System – Hot Water Heating

• Overview

As a closed piping system, it should be possible to maintain corrosion activity at under 0.5 MPY and even lower.  Unlike chilled water systems, there is no outer corrosion issue, and the only consequence of insufficient insulation or its poor quality installation is a higher heating cost.  Various forms of hot water heating exist such as:

• • • Reheat systems
    • High temperature hot water
    • Secondary hot water
    • Perimeter heating

Depending upon the geographical area, hot water systems used for comfort are typically shut down and unused for about half of the year.  Hot water heating systems are traditionally treated with chemical corrosion inhibitors to a much higher level than chill water and other closed systems based upon the expectation that higher temperatures accelerate the corrosion process.

• Common Problems

The primary concern for hot water heating systems exists to all closed piping systems – which is the production and accumulation of iron oxide rust product over time.  Even under the lowest corrosion levels below 0.5 MPY, substantial rust will be created after a few decades.   While the need for water filtration begins on the first day of operation, it is rarely installed as a precautionary measure.  Instead, water filtration is often a last resort measure realized after the fan coils are found clogged with rust.

Relative to the water treatment, a quite opposite opinion, shared by many in the HVAC community as well as even the chemical treatment field, is that the higher temperatures of such systems drive off the oxygen for such systems – thereby reducing or eliminating the need for chemical water treatment whatsoever.  CorrView has repeatedly disproved this flawed theory by documenting advanced failures which would never have occurred if high chemical treatment levels had been maintained.

Similar to a chill water system, the accumulation of rust product typically results in a loss of heat transfer efficiency.  Tenants begin to report cold complaints which then results in the engineering department to raise hot water supply temperatures.  Although investigation why this loss of heat transfer would be the proper response, a mouse click on the building’s control computer to raise water temperature is far easier, and the problem continues.  We have identified water temperatures as high as 210° F., which represents a scalding threat should a leak occur.  While there are many indicators to a corrosion problem at a hot water heating system, a supply water temperature anywhere near 200° F. is confirmation of that fact.

• Major threats

Although far lesser threats exist, most hot water systems distribute widely throughout a building property and therefore present a significant water damage threat should failure occur.  That threat increases with the height of any building.  Unlike chilled water systems, hot water heating systems will often have expansion joints installed every 12 or so floors – introducing one additional threat which increases with age and piping temperature.  Replacement costs are also very high.

Specific areas of concern are:

• • • Absence of effective water filtration

Once again, internal rust deposits are the primary threat to all hot water heating systems.  For this reason we strongly recommend side stream filtration to hot water heating as well as to any closed piping system.  Small basket style bag filtration systems, low cost and simple to install and maintain, are all that is required.  No form of automatic backwashing filter or centrifugal separator should be considered given that they are inappropriate for the application.

The installation of filtration to any new or older closed piping system is always advised, although rarely performed.  Instead, this step is only considered once an interior corrosion problem has become severe enough to interfere with operations.  In the photograph at left, hot water heating fan coils, so severely clogged with rust product that they regularly required the use of nitrogen to blast them open, finally forced the installation of a multi-cell filtration unit.

The massive volume of rust circulating throughout the system quickly overwhelmed the filter; somewhat cementing the filtering elements in place.  As a result, the filter was judged as ineffective and valved off from service.  No other type of unit was installed in its place.

• • • Galvanized steel pipe

The zinc protective finish of galvanized steel pipe is highly vulnerable to heat and should never be installed into a hot water heating system.  This problem is amplified where galvanized steel pipe has been directly threaded into a brass valve, with substantially deterioration taking place to the most vulnerable threads.  The end result is often a large scale failure where none would ever be expected.

• • • Copper piping

Copper pipe has a long history of being highly resistant to corrosion but many unknown conditions exist that can lead to premature failure.  Within hot water heating systems, the presence of rust product can settle into the smaller copper run-out lines to initiate a galvanic cell corrosion condition.  This is also common for domestic hot water systems where the boiler or heater tubes are deteriorating – then distributing rust particles downstream to cause the problem.

• Testing Focus

Basic filtration, properly installed, is the primary need of most hot water heating systems.  Maintaining chemical treatment to higher levels is also important, although inevitably, substantial rust deposits will appear under even the lowest corrosion conditions. Areas of interest for any ultrasonic investigation should be:

• • • All threaded pipe
    • Any galvanized steel pipe
    • Drain lines
    • Vent lines
    • Expansion tanks

A mild chemical cleaning and sterilization every 5 years is another recommendation which is rarely acted upon.

© Copyright 2023 – William P. Duncan, CorrView International, LLC