HOT-WATER HEATING, EXPLAINED
Understand what
the numbers mean.
A boiler heats water. A pump moves it through your home. These examples show how that water carries heat, how antifreeze gets diluted, and why valves need pressure to move water through them.
LESSON 4 / PUT THE NUMBERS TO WORK
Work through the numbers, then try your own.
Practise heat delivery, mixture changes and valve pressure loss. Each case supplies the values and assumptions, explains the calculation and links to the calculator.
A cold room: how much heat is the water carrying?
The question: does the measured water flow carry the room’s calculated heating requirement at this temperature drop?
- Calculated room requirement
- 12,000 Btu per hour — the heating power needed at the outdoor temperature used in the heating design.
- Water at the room heater
- 130°F entering, 110°F leaving: a drop of 20°F. Measurements are taken while operation is steady.
- Measured flow
- 0.8 US gallons per minute — less than one gallon passes through each minute.
Work through it
For water, approximately 500 × gallons per minute × temperature drop = Btu per hour carried. The 500 combines the weight and heat capacity of water with minutes per hour.
500 × 0.8 × 20 = 8,000 Btu per hour
That is 4,000 Btu per hour below the stated 12,000 requirement. At the same 20°F drop, carrying 12,000 would take 12,000 ÷ (500 × 20) = 1.2 gallons per minute.
Read the result: the water is delivering about 8,000 Btu per hour, compared with the room’s stated need of 12,000. That shortfall explains why this room struggles under the example conditions.
Apply it to the room: compare the measured heat delivery with the room’s heating need. Then check circulation and the radiator’s output. Changing flow also affects the temperature drop, so remeasure both when evaluating an adjustment.
Use “Heat carried” for measured operation and “Water flow needed” for the comparison. Source: Caleffi, circulation in hydronic systems (PDF) ↗
After a leak: how much antifreeze mixture must change?
The question: a repaired system holds 220 gallons, now measured at 20% glycol by volume. Its specified target in this example is 40%. How much evenly mixed liquid must be removed and replaced?
What the percentages mean: glycol is the antifreeze ingredient. A 20% concentration means 20 gallons of glycol in every 100 gallons of mixture. The example replacement is 100% glycol; an actual product’s strength must come from its data.
Work through it
Each exchanged gallon removes 0.2 gallon of glycol and adds 1 gallon. That increases glycol by 0.8 gallon while keeping total volume the same.
220 × (40 − 20) ÷ (100 − 20) = 55 gallons exchanged
Check the result: the original 44 gallons of glycol lose 11 gallons when 55 gallons of 20% mixture are removed. Adding 55 gallons of pure glycol gives 88 gallons in 220: 40%.
Try a weaker replacement: using a 50% premix would require exchanging about 146.67 gallons. Each gallon adds less glycol, so more of the original mixture must be replaced.
Finish the job: once the leak is repaired, use a compatible heating fluid at its labeled strength. Circulate the adjusted mixture and retest its concentration, freezing protection and corrosion-protection chemicals.
Amounts use the classroom example supplied for this guide. Concentrations are by volume. Dow: fluid concentration and product-property tools ↗
A restrictive valve: how much pressure does it use?
The question: a design calls for 15 gallons of water per minute through a valve. Its manufacturer lists Cv 7 at the intended opening. How much pressure difference is needed across this valve?
Cv explained: this number describes how easily water passes. Cv 7 means seven US gallons per minute of reference water gives a one-psi pressure drop. Psi means pounds per square inch. A larger Cv means less resistance at the same flow.
Work through it
Water’s specific gravity — its weight compared with an equal volume of reference water — is approximately 1 here. Divide flow by Cv, multiply the answer by itself, then multiply by specific gravity.
1 × (15 ÷ 7)² = 4.59 psi
Pump charts often show pressure difference as feet of head, an equivalent column of the pumped liquid. For this water example, 4.59 psi is about 10.61 feet of head.
Compare one change: at the same 15 gallons per minute, a valve with Cv 14 needs about 1.15 psi, or 2.65 feet of head. Twice the Cv gives one quarter of this pressure loss.
Apply it to pump selection: include the resistance of the rest of this circuit, then compare the total at the intended flow with the pump chart. Changing the valve changes circuit resistance, so the actual flow may change too.
01 / WATER CARRIES HEAT TO THE ROOM
How much water
moves the heat?
What the result means & how it is calculated
Water gives up heat as it cools. If each gallon gives up more heat, fewer gallons per minute can carry the same heating amount. The homeowner example holds that amount at 30,000 Btu per hour; Btu is a unit of heat, so Btu per hour describes heating power.
Heat = fluid factor × flow × temperature drop. The fluid factor accounts for how much a gallon weighs and how much heat it holds. We use approximately 500 for water. With product data, it is 60 × density (pounds per US gallon) × specific heat (Btu per pound per °F).
The answer describes heat carried by the water. The room still needs floor tubing, a radiator or another heater that can release that heat. Choosing a pump also requires the resistance of the pipes and parts, plus the pump manufacturer’s performance chart.
Source: Caleffi’s circulation guide (PDF) ↗02 / GLYCOL IS THE ANTIFREEZE INGREDIENT
What happens when
you add plain water?
What the result means & how it is calculated
Fresh mix: product gallons = final gallons × wanted glycol percentage ÷ product glycol percentage. Add enough water to reach the final total.
Changing a mixture: exchange gallons = system gallons × (wanted % − current %) ÷ (replacement % − current %). “Exchange” means removing that amount of mixed fluid and replacing it with the same amount of the chosen liquid.
The calculation assumes the starting fluid is evenly mixed and the added volumes combine without changing total volume. Use the product data and fluid tests to check freezing temperature and corrosion protection. Fix leaks and verify the mixture using the fluid maker’s test instructions.
Source: Dow’s heating-fluid information ↗03 / THE PUMP HAS TO PUSH THROUGH THE VALVE
Why does faster flow
need more pressure?
What the result means & how it is calculated
Flow is the volume moving each minute. Pressure drop is the pressure difference from the valve’s inlet to its outlet at that flow. The pump provides that difference.
Cv is a number from the valve maker that describes how easily water passes through it. A larger Cv means less resistance at the same flow. This homeowner example uses Cv 7: seven US gallons per minute of reference-temperature water produces a 1 psi drop.
SG (specific gravity) compares a liquid’s density with water. Water is about 1. The formula is: pressure drop in psi = SG × (gallons per minute ÷ Cv)². Squaring means multiplying that number by itself.
Head in feet expresses pressure as the height of a column of the pumped liquid. Pump performance charts often use it. Head ≈ 2.31 × psi ÷ SG.
The formula applies while the liquid stays liquid through the valve. Thick glycol adds resistance that this simple formula may underestimate, especially when cold. The valve maker’s fluid corrections help account for that extra resistance.
Source: Caleffi’s explanation of valves and pressure ↗Explore the examples or use Pro mode to enter your own values. Each field explains the measurement it needs.
CONNECT THE NUMBERS TO THE SYSTEM
Keep exploring.
CHECK YOUR UNDERSTANDING
Can you explain it?
How does the valve-loss calculation help with pump selection?
Try answering in your own words, then compare your reasoning.
Show the answer & why
It gives one part of the resistance along a heating circuit. Add the losses through the pipes and other parts on that same route at the intended flow, then compare the total with the pump chart. Branches running alongside one another are evaluated as parallel paths.