Antifreeze installationinto a heating boiler Heating Boiler Anti-Freeze Installation

  • ANTIFREEZE for BOILERS - CONTENTS: How & Why to add antifreeze to a boiler for freeze protection. What Kind of Anti-Freeze Is Used in Hot Water Heating Systems. How Much Antifreeze is Needed for a Heating Boiler? How to Measure Anti-Freeze Protection Level in a Heating Boiler. Effects of Anti-Freeze on the Thermal Conductivity of Water & on Building Heating Costs. Do I Need to Drain the Heating Boiler to Add Antifreeze? Do I need to flush & clean the Heating Boiler Before Adding Antifreeze? Where to Buy Heating Boiler Antifreeze & Anti-Freeze Hydrometers or Refractometers. Annual Maintenance Recommendations for Anti-Freeze Protected Heating Boilers & Systems
  • POST a QUESTION or READ FAQs about the procedure for adding antifreeze to a hot water heating system & its boiler

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Heating boiler antifreeze procedure:

This article describes how to add anti-freeze to a heating boiler and to the hot water or hydronic heating system piping, baseboards, convectors, radiators, etc. to protect the system from freeze damage.

We describe the actual procedure for adding anti-freeze to the system, and we answer questions about how much anti-freeze to add and what type of anti-freeze to use.

This article also discusses the risk of poisoning the building water supply piping if you use the wrong type of boiler antifreeze, and we explain the effects of using anti-freeze in a heating system on the thermal conductivity of water and on building heating system efficiency and operating cost.

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How & Why do You Add Antifreeze to a Boiler?

Input feed to a heating boiler riser pipe (C) Daniel Friedman

Heating boiler antifreeze, usually a non-toxic polypropylene glycol anti-freeze and water mixture is installed in hot water heating boilers and plumbing systems to reduce the risk of expensive freeze damage to the heating system piping and of course to avoid costly water or even mold contamination that occurs when an unattended building suffers burst heating piping and leaks.

Even if a heating system is in good and reliable working order, loss of electrical power or running out of fuel means loss of heat that in freezing weather means burst pipes and water damage to the building.

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Reader Question: How do you add anti freeze to you boiler and what kind do you use? - Don Rose 9/26/2012


Don, some help on winterizing a boiler as well as a whole building is at Winterize- Heat Off Procedure.

The basic procedure is to use a pony pump (a transfer pump) to add antifreeze (buy at your local heating supplier) to the water mix in the boiler until our test gauge shows that we've got enough antifreeze in the system to protect down to at least a bit below the lowest expected temperature.

Heating boiler drain (C) Daniel Friedman

The service tech uses a pony pump connected to a boiler drain or zone drain valve, typically using a pair of washing machine hoses to form a loop between a boiler drain, a bucked of antifreeze & water mix, and a zone drain.

Hose & Pump Connections for Adding Anti-Freeze to a Boiler

Typically one washer hose connects to a zone drain or to a drain on the hot water riser out of the boiler (our photo at above left)

A second washer hose connects to a boiler drain (photo at left).

The output side of the transfer pump is connected to one of these hoses; the other hose drains into a bucket from which we can use a hygrometer or refractometer to measure the antifreeze concentration in the heating system's water (now water-antifreeze mix).

The inlet side of the transfer pump is connected by a pick-up hose to an open bucket of water/antifreeze mix or more pure antifreeze supply.

The pump pushes antifreeze into the system from the bucket; keeping the drain end of the second hose below the level of antifreeze in the bucket avoids introducing air into the system and thus avoids having to bleed that air out.

Exact transfer pump hookup points and step by step procedures: Since the transfer pump hose connections are the same those used when forcing water through a hydronic system to correct an airbound heating system problem, you can see another example of this transfer pump to boiler hook-up

Little Giant pony pump

Watch out: if you are going to perform this boiler anti-freeze addition task frequently buy a good quality transfer pump If you buy a "cheap" transfer pump it may not last long when handling antifreeze mixture.

