How to Install a Hot-Water Heating System

A hot-water heating system installs by sizing the load, routing piping, setting the boiler, and commissioning with code-safe checks.

This guide walks you through the full process, from planning to first heat. You’ll see what to buy, where parts go, and how to start the system safely. The steps apply to a one- to two-story home with baseboard, panel radiators, or radiant loops. Always follow the appliance manual and your local mechanical code.

Plan The Layout And Load

Start with a heat-loss estimate. That tells you the boiler size and the flow each room needs. Use outdoor design temperature for your region, room dimensions, insulation levels, and window details. With a total BTU load, you can right-size the boiler and circulators instead of guessing.

Now sketch the system: boiler room location, near-boiler piping, main supply and return, branch runs to emitters, and where you’ll place valves, air removal, and the expansion tank. Keep the boiler near an outside wall with a clear vent path. Leave service space on all sides.

Core Parts You’ll Need

Pick matched components that fit the design. Use oxygen-barrier PEX for radiant loops, copper or PEX-AL-PEX for mains, and quality valves at every branch. A dirt/air separator keeps the water clean and quiet. A diaphragm expansion tank handles thermal growth. A 30-psi relief valve protects the closed system. Add shutoffs so each zone and device can be isolated.

Hydronic Components And Roles

Component	What It Does	Typical Location
Boiler (Gas/Oil/Electric)	Heats system water to setpoint	Mechanical room near vent and drain
Circulars/Zone Valves	Move water through zones	Near boiler or zone manifolds
Expansion Tank (Diaphragm)	Absorbs thermal expansion	Near air separator on the suction side of the pump
Air Separator/Vent	Removes micro-bubbles and trapped air	At hottest, lowest-pressure point on supply
Pressure Relief Valve	Opens if system pressure spikes	On boiler or supply per code
Backflow/Fill Valve	Feeds water and prevents reverse flow	Cold water makeup to return side
Isolation/Purge Valves	Service and air removal	Each zone return and near boiler
Emitters (Baseboard/Radiators)	Deliver heat to rooms	Along walls or radiant floors
Controls/Thermostats	Calls for heat and sets temps	Living spaces and boiler panel

Choose And Size The Boiler

Match the appliance to the building load, not the old nameplate. Oversizing leads to short cycles and noise. Pick a model with AFUE that meets your target and local incentives. Check venting type (chimney, direct-vent, or condensing sidewall) and the available gas or electrical service. High-efficiency units need proper condensate drainage and neutralization.

Before you order, verify that the heat exchanger output and pump curve can deliver design flow at the planned temperature drop. Manufacturers publish pump head and required flow for each boiler and heat emitter type.

For general guidance on efficiency ratings and boiler types, see the U.S. Department of Energy’s page on furnaces and boilers.

Installing A Hydronic Hot-Water System: Room-By-Room Plan

Break the house into zones that mirror real use: bedrooms, living areas, and basement loops. Long loops need balancing valves or home-run manifolds with flow meters. Keep loop lengths within the pump’s capability, and use the same tube size within a loop for even flow.

Emitter Layout Tips

Baseboard: place on exterior walls where heat loss is highest; leave clearance above for airflow.
Panel radiators: mount level, add thermostatic radiator valves (TRVs) for room control.
Radiant floors: maintain staple spacing per design; protect tubing at transitions and doorways.

Piping Rules That Save Time

Run supply and return as straight as the framing allows. Use sweep elbows, avoid tight bends, and strap PEX at regular intervals. Keep the air separator on the supply near the boiler outlet. Mount the expansion tank under the separator on the pump suction side (the “point of no pressure change”) to keep air quiet and bleeding simple. This placement matches common manufacturer guidance for air control and tank connection.

Place And Pipe The Boiler

Set the unit on a level pad. Confirm clearances for service doors, venting, and combustion air. Dry-fit vent sections before cementing or clamping. Slope condensate lines to a neutralizer and drain. On the hydronic side, assemble the near-boiler piping with unions at key devices for service.

