# Hot Water and Heat Submetering: How It's Measured and Billed

Source: https://submeter.com/submetering-hot-water/
Publisher: National Exemption Service (NES), https://submeter.com. Last reviewed September 2026.

How apartments submeter hot water and heat: meters on hot and cold lines, billing the energy to heat water, BTU meters, and when the setup pays off.

## Can you submeter hot water and heat?
Yes, you can submeter both hot water and heat in multifamily buildings, but the plumbing setup determines the hardware. Central boiler systems that supply domestic hot water to units use water meters on each unit branch line. Central hydronic heating loops rely on BTU meters, which measure water volume and temperature change to calculate heating energy. For properties where individual meters do not fit the piping, owners can bill heating costs through ratio utility billing (RUBS). In all cases, billing rules vary by state, county, and city.

If you manage an apartment community, condominium, or mobile home park, central utility bills cut into net income. Putting meters on hot water lines or heating loops shows who uses what and lowers property expenses. This guide reviews equipment setups, typical costs, and the rules for billing residents.
## How hot water submetering works in central systems
Central domestic hot water systems use one or more central boilers to supply hot water to the whole building. Cold water enters the boiler room, heats up, and runs through a dedicated loop. To measure use under this layout, an installer puts a meter directly on the hot water line feeding each unit.
This meter records the exact gallons of heated water flowing into the apartment. Hot water wears down standard plastic parts, so these pipes require meters rated for higher temperatures. Modern wireless submeters report several times a day to an offsite reading system.
Resident statements then reflect the actual gallons used. At National Exemption Service (NES), we track continuous flow through our MyReads portal to flag a running toilet or a dripping faucet before it drives up the bill. The total amount billed to residents never exceeds the property's master-meter cost. Common-area use for laundry, irrigation, pools, and clubhouses stays with the owner.

## Single meters versus dual meters for hot and cold lines
Plumbing layout decides whether an apartment needs one meter or two. Many newer apartments have one cold feed and one hot feed entering at a utility closet. Metering just the hot line tracks heated water, but most owners prefer to capture total water use through [water submetering](https://submeter.com/what-is-water-submetering/).
When a home has separate hot and cold incoming pipes, you can install [submetering meters](https://submeter.com/submetering-meters/) on both lines. The cold meter measures water for toilets and cold faucets, while the hot meter records water from the central boiler. Combining both reads gives you the total water use for monthly billing.
Older buildings with vertical plumbing risers work differently. In these properties, one unit might have three separate risers: one in the kitchen, one in a half bath, and one in the main bathroom. Opening drywall to put a meter on every one of those pipes usually costs more than the meters would recover. For those buildings, owners often turn to ratio utility billing (RUBS), which splits the bill without installing new hardware.

## Billing the energy required to heat domestic water
A water meter measures liquid volume, not the fuel used to heat it. Central boilers burn gas or use electricity around the clock to keep water hot. A standard water meter records gallons, not therms or kilowatt-hours.
You can recover this heating cost by dividing the boiler's monthly energy bill by the total gallons of hot water the building used. That calculation produces a heating cost per gallon.
Residents then pay for their water volume plus their share of the heating fuel. Once residents pay for their own water, water use typically drops 15 to 30 percent.

## Measuring space heat with BTU meters
Buildings with shared heating loops cannot rely on basic water meters. With hydronic heat, [submetering](https://submeter.com/what-is-submetering/) measures energy, not water. The water circulating through baseboard radiators or fan coils never leaves the closed piping loop.
Because residents do not drink or drain this water, you must measure the heat extracted from the pipe. An installer places a British Thermal Unit (BTU) meter on the unit's supply loop. A BTU meter uses three parts:
- Flow sensor. Measures the volume of water moving through the heating loop.
- Temperature sensors. Records the water temperature as it enters and leaves the home.
- Calculator unit. Multiplies water volume by the temperature drop over time to calculate the heat energy delivered to the room.
BTU meters give you an accurate way to track heating in central loop buildings. These meters cost more than standard water meters, so owners use them most often in condominiums and large buildings.