Watch out: for building water damage and loss coverage insurance issues. If you are winterizing a building by adding glycol based antifreeze you might have an insurance coverage issue with your insurance company and thus you might not be covered should a loss occur.

Check with your insurance company.

What Kind of Anti-Freeze Is Used in Hot Water Heating Systems - Boilers, Baseboards, Radiators, Radiant Heat Tubing, Convectors

Usually heating system antifreeze products use propylene glycol. As we explain here, there are some good reasons for using a non-toxic anti-freeze in heating boilers.

Watch out: Be sure you are buying the right antifreeze product: an anti-freeze intended for use in heating boilers, not automobiles or trucks - the chemicals are different, and as we explained above, automotive antifreeze (ethylene glycol) is toxic (poisonous). Not only is the care and handling of toxic chemicals more demanding, on some heating systems there can be a risk of accidentally introducing toxic antifreeze into the home water supply system.

The applications for propylene glycol anti-freeze such as that used in hot water heating systems include:

- Adapted from "Antifreeze & Heat Transfer Fluid", Noble Company [1]

How Might Anti-Freeze Leak Out of a Boiler & Into Building Water Supply Piping?

Should the heating boiler include a tankless coil used for making domestic hot water, and should there be a simultaneous loss of water pressure inside the tankless coil and a tankless coil perforation or leak, then the contents of the anti-freeze treated boiler can leak backwards into the building water supply piping. While this combination of events is unlikely, it is possible, and it has happened.

Watch out: some boiler antifreeze products are not suitable for use in heating systems (or plumbing systems) that use galvanized steel piping. Check with the manufacturer and check your system's heating piping.

How Much Antifreeze is Needed for a Heating Boiler to Protect from Freezing?

EZ Red Antifreeze Hydrometer

There is no simple right answer to the anti-freeze quantity question, despite some DIY articles saying you need 10 gallons of antifreeze for a typical 2800 sq.ft. home. The answer depends on the total volume of water that is in the hot water heating system; the volume of baseboards, radiators, pipes, pumps, expansion tank, and the boiler itself.

But nevertheless, it is very easy to know when you have added enough anti-freeze to a heating boiler, as we explain here.

What a heating service tech does is use an inexpensive anti-freeze hydrometer or refractometer designed to measure propylene glycol (PG) concentration to actually measure the antifreeze concentration in the antifreeze-water mix in the boiler.

The heating service tech will add boiler antifreeze, circulate it through the system to mix it thoroughly with the system water (or de-ionized water in new and some commercial installations), and then will check the mixture level by reading the freeze-protection temperature on the hydrometer/refractometer.

When the gauge indicates that freeze protection is down to the required temperature, you've added enough antifreeze. Record the test results (freeze protection temperature) and date that the antifreeze was added on to the heating boiler's service tag.

The amount or volume of antifreeze you need to add to the boiler to reach the desired freeze-protection temperature will also depend on the concentration of the anti-freeze product itself.

For example, to protect a heating system down to zero °F using NOBURST-100 you'll need to add enough anti-freeze to reach a 50% concentration in the system. [1]

How to Measure Anti-Freeze Protection Level in a Heating Boiler

Use a hydrometer or refractometer to test the water-anti-freeze mixture in your heating system. The hydrometer, if you've bought the right type, will read the freeze-protection temperature right off a scale within the instrument.

Watch out: not all hydrometers accurately measure both types of antifreeze chemicals (automotive system antifreeze, i.e. ethylene glycol and heating system antifreeze, i.e. propylene glycol. Make sure that your antifreeze concentration measurement is made using a propylene glycol (PG) refractometer or hydrometer or using a hydrometer that can (according to its scale and instructions) measure both types of anti-freeze chemical concentrations.

While we use a hydrometer or refractometer to measure the anti-freeze concentration in the hot water heating system, and while we will read the freeze-point to which the system is protected directly off of the scale on the hydrometer/refractometer, you can get a general idea of the quantity of antifreeze that you may need to add to the hot water heating system by converting the % concentration of antifreeze necessary to protect to a given low temperature into gallons.