Near-Boiler Assembly Order

Boiler supply outlet to air separator, with an automatic vent on top.
Tees to system supply manifold and to a bypass or hydraulic separator if used.
System pump(s) on the supply or return per the appliance manual, with isolation flanges.
Expansion tank under the separator; add a ball valve and drain for service.
Makeup water with a pressure-reducing fill valve and backflow preventer tied to the return.
Relief valve and discharge to a safe drain path as directed by code and the boiler manual.

Use dielectric unions when joining dissimilar metals. Label zones and valves. Support vertical risers so valves aren’t carrying the weight. On PEX, keep lines away from flue pipes and use sleeves through studs and plates.

Wire Controls And Thermostats

Low-voltage wiring ties the thermostats to zone valves or pump relays. Keep thermostat cables and 120-volt conductors in separate runs. Follow the boiler control schematic for end switches and safety circuits. Add outdoor reset if the boiler supports it; lower supply temperature on mild days cuts cycling and noise. Mount the outdoor sensor on a north or northwest wall out of sun and vents.

Safety Devices You Should Never Skip

Boiler limit controls and low-water cutoff where required by the appliance or code.
Backflow prevention on the fill line.
At least one safety relief valve on the hot-water boiler, sized and set per the rating plate.

For code language on testing hydronic piping and relief protection, review the IRC section on hydronic systems and testing found in the Hydronic Piping chapter.

Fill, Purge, And Pressurize

Close all isolation valves, then open one zone return at a time for purging. Use a hose at the purge valve to a drain bucket. Open the fill valve and push fresh water through the loop until bubbles stop and flow runs clear. Close the purge, then move to the next zone. Keep the boiler off during purging.

Cold pressure for a two-story home often lands near 12–15 psi; taller stacks may need a bit more. Pre-charge the expansion tank to match the cold system pressure before filling. With air removed and valves open, check all joints for weeps.

Leak And Pressure Test

Many regions require a hydrostatic test before the first fire. Use a test pump and gauge, and hold the pressure for the time your jurisdiction lists. Fix any drip before you proceed. This step proves joints and protects finishes later.

Fire Up And Balance

Power the boiler and follow the start-up steps in the manual. Set the high limit and temperature curve if outdoor reset is present. Verify that venting is tight and combustion air is clear. Watch for stable flame on gas models and steady current draw on pumps.

Now balance flow. Open all valves, bring the system to temperature, then check supply and return on each loop. Aim for a reasonable temperature drop through each emitter bank. Fine-tune with balancing valves so rooms heat evenly and pumps don’t cavitate. Bleed any panel radiators at their vents until quiet.

Before you leave the boiler room, test the home’s CO alarms. The U.S. Consumer Product Safety Commission recommends alarms on each level and near sleeping areas; see its guidance on carbon monoxide safety.

Set Controls For Comfort And Efficiency

Dial in outdoor reset so supply temperature tracks the weather. Lower water temperature extends run time and smooths room swings. If baseboard can’t meet load at the curve you pick, raise the slope a touch. If radiators click or harbor air, nudge the air separator vent and go through a short purge again.

Thermostat Placement Tips

Mount on an interior wall, away from direct sun and supply grills.
Keep cables away from line voltage to avoid interference.
Use TRVs to trim room-by-room comfort without rewiring.

Commissioning Readings And Targets

Log the first-day readings so you can spot drifts later. A simple table keeps service visits short and clear.

