Home / Trade / Hvac / Make-Up Valve Specification

Make-Up Valve Specification

Calculate the required GPM auto-fill rate and PRV valve sizing for hydronic heating systems based on boiler BTUH rating and loop pressure. Includes cold-fill PSI guidance, expansion tank relationship, and valve selection by flow capacity.

Hydronic System

Boiler Input Capacity

BTUH

The maximum firing input capacity listed on the manufacturer nameplate.

System Operating Pressure

PSI

Cold fill 'street' pressure. Standard 2-story homes are typically 12-15 PSI.

Feed Water Specification

Required Feed Rate

1.50GPM

ASME minimum required water replacement flow limit

Recommended Auto-fill Valve

1/2" Type B (Standard Residential)

Email Link Text/SMS WhatsApp

What is Hydronic Make-Up Water: The 1-for-100k Rule, PRV Sizing & Cold-Fill Pressure?

A closed-loop hydronic heating system (radiant floor, baseboard radiation, fan coils) is designed as a sealed, pressurized loop with zero intended water loss. In practice, micro-losses occur through automatic air vents, weeping relief valves, system bleeds, and expansion tank replacement. The pressure-reducing valve (PRV), also called the auto-fill or make-up water valve, automatically replenishes the loop from municipal supply whenever system pressure drops below the cold-fill setpoint. The ASME minimum sizing requirement ensures the valve can restore system pressure fast enough to prevent the boiler from dry-firing — a catastrophic event that destroys heat exchangers in seconds.

Mathematical Foundation

ASME Minimum Make-Up Water Flow Requirement

\text{GPM}_{\text{min}} = \frac{\text{BTUH}_{\text{input}}}{100{,}000}

\text{BTUH}_{\text{input}}

= Boiler gross input rating in BTU/hr, stamped on the boiler data plate. Use INPUT (not output/net). A condensing boiler rated at 180,000 BTUH input / 90% efficiency = 162,000 BTUH output. Use 180,000 in this formula, not 162,000.

100{,}000

= ASME-derived constant: 1 GPM of makeup flow capacity per 100,000 BTUH of burner input. Ensures the valve can introduce enough cold water to prevent dangerous dry-fire conditions even when the boiler is running at full output with a critically low system.

Cold-Fill Pressure Requirement

\text{PSI}_{\text{cold-fill}} = \frac{\text{Height}_{\text{ft}}}{2.31} + 4

\text{Height}_{\text{ft}}

= Vertical rise from the boiler (fill valve) to the highest point in the system: the topmost radiator, coil, or air vent. Each foot of height requires 0.433 PSI to raise water (1 PSI = 2.31 ft of head). Adding 4–5 PSI ensures positive pressure at the top to prevent air ingestion.

\text{Typical values}

= Single-story residential (8 ft rise): ~8 PSI cold fill. Two-story (20 ft rise): ~13–15 PSI. Three-story (32 ft rise): ~18–20 PSI. High-rise systems use booster pumps and zone PRVs. Never set cold-fill above 25 PSI or 10 PSI below the ASME safety relief valve setting (typically 30 PSI residential, lower of the two governs).

Laws & Principles

Valve selection by flow capacity: Standard 1/2” residential PRVs (Bell & Gossett FB-38, Watts 1170B, Caleffi 533) are rated for approximately 8–10 GPM at full flow. This covers boilers up to 800,000–1,000,000 BTUH. For commercial boilers above 1 MMBTU/hr (10 GPM requirement), a 3/4” or 1” PRV is necessary. Never undersize the PRV — a too-small valve allows pressure to drop below the low-limit cutoff and shuts the boiler off on low-pressure lockout, causing no-heat calls.
Expansion tank sizing interaction: The PRV and expansion tank work together as a pressure management pair. The expansion tank absorbs the volume increase as the system water heats from cold-fill temperature (~50°F) to operating temperature (~180°F). Without a correctly sized expansion tank, thermal expansion has nowhere to go, pressure rises, and the relief valve (PRV) opens and spills water. This then causes the fill valve to add fresh makeup water — every heating cycle. Each cycle introduces fresh oxygen and minerals, accelerating corrosion and scale. If a boiler's system relief valve opens regularly, the expansion tank is undersized or waterlogged, not the relief valve faulty.
Backflow preventer requirement: A backflow preventer (check valve rated for potable water protection) is required by most plumbing codes when connecting a hydronic system to municipal water supply. The glycol antifreeze used in many hydronic systems is a Category 4 hazard (toxic contamination) that must be prevented from back-siphoning into the potable supply. Most modern fill valves (Watts, Caleffi) include an integrated dual-check backflow preventer. Confirm local code requirements — some jurisdictions require a reduced-pressure zone (RPZ) device for glycol systems.
Low-pressure lockout and relief valve interaction: Residential boilers are typically set with a 15–30 PSI operating range. Below the low-pressure cutoff (~8–12 PSI), most boilers lock out on a pressure switch. The relief valve is factory-set to 30 PSI (ASME CSD-1 standard) and opens to prevent overpressure. Cold-fill should be set 10–15 PSI below the relief valve setting to provide adequate thermal expansion headroom. Setting cold fill at 20 PSI on a 30 PSI system leaves only 10 PSI of thermal expansion margin — marginal for most systems.

