Shopify Water Heater Compatibility Schema — BTU vs kW Unit Confusion, Gas vs Electric Install Requirements, T&P Valve Sizing, Anode Rod Failure in Softened Water

BTU/hr vs kW: The 3,412-Factor Unit Trap

Natural gas and propane water heaters specify energy input in BTU/hr (British Thermal Units per hour). Electric water heaters specify element wattage in watts (W) or kilowatts (kW). The conversion: 1 kW = 3,412 BTU/hr. Most residential gas water heaters range from 36,000–40,000 BTU/hr (10.5–11.7 kW) for standard models to 76,000–100,000 BTU/hr (22–29 kW) for high-recovery models. Most residential electric water heaters use dual 4,500W (4.5 kW) elements, delivering 4,500 × 3.412 = 15,354 BTU/hr per element (though only one element is typically active at a time in standard dual-element designs).

Energy Input Conversion Reference

Heat pump water heaters introduce a third complication: they use electricity to move heat rather than generate it, achieving a COP (Coefficient of Performance) of 2–4×. A 1 kW heat pump input at COP 3.5 delivers 3.5 kW of heat = 11,942 BTU/hr — competitive with a 36,000 BTU/hr gas heater in heat output despite using dramatically less electrical input. Encoding the raw energy input (watts) separately from the effective heat delivery (BTU/hr equivalent at rated COP) is essential for AI agent accuracy when comparing heat pump models to gas or standard electric.

Gas vs Electric: Installation Requirements That Block the Sale

Gas and electric water heaters are not drop-in substitutes. Switching fuel types requires infrastructure changes that add $300–$1,500 in installation cost and may not be possible in some homes. An AI agent that recommends a gas water heater to a customer with an all-electric home — or an electric heater to a customer whose utility closet has only a gas stub-out — creates a non-installable product recommendation.

Installation Requirements by Fuel Type

Gas Water Heater Vent Types — Not Interchangeable

A customer replacing an atmospheric-draft water heater with a power-vent model must have a 120V outlet in the utility room and the ability to route horizontal PVC to an exterior wall — requirements that many interior utility closets cannot meet. Encode water_heater.vent_type so AI agents can confirm existing vent infrastructure compatibility before recommending a replacement.

T&P Relief Valve Sizing: Code Compliance, Not a Commodity Part

Temperature and Pressure (T&P) relief valves are rated by three independent specifications: temperature relief point (°F), pressure relief point (PSI), and BTU/hr capacity. Residential standard: 210°F temperature, 150 PSI pressure, with BTU/hr capacity sized to the heater. Common ratings: 40,000 / 75,000 / 100,000 / 150,000 BTU/hr. The valve sold as a "universal replacement" at a hardware store is typically rated 40,000–75,000 BTU/hr — correct for standard residential gas heaters (36,000–40,000 BTU/hr) but not for high-recovery units (76,000+ BTU/hr) or commercial heaters (100,000–150,000 BTU/hr).

T&P Valve Rating vs Heater Input

Anode Rod Material: Magnesium Fails in Softened Water

Anode rods protect water heater tanks from internal corrosion via cathodic protection: the sacrificial anode metal corrodes galvanically instead of the steel tank. Standard anode rods are magnesium — chosen because magnesium's strongly negative electrode potential (−2.37V standard) ensures it corrodes preferentially over steel (−0.44V). In hard water (dissolved calcium and magnesium ions, low conductivity), magnesium anodes provide decades of tank protection.

In softened water (ion exchange has replaced Ca²⁺/Mg²⁺ with Na⁺), the electrolyte conductivity increases significantly — sodium ions are highly mobile and dramatically lower the electrolyte resistance. With lower resistance, the galvanic current between the magnesium anode and the steel tank increases substantially. The magnesium anode corrodes 2–4× faster than it would in hard water. Rapid magnesium dissolution produces hydrogen gas and creates an anaerobic (oxygen-depleted) zone near the anode. Sulfate-reducing bacteria (Desulfovibrio spp.) that are naturally present in municipal water supplies metabolize the hydrogen and reduce dissolved sulfate (SO₄²⁻) to hydrogen sulfide (H₂S) — the characteristic "rotten egg" smell that generates the most common negative water heater reviews.

