John Siegenthaler: Hydronics Workshop

The air-to-water heat pump summit brought contractors, suppliers, and educators together for hydronic technology

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Fri, 03 Apr 2026 07:00:00 -0400

The air-to-water heat pump summit was held on March 4th in Burlington, VT, and attended by over 120 people.

Why thermal mass and domestic hot water demand don’t always play nicely together

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Wed, 18 Mar 2026 09:00:00 -0400

When radiant slabs and high-demand fixtures share a heat source, thermal equilibrium doesn’t always favor the shower.

What’s it capable of? (part 2)

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Wed, 04 Feb 2026 00:00:00 -0500

In hydronic systems, the circuit usually passes through the boiler, which adds heat. When using a block heater and the boiler is off, it dissipates heat from the circuit. This heat loss must be considered when measuring the circuit's true "net" heating output.

What’s it capable of? (part 1)

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Fri, 09 Jan 2026 00:00:00 -0500

Measuring the flow rate through a circuit has always been more of a challenge compared to measuring temperatures. Very few circuits are equipped with permanently installed flow meters.

Injection mixing revisited (part 2)

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Mon, 08 Dec 2025 00:00:00 -0500

Last month, we reviewed the basics injection mixing. It’s a method for controlling water temperatures by regulating the flow of heated water “injected” into a circulating distribution system, while simultaneously removing an equal flow of cooler water from that distribution system. Any method that controls the rate of injection flow also controls the rate of heat transfer into the distribution system.

Injection mixing revisited: part one

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Mon, 17 Nov 2025 00:00:00 -0500

For boilers operating on natural gas or propane, flue gas condensation begins at inlet water temperatures below about 130 ºF. Boilers operating on low sulfur #2 fuel oil have lower dewpoint temperatures in the range of 110 ºF.

How mismatched loads in a dual-temperature heat pump system can limit performance

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Wed, 08 Oct 2025 00:00:00 -0400

At a supply water temperature of 103 ºF, the heating capacity of the heat pump is the same as the total heat dissipation ability of the distribution system. That’s where this system achieves thermal equilibrium. The floor heating zone is fine at this condition, but the output of the air handler is significantly lower than what was planned for based on the assumption that 120 ºF supply water would be available.

Methods from Millinocket: One town in Maine is leading the way in hydronics technology and transactive energy management

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Thu, 18 Sep 2025 00:00:00 -0400

Millinocket is near utility-scale wind turbine farms where strong winter winds can lead to over-generation of electricity, surpassing local grid demand. While it might seem intuitive to shut down some generation equipment during such times, the process is not as simple as flipping a switch.

Using antifreeze vs. water in hydronic circuits

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Mon, 04 Aug 2025 00:00:00 -0400

One concept that’s getting increasing attention among HVAC engineers is “resilient design.” It refers to approaches that - ideally - allow systems to remain operational under very abnormal conditions, or minimize effects that could cause major damage to systems when and if they are otherwise rendered inoperative.

How to integrate a heat pump with a boiler for dual temperature hydronic heating and cooling

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Wed, 02 Jul 2025 00:00:00 -0400

A building with slab-on-grade heating from a cast iron boiler plans to expand. A monobloc air-to-water heat pump will provide heating and cooling for the new space via a fan-coil unit, with the existing boiler serving as backup.

Rethinking Heat Transfer: Why 'Natural' Isn't Always Ideal for Air-to-Water Systems

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Mon, 16 Jun 2025 00:00:00 -0400

Air-to-water heat pumps are gaining popularity in North American hydronic systems, but many practitioners are discovering that they operate differently from boilers.

Using hydronics to leverage time-of-use electrical rates

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Fri, 02 May 2025 00:00:00 -0400

As electrification reshapes the HVAC market, hydronic-based heating and cooling systems stand ready to offer many design and performance benefits. One of them is the potential for thermal storage. After all, water is one of the best materials on earth for storing sensible heat.

Build what you need: Availability and customization of heat pump systems opens doors of possibilities

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Wed, 02 Apr 2025 00:00:00 -0400

The availability of heat pumps allows designers and installers of hydronic heating to include cooling. A simple method involves using a single air handler with a chilled water coil for a ducted distribution system, providing single zone cooling that can complement multi-zone hydronic heating in average houses.

Be prepared to advocate for hydronic-based heat pump systems

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Sun, 09 Mar 2025 23:00:00 -0400

Most heat pumps in North America extract heat from outside air to warm interiors, which is why we call them "air-to-air" heat pumps. However, this term is incomplete, as heat pumps can also provide comfort without relying on forced air delivery.

Heat pumps open possibilities for DWH and hydronic-based cooling

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Tue, 04 Feb 2025 00:00:00 -0500

Last month, we started with an air-to-water heat pump for space heating and added an auxiliary heat source and chilled water cooling. This month, we’re adding domestic water heating, similar to last month’s setup.