Step-by-Step Example Walkthrough

" Commission a new Weil-McLain Gold Plus boiler rated at 180,000 BTUH input in a two-story house with the highest radiator at 18 ft above the boiler room. "

GPM minimum: 180,000 / 100,000 = 1.8 GPM (standard 1/2” PRV suffices, rated ~10 GPM)
Cold-fill pressure: Height adjustment = 18 / 2.31 = 7.8 PSI
Add 4 PSI safety margin: 7.8 + 4 = 11.8 PSI ≈ 12 PSI cold-fill setpoint
System relief valve: 30 PSI (factory). Margin = 30 − 12 = 18 PSI for thermal expansion
Operating pressure at 180°F: cold-fill 12 PSI + ~8 PSI expansion = ~20 PSI operating (within range)

Final Result: Install a standard 1/2” B&G FB-38 (or equivalent) set to 12 PSI cold-fill. System operates between 12 PSI (cold) and ~20 PSI (hot) — well clear of the 30 PSI relief valve. PRV at 1.8 GPM minimum is dramatically oversized by the 1/2” valve's 10 GPM capacity, providing excellent emergency reserve.

Quick Answer: How do you size a boiler make-up water valve?

Minimum GPM = Boiler input BTUH / 100,000 (ASME 1-for-100k rule). Cold-fill PSI = (Height to highest point ft / 2.31) + 4. Example: 180,000 BTUH boiler, highest radiator 18 ft above boiler: min GPM = 1.8 GPM → standard 1/2” PRV (rated ~10 GPM). Cold-fill = 18/2.31 + 4 ≈ 12 PSI. Set PRV to 12 PSI; system operates 12–20 PSI, clear of the 30 PSI relief valve. Always use input BTUH from the boiler nameplate, not output or net.

Make-Up PRV Valve Sizing Reference by Boiler Input

Select the PRV size based on calculated minimum GPM. Choose the next larger valve size when calculated flow exceeds 70% of a valve’s rated capacity to allow headroom for line pressure variation. Always confirm valve Cv rating at actual supply pressure.

Boiler Input (BTUH)	Min GPM Required	PRV Valve Size	Common Models
≤600,000	≤6 GPM	1/2”	B&G FB-38, Watts 1170B, Caleffi 533 (standard residential)
600,000–1,000,000	6–10 GPM	1/2” high-flow	Watts LF25AUB-Z3, Caleffi 535 (larger Cv body)
1,000,000–1,500,000	10–15 GPM	3/4”	Watts LF25AUB-Z3 3/4”, Caleffi 536, Spirotherm PRV
1,500,000–2,500,000	15–25 GPM	1”	Commercial PRV with integral strainer, manual bypass
>2,500,000	>25 GPM	1-1/4” or larger	Commercial grade with motorized actuator, may require booster pump if street pressure is inadequate
GPM ratings are approximate at 60 PSI supply pressure. Lower supply pressure reduces actual GPM. A backflow preventer reduces pressure slightly (−1–3 PSI drop). For glycol systems, verify backflow preventer is rated for the glycol type. Most jurisdictions require a backflow preventer when connecting to municipal supply.

Pro Tips & Common Hydronic Make-Up Water Mistakes

Do This

✓Install a water meter on the make-up water line to detect system leaks before they become emergencies. A correctly sealed hydronic system should consume near-zero makeup water after initial commissioning and bleeding. A digital totalizing flow meter on the fill loop will show exactly how many gallons have been added since installation. More than 1–2 gallons per week on a residential system indicates an active leak or a weeping relief valve. Without a meter, small leaks go undetected for months — the system silently adds fresh oxygenated water, accelerating internal corrosion, until a component fails. Low-cost paddle-wheel digital meters ($40–80) make this monitoring trivial.
✓Verify the expansion tank pre-charge pressure matches cold-fill setpoint before placing the system in service. An expansion tank (bladder or diaphragm type) shipped from the factory is pre-charged to 12 PSI nitrogen. If you set cold-fill at 15 PSI, the tank pre-charge must also be adjusted to 15 PSI (deflate slightly with a Schrader valve). If cold-fill exceeds the tank pre-charge, the system pressure immediately compresses the bladder to nearly full and no expansion capacity is available. The system will then push water into the relief valve on every heating cycle. Confirm the tank pre-charge with a tire gauge (bladder side, system drained) and adjust to match cold-fill setpoint before filling.

