System – Dual Temperature

• Overview

Dual temperature piping systems, as the name implies, are 2 pipe HVAC designs carrying hot water during winter or cooler months, and chilled water during the summer and warmer months.  They are a simple and an extremely common engineering design found in residential, apartment, condominium, hospital, high rise office building, and many other building designs.  Multiple set of risers commonly travel up and down the perimeter building columns where they run-out to serve window located fan coil units.  A third gravity drain line exists to carry away the condensate created during the summer air-conditioning season.

The pipe runs either hot or cold as determined by building engineering or a building management program; its temperature determined by outdoor conditions.  With supply and return risers installed to every or every other perimeter column, the presence of 30 or more sets of risers over 40 floors separated into multiple vertical zones are not uncommon.

There are also 4 pipe systems arguably similar, but having 2 dedicated chill water and 2 dedicated hot water lines simultaneously serving the same air handler.  This configuration, common for hotels, provides tenants or guests substantially more comfort control, and during changeover seasons such as spring and fall, will provide one tenant the ability to cool their space while their next door neighbor can enjoy some warmth.

Although an excellent design in its simplicity, multiple faults exist:

• • • The piping was installed in a high moisture environment at the building envelope
    • Standard schedule 40 steel pipe was used having limited wall thickness
    • The pipe was threaded – further reducing wall thickness
    • The pipe was never protected by any form of anti-corrosion coating
    • Insulation was generally inadequate and too thin
    • Insulation was poorly installed
    • The quality of work hidden behind walls was never expected to see the light of day
    • Such piping systems, like any other, were never intended to last indefinitely

Damage to a dual temperature piping system, similar to any chilled water or cold water system, is greatly influenced by the amount of moisture in the area.  This makes dual temperature systems, typically located at the perimeter columns and substantially exposed to the outside atmosphere, highly vulnerable – with the problem more severe as the building ages.  Dual temperature systems located closer to the central core of the building and through areas where air-conditioning exists, although extremely rare as a design, typically show far more favorable results.

• Common problems

The most common and also most serious issue related to dual temperature piping systems remains completely hidden until a failure occurs.  Failures due to a pinhole leak will provide evidence to a problem that is typically too far advanced to correct in any way.  More serious failures, however, are typically in the form of a total pipe thread separation, and have the capability of causing catastrophic damage to not only the floor where a failure occurred, but too many floors below.

Most dual temperature investigations do not result in good news.  A serious problem is usually documented along with evidence that it is systemwide – thereby requiring massive building disruption and expenditure given the only option being replacement.  Since that the most significant damage is often caused to the exterior of the pipe, there are no potential corrective actions available, such as filtration.  Re-insulating the pipe, which is the first response, will provide no benefit whatsoever.  Painting the pipe is worthless.  Coating the pipe exterior is similarly worthless unless all rust has been removed, which in itself is typically impossible.  An internal coating or lining, as commonly recommended, is simply not feasible since the problem is external.

As a closed piping system, internal corrosion activity should be maintained to a minimum.  For most older dual temperature systems, however, corrosion may not have been adequately maintained, and a second internal corrosion front may equal that occurring externally.  Where both areas of heavier deterioration line up opposite each other, failure occurs.

• Major threats

There are many significant threats common to such systems:

• • • • Schedule 40 pipe

With extremely rare exception, all such piping systems were installed using standard schedule 40 pipe.  At any size 2-1/2 in. and smaller, this pipe is threaded, and therefore has lost approximately 50% of its original wall thickness day one.  Copper pipe is rarely found.

• • • • Insufficient insulation

Dual temperature risers were installed into generally tight location at the perimeter columns where the application of necessary thicker insulation was the least concern.  In many investigations we have performed, exceptionally thin and inappropriate 1 in. fiberglass was installed even though it was expected to protect pipe in an environment of high humidity.

• • • • Poor quality installation

The quality of installation of pipe insulation for dual temperature systems has been traditionally poor.  Installed behind walls where the lack of workmanship was never expected to see the light of day, many examples are so poor that they would never have been tolerated within a well traveled mechanical room.  Unseamed joints, gaps between insulation sections, and no insulation at take-off tees are just a few of the deficiencies commonly identified.  For many locations where failures have occurred, loose attic style fiberglass insulation was simply wrapped around the tee connection at every floor and held in place using twine – a common method of installation decades ago.

• • • • Improper insulation and planning

Even under the best installation practices, the fiberglass insulation typically specified for dual temperature risers is unsatisfactory for the application.  Sealing tapes at its outer covering are still considered moisture barriers throughout the industry, when in fact they are quite inadequate.  Proper installation would have first required a high solids zinc-based waterproof coating or paint applied to the pipe followed by the installation of thick hard cell foam glass covered by a hard shell outer vinyl material – none of which were ever provided to such piping systems.

• • • • Condensate return

Most older condensate piping systems were constructed using schedule 40 galvanized steel pipe; newer condensate lines are commonly constructed using PVC.  Given the common operating problem of airborne capture debris, microbiological growths, and other piping constrictions which in turn produced condensate pan overflow, biocide tablets and often heavy-duty cleaners were commonly used.  This in turn helped destroy the galvanized zinc finish which then led to a deterioration of the underlying steel and ultimate constriction of the pipe by an expanding rust product.

