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Steam radiator (C) Daniel FriedmanHeating Boiler Radiator Volume, Sizing, BTUs & Capacities

Heating system radiator sizing, volume, capacity, and BTUs: this article explains How to calculate the internal volume of a hot water or steam radiator. How many BTUs are provided by radiators of different types & sizes?

Types of heating radiators used in buildings. Basic Radiator Water Volume vs BTU Output Data: how much water is required per BTU of heating radiator output. Basic temperature assumptions for water and air temperatures when sizing heating radiators.

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How much water is in a heating radiator? Radiator Volume Calculations

Plate warming home heating radiator (C) Daniel FriedmanReader Question: 6 Oct 2014 Doug said:

We want to add propylene glycol to our hot water heating system and are trying to calculate the amount of water in the system in order to calculate how much antifreeze is required.

How do you calculate the amount of fluid a radiator holds. We have 28 radiators of varying sizes! Thanks for your help

Reply:

Doug,

In the Continue Reading at link at the end of this article please see

ANTIFREEZE for BOILERS for advice on how to add antifreeze to a hot water heating system.

You don't really want to try calculating radiator volume. It's ugly, as you'll see in the article below and as other readers will imagine just looking at the kitchen plate warmer radiator shown in our photo at left (next to the attractive real estate agent).

Since I too found it almost impossible to find radiator volume data or calculations among the various radiator manufacturers (for reasons we explain in this article) we'll give some approaches to finding radiator internal volumes and capacities as well as BTU data here.

The volume of water contained in a hot water heating radiator depends on its internal volume - something that’s not commonly given since heating equipment manufacturers, engineers, designers, and installers are more interested in the BTUh output of the radiator or possibly its surface radiating area.

And as you point out, radiator water volume cannot be easily measured from looking at the outside of the radiator, as its surfaces and shapes vary so widely.

Heating convector wall recessed (C) Daniel FriedmanIn contrast, calculating the volume of water in a heating baseboard system using finned copper tubing would be much easier - since we have a known and consistent shape, a simple cylinder of a given internal diameter and length.

For details and examples of calculating the volume of a cylinder such as a well casing or a pipe see WELL DYNAMIC HEAD & STATIC HEAD DEFINITION

There we see that for a simple round pipe, its internal volume is calculated as:

Vcyl inches = 3.1416 x r2 inches x h inches

If you are determined to try calculating the volume represented by your radiators I’d suggest making a guesstimate of the equivalent round cylindrical diameter and height of each of the vertical tubes that make up each radiator section, then multiply that by the number of sections to obtain the volume for that radiator.

For example, for most old large cast-iron radiators you could guesstimate at 1.25 inches of equivalent internal diameter.

Determining How Much Antifreeze to Add to a Hot Water Heating System Without Measuring Water Volume

I would not take the approach you describe: calculating radiator and piping volumes (to which you must add the volume of every other component of the heating system: valves, expansion tank, boiler itself, flow controls, etc. You’re making a horribly difficult task that probably won’t give the correct answer anyway.

Instead I’d do what a heating service technician does when adding antifreeze to the system to get the right quantity of antifreeze in the mix. We describe this procedure in complete detail
at ANTIFREEZE for BOILERS - here is an overview:

  1. Find or make appropriate plumbing connections to allow insertion of a water-antifreeze mix into the boiler. This means connecting a pony pump output to a boiler input valve such as a heating zone drain valve, connecting the pony pump input to a hose immersed in a bucket of water-antifreeze mix, and also connecting a boiler drain or equivalent to a hose whose output end is kept below the water level in the bucket (to avoid introducing air into the heating system).
  2. Start the pony pump circulating water through the entire heating system. Be sure any other boiler controls or valves that need to be open are in the open position.
  3. Measure the antifreeze protection level of the water in the bucket.
  4. Add antifreeze to the bucket as needed, allow that mix to circulate through and mix with the water in the hot water (hydronic) heating system loop and boiler.
  5. When the desired freeze protection level is indicated on your antifreeze mixture tester (get a simple test tool available from your heating supplier or your automotive supplier) stop adding antifreeze.
  6. Circulate the water for a time to be sure that the antifreeze is thoroughly mixed and double check the antifreeze level.
  7. If you messed up and got air into the system piping you’ll need to remove it to avoid air bound radiators or piping.

    See AIR-BOUND HEATING SYSTEMS

Heating Radiator Types & Capacities

How much water is contained in a heating radiator? Typically plumbers and installers don’t care and don’t think about water volume since knowing the water volume is not going to give a useful answer to the heating system’s output or capacity anyway.

Why not?

Because the radiator output in BTUs per hour depends not just on the water volume in the radiator but also on the water temperature, flow rate, the surface radiating area of the radiator (see EDR below), the surrounding air temperature, air movement across the radiator surfaces (affected for example by radiator enclosures, furniture location etc).

But for people who ask about radiator water volume anyway, here’s the data with some help from Fernox and Columbia (references below).

Basic Radiator Water Volume vs BTU or KW Heat Output Data

For these volumes, the assumed water temperatures, flow rates air temperature assumptions and other factors are discussed below.