Avoid This

✗Don't set cold-fill pressure above 25 PSI on a system with a 30 PSI relief valve — it will open immediately on each heating cycle. The 30 PSI ASME relief valve is a safety device, not a system regulator. It opens at 30 PSI to prevent vessel rupture and should open only in emergency overpressure events. Systems set with inadequate cold-fill-to-relief margin (less than 10 PSI) will regularly see the relief valve drip during normal heating cycles as system pressure rises from thermal expansion. Each time the relief valve weeps, it releases water and a fresh, oxygenated cold makeup charge enters through the PRV. This cycle accelerates system corrosion dramatically. The relief valve also becomes prone to calcium deposits and may fail to reseat properly.
✗Don't use boiler output (net) BTUH to calculate minimum GPM — always use the gross input rating. Gross input is the total burner firing rate. Net output (DOE output or IBR net rating) subtracts the jacket losses and is the heat actually delivered to the building. On a condensing boiler at 95% efficiency, the difference is small (180,000 input vs 171,000 output). On a cast iron boiler at 80% efficiency, it’s significant (180,000 input vs 144,000 output). The ASME 1-for-100k rule is based on gross input because in an emergency low-water event, the burner fires at full gross rate and the makeup valve must keep pace with the full burner heat input to prevent dry-fire. Using net output would undersize the valve by 5–20%.

Frequently Asked Questions

Why does the boiler relief valve keep dripping after installation?

A relief valve that drips on every heating cycle is almost never caused by a faulty relief valve (though the valve may eventually fail to reseat after repeated opening). The root causes are: (1) Cold-fill pressure too high: If cold-fill is set at 20 PSI on a 30 PSI system, only 10 PSI of thermal expansion headroom exists. Water expanding from 50°F to 180°F increases volume by approximately 3.5% — enough to push a mid-sized system above 30 PSI without adequate expansion capacity. (2) Undersized or waterlogged expansion tank: A bladder tank that has lost its nitrogen pre-charge or is too small for the system volume can't absorb thermal expansion. Test the tank with system pressure drained: if no air pressure is felt at the Schrader valve, the bladder has ruptured (tank full of water) and must be replaced. (3) Closed or throttled system valve: A closed zone or isolation valve trapping a water column during expansion. Check all zone valves are open before troubleshooting expansion issues.

What is the correct cold-fill pressure for a multi-story system?

Cold-fill must overcome the static head of water from the PRV to the system’s highest point. Formula: PSI = (Height in feet / 2.31) + 4 to 5 PSI buffer. Real-world examples: Single-story, 8 ft rise: (8/2.31) + 4 = 7.5–8 PSI. Two-story, 22 ft rise: (22/2.31) + 4 = 13.5 PSI. Three-story, 34 ft rise: (34/2.31) + 4 = 18.7–19 PSI. Four-story, 46 ft rise: (46/2.31) + 4 = 24 PSI. At four stories on a 30 PSI relief system, cold-fill at 24 PSI leaves only 6 PSI for thermal expansion — the system may need a 45 PSI or higher rated boiler and relief valve, or a separate booster zone. For buildings above ~6 stories, zone PRVs at each floor level are standard practice — the boiler loop runs at high pressure; individual zone PRVs step it down to a safe range for each floor zone.

Does a glycol antifreeze system require a different make-up valve?

Yes — glycol systems require several additional considerations: (1) Backflow protection: Glycol (propylene glycol for food-safe, ethylene glycol for industrial) is a Category 4 cross-connection hazard. An RPZ (reduced-pressure zone) backflow preventer is required by most codes when connecting a glycol system to potable water, not just a simple check valve. Confirm local code. (2) Manual make-up instead of automatic: Auto-fill valves will dilute the glycol solution with plain water when the system is low. In a freeze-protection system, diluting the glycol concentration below the design freeze point defeats the purpose. Many glycol systems use a manual fill valve (closed normally) and a dedicated glycol/water blend tank maintained at the correct ratio. (3) Expansion volume increase: Glycol-water solutions expand more than pure water on heating (slightly higher coefficient of thermal expansion). Size the expansion tank 10–15% larger than a water-only calculation for a 50/50 glycol mix.

What is the difference between a PRV (auto-fill) and a pressure relief valve?

Despite sharing a name abbreviation (“PRV”), these are opposite functions: Pressure Reducing Valve (PRV / auto-fill): Adds water to the system when pressure drops below the setpoint. Steps down high street supply pressure (60–80 PSI) to a low, controlled hydronic pressure (12–20 PSI). Normally open (flowing) only during makeup events. Located on the cold-water supply line entering the system. Pressure Relief Valve (safety relief valve): Removes water from the system (emergency discharge) when pressure exceeds the safety setpoint (typically 30 PSI residential, 125 PSI commercial per ASME). This is a one-way emergency safety valve. It must be tested annually (lift testing) and replaced every 6–10 years whether it has activated or not. Located on the boiler vessel above the water line. Both are required on every properly designed hydronic system. They address opposite failure modes: the PRV prevents low-pressure dry-fire; the relief valve prevents overpressure vessel rupture.