The indirect DHW solution


This commercial indirect storage tank is connected to an external heat exchanger.

By Roy Collver

The new decade seemed like a good time to review some hydronics basics and look at current trends to see what has been evolving in the industry. Production of domestic hot water (DHW) in hydronically heated houses and commercial buildings has taken some interesting twists over the last 20 years, so it seemed like a good lead-in.

When using hydronics to heat or cool a building in most of the country, there are a number of popular options used to generate DHW:

  • Boiler with Indirect storage tank (internal heat exchange coil)
  • Boiler with storage tank and external heat exchanger
  • Boiler with separate, dedicated DHW heater
  • On-demand combi systems (with or without potable water separation, with or without storage tank)

Heat pumps of various flavours are also being used with success. I tend to classify a heat pump as a low-temperature boiler.

One appliance can do it all

I have seen some rather strange trends lately – and when I started researching this article, I found that other people are seeing them as well. The oddest application, in my opinion, is when a boiler is installed in a home to look after the heating load (usually radiant floor), with the DHW load handled by a separate dedicated DHW heater – either a traditional direct-fired tank-type or an on-demand heater.

Going even further, some installers will use a gas-fired forced-air furnace to provide upper floor heat, air conditioning and ventilation. This is being reported across the country – three combustion appliances with their associated vents, gas lines, etc., when one would do the job. I’m not sure why anyone would want to do this residentially, although there are some commercial projects where it can make sense.

Attending a hydronic heating course at your local wholesaler is a good way to learn all about indirect tank installation. This is Desco’s new training lab at the company’s Mississauga, Ont. headquarters.

There are affordable, reliable, high quality condensing boilers available with excellent sales and technical support in every region. These boilers can do all the heating and DHW, with one vent, one gas line, one condensate drain and an on-board control. Radiant floor heat, air handler and DHW loads can all be treated as individual zones from one combustion appliance. Most of these boilers can modulate their input to match the load requirements as needed. Looking further down the road, combustion heating equipment is going to fade away and, come the day, you will be able to swap out the boiler for a heat pump or other heat generator and keep the other equipment in service.

Selling quality in a budget world

I recently had a chat with Ron Robinson, home comfort specialist (what a great title) at Atlas Care, Oakville, Ont., a 39-year hydronic heating veteran. He filled me in on the picture in Ontario, although many of his comments apply across the country as well.

Atlas Care, and most other service-oriented companies, deal primarily with repair and retrofit. Unfortunately, the trend toward real estate flipping increasingly influences what customers are willing to spend on HVAC equipment. Higher quality upgrades often take a back seat to lowest first cost, as has been the case for forever in new subdivision housing.

Owners balk at putting in quality equipment because they are only going to occupy the house for a few years and don’t see a personal payback. Sad that so many buyers paying over half a million dollars for a house will get stuck with a mediocre “minimum code” heating and DHW system because the previous owner or builder did not see value in passing on a quality system to them.

To add insult to injury, buyers often discover that the exorbitant price did not include ownership of the HVAC equipment and they are on the hook for perpetual rental charges. Robinson and his associates try hard to get people to see the value in an upgrade, and they are there to help out when sketchy systems often fail to make the grade. Service companies today need to spend a good deal of time managing customer expectations – making them aware of the limitations of their existing equipment and explaining the benefits of upgrading.

Indirect makes sense

After 30 years or so of working primarily in hydronics, I have concluded that the premier, most reliable and flexible method of producing DHW when a boiler is used for heating, is to utilize an indirect fired storage tank with an internal heat exchanger coil. The advantages are many:

  • A true instantaneous DHW source – hot water on-call
  • High thermal efficiency, matching the boiler’s rating and eliminating boiler short cycling
  • Low standby losses – newer equipment is highly insulated
  • Longevity – high-grade stainless units will likely outlast the boiler
  • Largely maintenance-free and reliable
  • Inexpensive installation – no vent, no fuel supply, minimal wiring and piping
  • Easy to fit in with DHW recirculation piping

Some installations just don’t make sense for an indirect tank. Piping issues in this small apartment building with its 45-year-old oil boiler made an electric storage tank the simple DHW solution.

There are many options. Storage volume and heat exchanger size can be chosen to match customer needs, and nothing can match their performance for rapid, instantaneous delivery of large DHW volumes.

Indirect tanks need some space in the mechanical room, but with no vent, they can be tucked away. Those who list high first cost as a disadvantage are usually not looking at the whole picture. Installed cost can be considerably less than that of an on-demand unit, or even a direct-fired tank these days. Given that you can expect to replace a direct-fired unit once or twice during the life of an indirect – there is no bargain to be had there.

Other approaches

Despite my preferences for indirect tanks, there are many factors at play that lead to suggesting other approaches, and there are some good operational reasons to pick an on-demand unit in various commercial applications. Both technologies have their place if you play to their strengths. Schools, office buildings, and some restaurants, for example, may have long periods outside of the heating season where there is no DHW requirement. On-demand heaters are an excellent choice for such applications.

On-demand units came into their own with the advent of load-responsive deep modulation. In applications where there is a continuous but variable draw of DHW, these units perform very efficiently and reliably. Customers, however, must be made aware of their limitations.

Yes, they can generate DHW all day long – provided the load does not exceed the maximum heat input available. Careful engineering will ensure this does not happen. Many on-demand manufacturers now provide options to connect to DHW storage and DHW recirculation systems – at which point the DHW heater starts to look a whole lot like a boiler.

I have written extensively about combi systems, and well-designed ones are performing beautifully all across the country. Expect to see more of them as residential heating loads continue to shrink while our appetite for lots of DHW remains. For hard-working on-demand units, especially in hard water areas, premature failure and reduced service life can be a problem. Equipment is getting smaller, more compact and lighter, with less robust components. Higher fail rates mean unhappy customers, but proper sizing and diligent maintenance programs can help keep them going.

It is important to look at every prospective hydronic DHW system through the lens of your customer’s needs and expectations. Once you know what they expect, make sure your design work takes every factor into account, including entering groundwater temperature and quality. And keep close to the builder or renovation contractor so you don’t get surprised if the owners change things up by adding an unexpected load. Some of those “car wash” showers can use 10 to 20 gallons per minute.

In the next issue, we will look at the nuts and bolts of indirect tank selection and installation.


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