## Meter placement in units with individual water heaters
Many garden-style communities skip central boilers entirely. Each apartment has its own gas or electric tank in a closet or utility room. In these buildings, [water, gas, and electric submetering](https://submeter.com/water-gas-electric-submetering/) follows a simpler path.
You do not need a high-temperature meter on the hot side. An installer places a standard cold water meter on the cold supply line entering the tank, or on the main cold line feeding the whole unit. Every drop of hot water that leaves the tank enters as cold water first, so metering the intake line captures all hot water use.
Access to that pipe determines the installation price. A standard [submetering cost](https://submeter.com/how-much-does-submetering-cost/) typically averages $150 to $300 per unit across a standard community. If a water heater sits in a cramped closet and blocks the main line, a plumber has to drain and move the tank to reach the pipe. That extra labor makes those units cost noticeably more.
| Building setup | What gets metered | How it is billed |
| --- | --- | --- |
| Central boiler with single hot branch per unit | Gallons of hot water delivered to the residence | Master water rate applied to gallons; boiler fuel allocated by hot water volume |
| Central boiler with separate hot and cold branch lines | Gallons on both cold feed and hot feed | Combined volume billed for water/sewer; central boiler fuel allocated across hot volume |
| Central hydronic heating loops (baseboard or fan coils) | Fluid flow rate combined with supply and return temperatures | Exact BTUs converted to kilowatt-hours or therms and billed against utility rates |
| Individual water heater inside each apartment | Gallons entering the unit or the cold inlet of the heater | Water billed by volume; the energy to heat it is on the unit's own electric or gas service |
| Central boiler with multiple vertical risers per unit | No practical physical point for a single meter | Ratio utility billing (RUBS) divides master fuel and water bills by occupants or area |

## When submetering hot water and heat makes financial sense
Submetering works best for communities with 80 or more units, including apartments, condominiums, townhomes, student housing, and mobile home parks. Smaller buildings are often better served by a small-property provider. Most properties earn back the install cost in 12 to 18 months from the utility costs they recover, and some sooner.
At one 337-unit community, NES added meters as leases renewed, so for a while both setups ran side by side. The 168 submetered units averaged 3,868 gallons per unit per month, 34% less than the 5,846 gallons used by the 169 units not yet metered. Their average bill was $54.22 a month against $84.72, 36% lower, or about $30 per unit. Across all 337 units, that gap is worth roughly $123,000 a year.
Billing for heat and the energy to heat water has its own rules, and they vary by state. Some places limit it or require approval first, so check what [the law allows](https://submeter.com/is-submetering-legal/) at your address. Our in-house compliance team does that before we quote.
We offer financing for properties that qualify. We also install nonproprietary meters backed by a warranty that you own outright, so you are never locked in to NES.
Central boiler or individual water heaters?. Tell us how your building is plumbed and send 12 months of bills. A specialist replies within one business day.

## Questions
**Do I need separate meters for hot and cold water?**
You need two meters only when hot and cold water enter through separate distribution lines and you want to bill for both. If you only meter the hot line, cold water use stays on the property's tab. In apartments with individual water heaters, a single meter on the cold line captures all the water entering the home, including what feeds the tank.

**How is the cost of heating water billed?**
Central water heating costs are allocated using hot water meter reads and the master boiler bills. We divide the boiler's gas or electric bill by the total gallons of hot water used across the property to find a cost per gallon. Each resident then pays that rate based on their own hot water use. The total billed never exceeds the master bill.

**What is a BTU meter?**
A BTU meter measures thermal energy in hydronic heating systems. It pairs a flow sensor with temperature sensors on the supply and return lines. The meter multiplies water volume by the temperature drop to calculate the heat energy delivered to the apartment. This lets owners bill residents for actual heat use on shared heating loops rather than splitting bills by square footage.

**Can heat be submetered in older buildings?**
Heat can be submetered if each unit sits on its own isolated heating pipe. In older buildings with vertical risers passing through multiple units, installing physical meters is rarely practical. For those properties, owners use ratio utility billing (RUBS). RUBS splits the master fuel bill using a formula like square footage or occupant count, without any new equipment.

**Does moving a water heater change the install cost?**
Moving a water heater increases the install cost because of the extra plumbing labor. When a water line is easy to reach, an installer puts in a meter in under 30 minutes. If a tank sits in a tight closet and blocks the pipe, a plumber must drain and move it. That extra work raises the per-unit price above the standard $150 to $300 average.

**Is billing for heat allowed in my state?**
Rules for billing heat and hot water energy vary by state, county, and city. Some places limit it, cap fees or require approval before the first bill. NES serves properties in more than 43 states. Our compliance team verifies the regulations for your exact address before issuing a quote, confirming that your billing program follows all local requirements.