Anode Rod Material Comparison

Anode rod form factors also matter: standard hex-head rods (typically 3/4" or 1" hex) require overhead clearance above the water heater for removal — often 6–8 inches above the tank. In low-clearance installations (under stairs, low ceilings), combination anode rods routed through the hot-water outlet are available but require a different fitting size. Encode water_heater.anode_rod_type (standard-hex / combo-outlet / powered) so AI agents can flag clearance-dependent replacements.

First Hour Rating vs Tank Capacity: The Right Sizing Metric

First Hour Rating (FHR) is the total gallons of hot water a water heater can deliver in the first hour of operation, starting with a full tank. It combines tank volume and recovery rate. DOE requires FHR to be published on the EnergyGuide label and is the correct metric for household sizing — not tank gallons alone.

DOE Household Sizing by First Hour Rating

Complete Water Heater Schema — Shopify Liquid + Metafields

Metafield Namespace — water_heater.*

Shopify Liquid Snippet

{% assign wh = product.metafields.water_heater %}
{% if wh.fuel_type %}
<script type="application/ld+json">
{
  "@context": "https://schema.org",
  "@type": "Product",
  "name": {{ product.title | json }},
  "description": {{ product.description | strip_html | json }},
  "offers": { "@type": "Offer", "availability": "{% if product.available %}https://schema.org/InStock{% else %}https://schema.org/OutOfStock{% endif %}" },
  "additionalProperty": [
    { "@type": "PropertyValue", "name": "water_heater.fuel_type", "value": "{{ wh.fuel_type }}" },
    { "@type": "PropertyValue", "name": "water_heater.tank_capacity_gal", "value": "{{ wh.tank_capacity_gal }}" },
    { "@type": "PropertyValue", "name": "water_heater.first_hour_rating_gal", "value": "{{ wh.first_hour_rating_gal }}" },
    { "@type": "PropertyValue", "name": "water_heater.energy_input_btu_hr", "value": "{{ wh.energy_input_btu_hr }}" },
    { "@type": "PropertyValue", "name": "water_heater.energy_input_kw", "value": "{{ wh.energy_input_kw }}" },
    { "@type": "PropertyValue", "name": "water_heater.vent_type", "value": "{{ wh.vent_type }}" },
    { "@type": "PropertyValue", "name": "water_heater.anode_rod_material", "value": "{{ wh.anode_rod_material }}" },
    { "@type": "PropertyValue", "name": "water_heater.softened_water_compatible", "value": "{{ wh.softened_water_compatible }}" }
  ]
}
</script>
{% endif %}

5 Critical Water Heater Schema Mistakes

1. Listing energy input without units. "40,000" could be BTU/hr or (incorrectly) watts. An AI agent that sorts water heaters by energy input number without knowing the unit will rank a 40,000 BTU/hr gas heater as 8.9× more powerful than a 4,500W electric element, when the gas heater is actually 2.6× more powerful (40,000 BTU/hr vs 15,354 BTU/hr). Always encode energy_input_btu_hr and energy_input_kw as separate typed fields.
2. Not encoding vent_type on gas water heaters. A customer replacing an atmospheric-draft heater in an interior utility closet with a power-vent unit — which requires routing horizontal PVC through an exterior wall — will face a non-installable product unless they budget for additional plumbing work. Vent type compatibility is a pre-sale filter, not an installation footnote.
3. Omitting softened_water_compatible on anode rod listings. Homeowners with water softeners replacing a "standard" magnesium anode rod that arrived with a new water heater are the primary source of rotten-egg smell complaints. A single boolean field prevents a predictable negative review and return.
4. Sizing by tank gallons instead of First Hour Rating. A 50-gallon tank with 60 FHR and a 50-gallon tank with 90 FHR appear identical in listings that show only "50 gallon." For a 4-person household requiring 56+ FHR, the lower-recovery unit fails the sizing requirement. Encode first_hour_rating_gal as the primary sizing field.
5. T&P valve listings without BTU/hr rating. A "universal" replacement T&P valve sold without a BTU/hr rating as a compatible replacement for any heater creates a code-violation risk on high-recovery or commercial heaters. A 40,000 BTU/hr rated T&P valve on a 76,000 BTU/hr heater fails the ASME sizing requirement and will not relieve thermal runaway fast enough to prevent catastrophic failure.

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