Although most dual temperature piping systems fail simultaneously with their condensate counterparts, the failure of one system should be understood to closely proceed the failure of the other.

• • • • Internal corrosion

Although corrosion activity at a closed system can be easily maintained at below 0.5 MPY, the steps required to maintain this low level of corrosion loss are rarely performed.  With generally no filtration provided, decades of iron oxide rust product not only reduces heat transfer, but also produces far greater secondary corrosion conditions.  Although the primary threat to most dual temperature piping systems is caused by external deterioration, their failure can be easily accelerated if internal corrosion activity is not maintained to minimum possible limits.  Effective water filtration, properly installed and maintained, is essential.

• • • • Lack of water filtration

As for any other closed system, dual temperature pipe will accumulate vast volumes of iron oxide rust as it ages, and in fact more that some larger chill water systems simply due to its increased inside surface area.  Water filtration is rarely installed as a proactive measure, and is typically turned to in desperation once the sheer volume of the corrosion problem can no longer be ignored.

• • • • Low water temperature

For most dual temperature systems, a clearly defined downward spiral exists.  Internal corrosion produces large volumes of rust product which then settle within the lower flow coils to reduce heat transfer efficiency.  As tenant cooling complaints increase, and rather than investigate and correct the heat transfer loss caused by rust, chilled water temperatures are lowered during summer months to offset internal rust related losses which in turn reduces the temperature of the pipe itself.  This lower temperature pipe then more effectively condenses area moisture which accelerates its deterioration.

The problem arises from it being far easier to turn down the chill water supply temperature at the refrigeration machine than to investigate and potentially correct an internal corrosion condition and remove the rust deposits which have been created following three decades of operation.  Any dual temperature system providing a chill water supply temperature of below 40° F. is evidence of an internal corrosion condition.  Hot water temperature is increased during the summer months in order to compensate for the same deficiency.

• • • • Mold

Wet moist fiberglass insulation is an ideal medium for mold growth.  Although its spread is greatly limited by its enclosure behind walls, the issue remains a top litigation issue should infection spread.  For older buildings, asbestos insulation may exist.  While the presence of asbestos

• Testing Focus
  

With the rare exception that copper pipe may have been installed, almost every dual temperature piping system is at risk.  For 4 pipe systems comprising 2 hot water risers and 2 chilled water risers, the exact same threat exists to the two chilled water risers.  Where system wide failure has occurred to the chill water pipe, and with the hot water pipe still in good condition, some building owners have abandoned their original 4 pipe system design; adapting their their still good hot water piping for now dual temperature service.  While less adjustable to the heating and cooling needs of the tenants, the massive costs saving will often offset that loss of control.  As another alternative, raising the water temperature helps to retard the corrosion progress at the expense of personal comfort.

Primary areas of interest for any UT investigation are:

• • • The actual perimeter risers
    • Express riser
    • Upper and lower header piping
    • Large diameter lines
    • Tee connections at the risers
    • Smallest threaded pipe
    • Expansion joints
    • Condensate piping
    • Run-out piping
    • Drain lines
    • Vent lines

With rare exception, no building owner, operating engineer, or tenant looks for problems before a major failure occurs, and in fact, no corrective actions are possible to better insulate the pipe.  Age is the major threat for all dual temperature systems, and often begins to indicate problems after 25-30 years of service depending upon multiple factors.  Any major building renovation should always address the question of the condition of such systems if present.  An ultrasonic investigation will document the level of deterioration present and estimate remaining service life.  Where the surface conditions of the pipe is so severely deteriorated that no one is willing to even remove the insulation or handle the pipe, the answer to its condition is already obvious.

Accurate wall thickness measurement requires the removal of any external surface rust down to bare metal, which for all investigations of dual temperature piping systems is risky.  We typically begin at piping areas of least surface rust, remove that rust, take a wall thickness measurement, judge its condition, and then move toward areas of heavier surface rust further rust removal can be safely performed.  Its a procedure very similar to checking the thickness of the ice before walking further out onto the ice.

In the examples below, we deemed ultrasonic testing as an unnecessary risk and declined to remove the rust necessary for accurate thickness measurement.  Building engineers also declined to help remove the rust.  At left, we ultrasonically measured a remaining wall thickness of 0.086 in. taken at a smooth area of pipe representing internal corrosion.  Immediately adjacent, a pit depth measurement of 0.098 in. defines a failure condition should both events line up and therefore no further need for testing.

Center and right, sections of 1-1/4 in. dual temperature pipe deemed to questionable to safely test were removed and shipped to our office.  Mild surface rust revealed a 1/4 in. size hole in one example.  At right, the condition of the pipe sample as received following its UPS transit  – no further testing necessary.  Of course it is important to note that prior to the removal of all three pipe samples, they were still in service and operating at over 75 psi – a disaster waiting to happen.

Depending upon the level of deterioration, various rehabilitation measures may be possible although in most examples we have been involved, the cost to repair the pipe has far outweighed the cost for replacement.

Corrview International, LLC maintains a large Photo Gallery specifically related to dual temperature piping in addition to other corrosion related issues. To visit our gallery on Dual Temperature, please click here

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