Adding functionality to heat pump systems

Thu, 02 Jan 2025 00:00:00 -0500

Standardized piping layouts for air-to-water heat pumps can improve acceptance by facilitating a basic heating system that uses a monobloc heat pump with a 30% propylene glycol solution for freeze protection.

Cutting-edge hydronics technology delivers superior efficiency and comfort

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Wed, 04 Dec 2024 00:00:00 -0500

Hydronically-heated concrete floor slabs offer comfort but can be slow to warm up. A proposed solution uses a heat pump and variable-speed circulators to provide rapid heating through air handlers, ensuring quick warmth during cool days while maintaining energy efficiency.

How to create your own flush cart for hydronic heating systems

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Mon, 04 Nov 2024 00:00:00 -0500

In 1986, I installed a 1.5-ton geothermal heat pump in my home using two 500-foot horizontal HDPE earth loops. After digging a trench, I connected the loops to the pump and opted for a 2 HP swimming pool pump to fill and flush the system, as a garden hose couldn't provide the needed flow rate.

The convergence of hydronics

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Fri, 04 Oct 2024 00:00:00 -0400

In 2024, the North American air-to-water heat pump market saw significant expansion, with nine new offerings introduced at the January AHR show by companies new to this category. By year's end, at least 18 companies either have or plan to offer these pumps in the U.S. Read the list of these companies as of mid-2024.

Switching heat pumps between domestic water heating and cooling

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Thu, 05 Sep 2024 00:00:00 -0400

Air-to-water heat pumps and water-to-water geothermal heat pumps provide warm water for space heating, domestic water heating, and chilled water for cooling. Switching a hydronic heat pump system from space heating to cooling and vice versa seasonally is relatively simple. The system operating mode is changed at the thermostat or a dedicated system mode switch with heat, off, and cool settings.

Comparing air-to-water versus ductless heat pump systems

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Fri, 02 Aug 2024 00:00:00 -0400

I attended a conference hosted by the Building Performance Association and presented on air-to-water heat pump systems. Many professionals were interested in heat pumps, especially ductless air-to-air systems. One person mentioned the difficulty of finding qualified installers for air-to-water heat pumps and noted the higher cost compared to ductless air-to-air systems.

Follow established piping layouts to prevent reverse thermosiphoning

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Mon, 01 Jul 2024 00:00:00 -0400

Flow direction is crucial in hydronic systems. Most designs have a fixed flow direction, but there are situations where reversals can occur. It's important to address these proactively to ensure smooth operations.

Do you understand hydraulic separation?

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Wed, 29 May 2024 00:00:00 -0400

Primary/secondary piping is a way to achieve hydraulic separation by connecting multiple hydronic circuits with their own circulators to prevent interference. Using a pair of closely spaced tees is a common method to achieve this.

Combining “low-tech” heating with state-of-the-art heat pump technology

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Wed, 01 May 2024 00:00:00 -0400

In rural upstate New York, residents harvest trees for firewood using chainsaws and hydraulic splitters. I burn about two face cords annually in my wood stove, enjoying its warmth during winter and relying on it during power outages caused by outdated utility lines.

Combining a wood-fired boiler with a hydronic distribution system

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Wed, 27 Mar 2024 01:00:00 -0400

I’ve written many PM columns discussing how air-to-water and geothermal water-to-water heat pumps can be combined with hydronic delivery systems to deliver efficiency AND superior comfort. This combination is a great solution for many consumers.

Does the boiler need to change when adding a heat pump?

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Fri, 01 Mar 2024 00:00:00 -0500

There is a growing interest in adding air-to-water heat pumps to existing hydronic heating systems. While I support their use, I do not recommend replacing a perfectly good cast-iron boiler that still has a long service life.

Options for connecting heat pumps to existing boiler systems

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Thu, 01 Feb 2024 00:00:00 -0500

Find out about piping options for retrofitting an air-to-water heat pump into an existing hydronic system.

The ‘ins’ and ‘outs’ of thermal storage

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Tue, 02 Jan 2024 00:00:00 -0500

Hydronic-based heating and cooling systems stand ready to offer plenty of design and performance benefits. One of them is the potential for thermal storage.

A simple, repeatable and scalable approach to hydronic distribution

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Thu, 30 Nov 2023 00:00:00 -0500

One of the best things about hydronic heating is there is virtually no limit to how piping systems can be customized. One of the worst things about hydronic heating is that there is virtually no limit to how piping systems can be customized.

What’s possible with hydronics and heat pumps?

hydronicpros@gmail.com (John Siegenthaler, P.E.) — Wed, 01 Nov 2023 00:00:00 -0400

Last month, we began discussing the HVAC system installed at my church’s newly-constructed parsonage. In this month’s column, we’ll complete the picture.