeping the boiler long-term? Pay for the stainless SuperStor Ultra’s limited lifetime warranty and skip the anode-rod chore for good.
- Want zero anode-rod maintenance but don’t need the longest warranty? The HDPE-lined Amtrol BoilerMate does that at a lower total-ownership hassle, if not the lowest price.
- Tightest budget, or unsure how long the current boiler sticks around? The glass-lined Contender costs less up front — just put the anode-rod replacement on a 2-4 year calendar reminder.
- No boiler in the picture at all? An indirect tank isn’t the right product — see our best gas or best electric tankless picks instead, both of which are fully self-contained.
- Currently running an old tankless coil off your boiler? That’s the setup this upgrade specifically targets — read our tankless coil water heater guide for the summer-efficiency math that makes the switch worth it.
- Weighing a combi boiler instead? A combi boiler handles space heating and hot water together with no separate tank at all — see our tankless water heater vs boiler comparison for how that differs from both an indirect tank and a standalone tankless unit.
The bottom line
An indirect water heater only makes sense if there’s a boiler in the picture — it’s a storage tank that borrows the boiler’s heat instead of firing its own burner, which is the opposite approach from every other unit covered on this site. If you’re keeping the boiler for the long haul, the HTP SuperStor Ultra SSU-45’s stainless tank and limited lifetime warranty is worth the higher price. Want to skip anode-rod maintenance without paying for the longest warranty term, and don’t mind the highest sticker price here? The Amtrol BoilerMate CH-41Z’s HDPE liner does that. On a budget, or not sure the current boiler is a 20-year bet, the HTP SuperStor Contender SSC-35 costs less up front — just calendar the anode-rod swap. If there’s no boiler at all, none of this applies to you: go straight to our best tankless water heater roundup for a self-contained unit instead.