Hot Water Heating System Freeze Protection Chart - Antifreeze Concentration Data
Example Heating System Anti-Freeze Product % Concentration of Anti-Freeze in the Heating System Approximate Freeze Point Approximate Fluid Flow Point
Note 3
Approximate Burst Protection
Note 3
Volume of Anti-Freeze Per Gallon of Heating System Water
Polypropylene glycol or Ethylene glycol anti-freeze
Notes 1, 2
RhoGard™ 40% +12F -0F -10F .4g / .6g
RhoGard™ 50% +5F -10F -55F 1g / 1g
RhoGard™ 100% -60F -70F -100F 1g / 0g
Noburst® -100 50% 0F -10F -60F .5g / .5g
Noburst® -100 75% -30F -40F -10F .3g / .7g
Noburst® -100 100% -100F -80F -60F 1g / 0g


1. For heating system or plumbing system anti-freeze products based on propylene glycol, the level of freeze protection is a function of the concentration of anti-freeze in the heating system water, typically ranging from 40% anti-freeze to 100% anti-freeze.

There are some variations in the absolute level of freeze protection given by various products due to the inclusion of special additives and by variations among manufacturer's formula.

Some of this data reported by the antifreeze manufacturers cites data obtained from Dow Chemical Company; Noble Corp. points out that all of the freeze protection data assume a [starting?] fluid temperature of 150 °F.

2. These are example antifreeze concentrations.

Product literature from the manufacturer gives the effects of other concentration levels. Also, these companies offer a variety of anti-freeze products with varying performance and freeze-protection levels besides the example products shown here. [1][2][3]

3. Fluid Flow Point defines the temperature below which the heating system fluid may no longer flow reliably and Burst Point defines the temperature below which the piping system or other components are at risk of actual bursting from the expanding pressure of freezing fluid

Of course chances are you will have no idea how much water was in your hot water heating system to start with, unless you drain and measure the system contents, or (unlikely and too difficult) add up and calculate all of the internal volumes of all of the system components.

Now you can see why a hydrometer or refractometer is the way to go in measuring the antifreeze concentration and level of freeze protection for the heating boiler & its piping, radiators, baseboards, etc.

Effects of Anti-Freeze on the Thermal Conductivity of Water & on Building Heating Costs

We care about the thermal conductivity of water because that figure describes the heat transfer efficiency of the the heating system - how efficiently we transfer heat from the hot water in the system through the metal tubing and fins of a copper baseboard or a cast iron radiator into the building air and into the area being heated.

If we reduce the thermal conductivity of water (by lowering its temperature or by changing its chemistry by the addition of antifreeze) we increase (usually just a bit) the cost of heating the building.

In a nutshell, using antifreeze in a building hot water heating system means a reduction in thermal conductivity of the heating liquid by around 30% (comparing straight water to 50% concentration of antifreeze in the table below). The effect is about a 10% decrease in relative heating system efficiency. In short, you will pay a little more to heat the building for the benefit of avoiding the high cost of burst heating pipes and related water damage.

Watch out: Don't add more antifreeze than necessary as doing so can reduce the effectiveness of heat transfer in the heating system unnecessarily.

Table of Heat Transfer Properties of Water Modified With Anti-Freeze Products 1
Liquid / Mixture Relative Heating System Efficiency Thermal Conductivity as
Freeze Protection
Water 1.00 0.35 + 320F
Ethylene Glycol
50% concentration 0.90 0.24 - 340F
75% concentration 0.77 0.18 - 600F
100% concentration 0.69 0.15 + 50F source data error?
50% concentration 0.93 0.27 - 600F
75% concentration 0.83 0.22 - 800F
100% concentration 0.80 0.17 - 1000F

1. Adapted from Noble Company "Antifreeze & Heat Transfer Fluid" [1]
2. Ethylene glycol data is from Noble's citation of data for DowTherm brand Ethylene Glycol, trademark and product of the Dow Chemical Company [1]
3. This data describes a function of specific heat and fluid density and assumes that there is no change in fluid flow data - that is, the flow rate in GPM remains unchanged. [1]

Do I Need to Drain the Heating Boiler to Add Antifreeze?