Basic definitions & Concepts for Residential Heating Radiators

Hot water radiators are (for purposes of assessing the radiator’s heat output) assumed to be operating with 180 degree water, and of cast iron construction (or of finned copper tubing) have a heat output rating that assumes 70 degree surrounding air.

The heat output of a radiator is a function of the radiator’s surface area, the temperature of the water in the radiator, and the temperature of surrounding air (greater difference between air and radiator temperature means faster heat transfer into the air).

Steam radiators in most residential systems operate using low pressure steam at under 1 psi and at temperatures as high as about 215F.

The heat output of a radiator in BTUs per hour increases linearly as a function of the water or steam temperature inside the radiator (if we assume that the air temperature outside the radiator is for this analysis kept constant at say 70F). Most hot water or hydronic radiators are assumed to operate at about 170 BTUh per square foot of Equivalent Direct Radiation or EDR (in essence the square foot area of the radiator’s heat emitting surface).

Heat increase or decrease with different radiator cover designs (C) InspectAPedia & ITTThe heat output of an entire heating system in a building is then calculated by multiplying the total square feet of EDR provided by all of the radiators by 170 BTUh (for hot water heat) or by 240 BTUh (for steam heat).

Watch out: this heat capacity calculation describes the heat delivery rate to the building in BTUs per hour but does not account for the building heat loss rate, a figure that will vary widely among buildings depending on outside temperatures, wind, building insulation, building air leaks, number of windows, even building shape and other factors.

Our sketch at left illustrates the impact on heating radiator BTU output depending on the type of radiator cover that might be installed. This is just one of the factors that impact the BTU output of a building heating radiator - which is why just using standard radiator sizing tables can be misleading.

Typical Radiator Sizes

Radiators in older homes were and still are designed as column type radiators (older radiators with columns up to 2.5” wide), more recent tube type radiators (sections are about 1.5” wide), and wall type radiators. Column and tube radiators vary in height (typically about 13” to 45” high) and in width (depending on the number of radiator sections that have been connected together).

For a heating radiator sizing chart that will help choose the number, size, and location of heating radiators see this PDF: RADIATOR SIZING & RADIATION TABLES from UTICA Boilers

References for Heating Radiator Volume Content Calculations & Sizing

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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2021-11-14 by (mod) - US gallons-to-liters vs Imperial Gallons-to liters

Thank you for the careful reading, Shalom, we've edited the article to make clear both US Gallons / liters and Imperial or Canadian Gallons / liters.

We appreciate that you took the time to write: working together makes us more-accurate.

On 2021-11-14 by Shalom

You've got your conversion wrong for gallons. You say "for North Americans" it's 4.546 liters per gallon, but that's only north of the border in Canada, where they use Imperial gallons; in the USA, it's 3.785 liters per gallon.

On 2018-11-03 by (mod)

Lars

Thank you very much for a most-helpful real-world example of heating system volumes and freeze protection. I'll be sure to keep your comment with this article.

Daniel

On 2018-11-03 by Lars Bagman - real world example of BTU output vs radiator and boiler hot water volume - freeze protection

One data point on cast iron radiator hot water heating system volume, I have a 2,000 sq ft 2 story home built circa 1940 and recently drained the system to temporarily disconnect one first floor radiator for later relocation after a room addition is completed.

I have 11 cast iron radiators, all column type 1" thick by 5" wide by 24" tall, one 6 ft long, a couple 4 ft long, most others are 3 ft long with a couple 2 ft long in the bathrooms.

Since the system hot water is treated with corrosion inhibitors that will last a very long time in the relatively mild conditions of use, I decided to collect and recycle the treated water. The volume collected was ~60 gallons but this does not include the basement level system which includes a vintage Bethlehem Dynatherm 48 inch 2 pass fire tube boiler converted for hot water operation (recently upgraded with a modern flame retention burner with electronic controls, 91% Eff.)

The boiler is 78 years old and with proper maintenance should last beyond my lifetime.

The volume within the boiler itself is specified by the manufacturer as 52 gallons. Allowing another 8 gallons or so for the un-drained basement level distribution piping would give a total system water volume of approximately 120 gallons which should be close enough for estimating the amount of antifreeze needed. After all, this in not anything close to prescription medications where a few milligrams would make a significant difference.

In my situation I would need to replace 35% of the 120 gallons of water (42 gallons of propylene gylcol) to lower the freezing point to +3F, 45% (54 gallons) to lower the freezing point to -15F, or 50% (60 gallons) to lower the freezing point to -20F. I would just go ahead and do the 60 gallons.

You could buy test equipment to be sure, but I wouldn't bother. How certain can you be that you will experience a low temperature of -10F but not -15F?

The indoor temperature will lag behind a temporary drop in outdoor temperatures so a temporary overnight -25F outdoor temperature would not be expected to drop the indoor temperature below the 24 hour average, unless you leave the windows open. In a very cold climate where the temperature dropped to -25F and stayed there for a few days, the indoor temperature would be pretty close to the outdoor temperature.


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