Open Loop Hot Water Systems

Photo of a Honeywell Mixing Valve

What should we say about these systems?

Do you ever get those anxiety dreams at night? The ones where the train is coming for you and the lights are bearing down but you can’t get off the tracks because your muscles respond like molasses. That is how I feel every time I look at an open loop hot water heating system.

What is an open loop hot water system?

As a refresher, an open loop heating system is a water-based heating system in which the same hot water that heats the building can also be run for domestic supply – for showers and sinks…..  Before we wade into the murky gray area of what we, as home inspectors, should be saying about these systems and why, let’s go over how to identify an open loop hot water system and then look into some history of how we got to where we are today.

 Tricks for Identification

Distinguishing open loop hot water systems from closed loop systems can be complicated.  One of the most basic characteristics of hot water heating systems is to see if there are two water heaters or one. A building with two water heaters likely has one dedicated solely to heating and is thus a closed loop boiler heating system.  If the building has only one hot water heater for both heating and domestic supply, you need to look more closely.

Photo of a Honeywell Mixing Valve
Honeywell Mixing Valve

If you see a mixing valve, you are probably looking at an open loop system. The mixing valve is where the hot water for heating is mixed with cold water to set the temperature of the domestic hot water supply. In this configuration the water heater can be set to heat to 140 degrees F or more for radiators and then tamped down with the mixing valve to 120 degrees F for domestic use.


Photo of a Heat Exchanger
Heat Exchanger

Another distinguishing characteristic to look for are these flat, pan-shaped heat exchangers. If you see one of these you are likely looking at a closed loop system. This is because this heat exchanger separates the domestic water from the heating water.  You could see both a mixing valve and a heat exchanger – in this case you would likely be looking at a closed loop system.

Another trick for identifying an open loop system is to see if the circulation pump that runs water through the heating coils is connected to a 24 hour timer. The timer should be set for 15 minutes a day or more to push water through the pipes all summer – this is very important to prevent stagnant water from sitting in those pipes all summer, which could then get flushed into your domestic water supply when the heat comes back on.

Finally, look at the water heater itself. Some water heaters separate the two types of water inside the heater.  These types of water heaters typically have a, “Heating Inlet,” and Heating Outlet,” as well as inlets and outlets for domestic supply – so you will see at least 4 pipes connected to the water heater. These types of systems are generally closed loop.

Armed with a few basic techniques for identifying an open loop heating system lets look at some history to understand why these are being installed now and how they differ from traditional closed loop boilers.

A little history

I grew up in a house in New England that had a traditional closed loop boiler system with galvanized steel pipes and cast iron radiators. The system was state of the art when it was designed and installed at the turn of the last century and, somewhat amazingly, it was still working well 80+ years later when I was living in the house.  In theory, with this old closed loop boiler, if it had never leaked over the years, I would have been able to open up the distribution piping system and watch water come out of the pipes that was put in when Theodore Roosevelt was in office….antique water.

The noteworthy attribute of this antique water is how much less corrosive it is than the same system in which new water is constantly being introduced. An open loop system delivers a constant supply of new oxygen-rich water and is therefore more corrosive than its closed loop cousin where the same water circulates. Where I live and work now, in the Seattle area, I sometimes see 100 year old galvanized steel boiler piping systems that are still in service in a closed loop heating system and yet the same steel pipe that was used for the domestic water supply has long ago failed and been replaced. That is the difference in corrosive power between closed loop and open loop systems.


So if closed loop hot water systems work so well, why did we go away from doing closed loop systems?

I don’t know what types of heating systems are used in your part of the country, but out here, these hydronic hot water heating systems with the Turbonic-type radiators just exploded into our houses starting in the 1990’s.  The flood of these heating systems rode in on the tide of new townhome construction that swept into many of our neighborhoods in this era and the reason these systems got installed is simple: they are cheap.

Photo of Turbonic-type radiator
Turbonic-type radiator

As these systems became more prevalent I found myself increasingly alarmed and confused by the lack of standards for installation. Each of these systems seemed to present a new confounding array of pipes, tubes, manifolds, heat exchangers, timers, relays, mixing valves, solenoids, thermostats and water heaters. Occasionally I run into one of these systems that looks so chaotic I am reminded of a rugby scrum of octopi. Even if an inspector lacked all technical knowledge about hydronic heating, some of these systems simply look unreliable.

Photo of a jumble of pipes
A jumble of pipes!

The odds of this jumble of pipes and wires actually working? I recall that my magic 8 ball had one triangle that read, “outlook not so good.”

Adding to the confusion about these hydronic heating systems is this matter of open loop versus closed loop and the potentially corrosive effects of configuring these as open loop systems, not to mention the greater risk from Legionella forming in the pipes of open loop systems due to the stagnant water issue mentioned above.

The fallout seems to be happening slowly. Every few months a new recall or class action lawsuit bubbles to my attention.

Photos of a Kitec brand tubing system
Images show a Kitec brand tubing system

This started with the now infamous Kitec and the IPEX system:

This was followed in no particular order by:

  • Zurn and Q-PEX fittings:
  • Ultra PEX:
  • Uponor:
Photo of a Kitec Manifold
Kitec Manifold

You can go online now and find posts and blogs where professionals and laymen alike lambaste open loop systems as disasters waiting to happen and yet these are still widely installed systems that could accurately be described as, “industry standard.”


So in light of all this history and knowledge we carry around, what should we be saying to our clients about open loop hot water systems?

Obviously, if you are looking at a tubing system that has been involved in a class action law suit and you are aware of this, the reporting gets a little easier. But what if there are no recalls or lawsuits that you are aware of? This is where I feel the heat from that spotlight on the train burning the nape of my neck and I start to thinking about home inspectors in the early 1990’s inspecting new construction homes with LP siding.

While we are technically not bound by our standards of practice to report on recalled products I do believe it is our job to educate our clients about what they are considering to buy. Explaining the drawback of open loop heating systems: namely that they are newer, unproven systems that seem to be having reliability issues due to the corrosive effects of the water as well as potential health issues related to having stagnant warm water in the pipes, would seem a professional duty.

  • I always recommend having these systems serviced by a professional hydronic heating contractor
  • I always Google the tubing system in use in the house to see if there is any sign of litigation.
  • I recommend that my clients obtain opinions and estimates for converting their open loop system to closed loop for the reasons discussed here.

I have seen estimates that range from $2200-$2800 to convert to closed loop – these prices may vary regionally. A logical time to make this conversion is when the water heater reaches the end of its 10-12 year service life and replacement of the existing water heater should be on the table anyway. However, with so many variables in how these systems are installed I find it difficult to say with any accuracy how urgent such work is and how reliable some of these systems will be even after conversion to a closed loop system.

I have only glanced over some of the technical issues with these systems in an effort to have a manageable article that is provocative and at least moderately enjoyable to read. I hope I have not over-simplified but also provided a platform for discussion. As home inspectors in other regions of the country may be running into these problems, I welcome any differing opinions on how other inspectors handle these systems. See my blog @


Dylan Chalk is the owner of Seattle-based Orca Inspection Services LLC – He is the founder of ScribeWare inspection report software offering innovative and simple report-writing solutions – He is also the author of The Confident House Hunter – a book to teach home buyers how to look at and understand houses: Cedar Fort Press Due out August 2016 –



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