Reading	Typical Target	Notes
Cold System Pressure	12–15 psi (two-story)	Match tank pre-charge to cold pressure
Relief Valve Rating	30 psi	Pipe discharge to a safe location
Supply Temperature	140–180°F (baseboard)	Lower with outdoor reset when load allows
ΔT Across Loop	10–20°F	Balance valves to even room heat
Air Separator Vent	No spitting once hot	Crack until air clears, then close if manual
CO Alarm Test	All units respond	Press test, replace batteries on schedule

Permits, Codes, And Inspections

Most areas require a permit for a boiler swap or a new hydronic install. Plan for a rough inspection before closing walls and a final check at start-up. Relief valves, discharge piping, makeup water protection, and hydrostatic testing all have specific rules. Your inspector will reference the local adoption of the residential mechanical code and any amendments. Reading the hydronic chapter ahead of time keeps red tags off your job; see the IRC’s hydronic piping test section for an example of the test method and pressure.

Troubleshooting Common Start-Up Issues

Noisy Baseboard Or Radiators

Popping or rushing sounds usually point to air. Vent at the highest points and confirm the air separator sits on the supply near the hottest outlet. If noise remains, lower the pump speed a step and retest.

Short Cycling

Oversized input or high water temperature can cause rapid on/off. Lower the setpoint or reset curve, add a buffer tank if the load is tiny, and verify thermostat wiring so you don’t stack calls.

Uneven Rooms

Check flow first. Tighten the fastest loop with its balancing valve. Verify that doors and area rugs aren’t blocking baseboard convection. On panel radiators, set TRVs to the same number to start, then fine-tune one click at a time.

Care And Upgrades

Annual service keeps bills steady and parts clean. Tasks include combustion checks, vent inspection, expansion tank test, relief valve lift test if allowed by the manual, and a quick system water check. Sludge or magnetite calls for a dirt separator or magnetic filter.

Simple upgrades bring gains without major plumbing. Outdoor reset, smarter circulators with ECM motors, and TRVs raise comfort while trimming energy use. For broader background on tuning central hydronics, the Department of Energy offers notes on optimizing hydronic heating.

Step-By-Step Summary You Can Follow

Before Materials Arrive

Run a room-by-room heat-loss calc and pick the boiler size to match.
Draw the zone map, pipe routes, and device locations.
Pull permits and confirm vent paths and clearances.

During Rough-In

Set the boiler and dry-fit venting and condensate lines.
Mount the air separator and tank at the pump suction side.
Pipe mains and branches with isolation and purge valves.
Wire thermostats, pump relays, and safeties per the schematic.

Before First Fire

Pre-charge the tank, fill the system, purge each loop, and set cold pressure.
Hydrostatic test if required, then fix any leaks.
Confirm relief valve size and discharge routing.

Start-Up And Tuning

Set limits and outdoor reset; verify venting and combustion air.
Bleed emitters, balance flows, and log temperatures and pressure.
Test CO alarms and label service valves and zones.

Safety Notes You Should Post Near The Boiler

Keep flammables out of the room.
Do not cap or plug a relief discharge.
Replace the expansion tank if the air side loses charge.
Service the appliance each year; the CPSC urges yearly checks for fuel-burning equipment and working CO alarms on each level.

When To Call A Pro

Gas piping, vent sizing, and combustion tuning carry real risk if done wrong. If you lack test gear for combustion or you’re unsure about code paths for vents and drains, bring in a licensed contractor for those parts. You can still handle layout, emitters, and trim work to keep the project on budget.

Materials Checklist

Use this as a shopping pass before you start. Adjust sizes to match your plan and local code.

Boiler with vent kit and condensate parts where needed
Circulators or zone valves with isolation flanges
Air separator with automatic or manual vent
Diaphragm expansion tank with service valve and drain
30-psi relief valve and discharge piping
Pressure-reducing fill valve and backflow preventer
Isolation and purge valves for each zone
Oxygen-barrier PEX or copper, fittings, hangers, and sleeves
Thermostats, cable, and relay panels
Pipe insulation for all accessible mains
Combustion air kit and vent terminations
Drain hose, bucket, and gauges for start-up

Final Pass

Once heat is stable, label everything, snap photos of the near-boiler piping and the open panel, and store a printed start-up sheet by the unit. That single step saves hours on future service calls and keeps the system running quietly through the season.