No. It is possible to add antifreeze to an existing system using a transfer pump. But if the system is to be cleaned first, that step will require draining as we explain next.

Watch out: some "do-it-yourself" websites offering tips on how to add boiler antifreeze are giving bad advice. For example, it is not necessary to drain the boiler to add antifreeze, and doing so will significantly increase the time, cost, and trouble of the job as you'll have to deal with air purging the system as well as disposing of boiler drainage.

Do I need to flush & clean the Heating Boiler Before Adding Antifreeze?

You will also read a recommendation by boiler antifreeze manufacturers such as Noble & Rhomar that the boiler and entire hot water heating system should be emptied, flushed, and de-scaled. This step may make sense for commercial heating systems or for a boiler with known scaling problems but it may not always be necessary for residential heating boilers.

If you are going to follow the manufacturer's recommendations you may have to flush and de-scale and clean the boiler before adding the anti-freeze product. This means a lot more time, cost, and products - a lot more work, which is why the techs I've watched don't take this step unless there is already a reason to believe the boiler needs such cleaning.

Certainly the service technicians we've watched installing antifreeze into a hydronic heating system do not generally take this step.

The hot water heating system that was freeze-proofed during the service procedure that we photographed for this article included a boiler that at the time was about five years old. That system has now been in service without difficulty of any sort for an additional decade.

Manufacturers of hydronic heating antifreeze products such as Noble Co. & Rhomar Water Management include in their product literature a recommendation to thoroughly clean and flush both new and existing hot water heating systems before adding their products.

More about the concern for scale and bacterial formation in anti-freeze-protected heating systems is
at Other Risks from Using an Anti-Freeze Mixture in a Heating Boiler System below in this article.

Where to Buy Heating Boiler Antifreeze & Anti-Freeze Hydrometers or Refractometers

The type of antifreeze used in heating boilers often includes an anti-corrosion additive and may include a de-scaling product, generally sold in 5-gallon pails. Usually heating boiler antifreeze consists of propylene glycol while automotive antifreeze, which requires different properties, consists of ethylene glycol (a toxic chemical).

Here are boiler antifreeze and boiler anti-freeze hydrometer sources:

Contact us to suggest products for the above list. No fees are involved. has no financial relationship whatsoever with the products or procedures and services discussed at this website.

Annual Maintenance Recommendations for Anti-Freeze Protected Heating Boilers & Systems

Use the same hydrometer / refractometer to check the level of freeze protection at annual boiler service time

Inspect all accessible areas and components and piping of the heating system for leaks at time of annual service, or if you observe an unexplained drop in system pressure.

Watch out: most hydronic heating systems include an automatic water feed valve that will add make-up water to the system should there be a water leak or loss. As a result, if your heating system suffers a small leak that goes un-noticed for some time, because make-up water is being added to the system to keep the water volume at a safe and functional level, the amount of anti-freeze in the system will become diluted. For that reason, the level of freeze protection afforded by the anti-freeze that remains will be diminished and the risk of freeze damage increased.

Other Risks from Using an Anti-Freeze Mixture in a Heating Boiler System


Continue reading at PUMPS, PONY PUMPS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see BACKFLOW PREVENTER VALVE, HEATING SYS - protect building occupants from toxic boiler water

Or see CHEMICAL TREATMENTS, BOILER for a discussion of the types of boiler treatments available, how & why they are used


Or see RADIATORS - home and also see RADIATOR VOLUME & BTUs

Or see WINTERIZE - HEAT ON PROCEDURE or if you intend to leave heat off completely see WINTERIZE - HEAT OFF PROCEDURE

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