Combined hydronic heat and hot water systems (often called combo systems) represent a fascinating intersection of building science, mechanical engineering, and practical installation know-how. These systems use a single water heating appliance to provide both domestic hot water and space heating, typically through radiant floors or fan-coil units. While the concept is elegant in its simplicity, the real-world implementation of combined hydronic heat and hot water systems presents a host of technical challenges that demand careful attention to design, component selection, and system controls. Drawing on insights from leading building science experts, this article explores what works, what doesn’t, and how to get tankless water heater combo systems right.
Understanding Combined Hydronic Heat and Hot Water Systems
Combo systems typically use a tankless water heater or a condensing tank-type water heater to serve dual purposes: supplying hot water to fixtures and providing heated water to a hydronic distribution system such as radiant floor tubing or fan-coil units. The appeal is obvious , one appliance replaces two, potentially saving space, reducing upfront costs, and simplifying mechanical rooms. However, as building science expert Michael Chandler noted after attending a Building America Water Heater Expert Session, everything he thought he knew about these systems was up for debate.
The Core Components of a Combo System
A well-designed combined hydronic heat and hot water system typically includes several key components working in concert:
- A primary heat source, most commonly a tankless water heater or condensing tank-type heater
- A tempering tank or buffer tank that stores heated water and smooths out demand fluctuations
- A recirculation pump that moves water between the heat source and the tempering tank
- A hydronic distribution system, such as radiant floor tubing or fan-coil units
- Controls and sensors that manage temperature setpoints, pump activation, and safety sequences
Each component must be carefully sized and matched to the others. Using a tankless water heater with a tempering tank, for example, requires attention to flow rates, temperature differentials, and pump sizing to avoid common efficiency penalties.
Why Building Scientists Are Rethinking Combo Systems
The expert session at Joe Lstiburek’s Building Science Summer Camp brought together manufacturers, researchers, and seasoned installers to hash out the real-world performance of combined hydronic heat and hot water systems. What emerged was a nuanced picture: while combo systems can work well under the right conditions, they are far more sensitive to design details than many practitioners realize. Issues ranging from reduced efficiency during low-load operation to accelerated tank corrosion and clogged intake filters were discussed in detail.
Key Challenges in Tankless Water Heater Combo Systems
Perhaps the most discussed challenge at the expert session was the efficiency penalty that tankless water heaters experience when used in combo applications. Unlike a simple domestic hot water setup where draw patterns are intermittent, a combo system may require the water heater to maintain a tempering tank at a constant temperature, leading to extended low-fire operation.
The Efficiency Penalty of Low-Load Operation
When a modulating tankless water heater is tasked with maintaining a tempering tank at 110°F to 120°F for radiant floor heating, it often operates at its lowest firing rate , analogous to driving a car at 15 miles per hour in top gear. Experts at the session reported measuring on-demand water heater efficiency as low as 50 percent when choked down in this fashion. The problem is compounded by soot buildup on heat exchangers when burning propane at these low-fire conditions.
The temperature of the return water matters enormously. When water returning from the tempering tank exceeds 120°F, the unit ceases to condense, eliminating one of the primary efficiency benefits of a condensing water heater. Data at the session showed efficiency vs intake temperature: 91% at 100°F, 89% at 110°F, 88% at 120°F, and 86% at 130°F.
Cold Water Sandwich and Flow Rate Issues
The “cold water sandwich” problem , a burst of cold water between hot-water draws , is a well-known annoyance with tankless water heaters. Manufacturers have made progress in shortening ignition sequences. Hugh Magande from Rinnai discussed improvements that reduced cold-start ignition from 10 seconds to as low as 2 seconds on newer units, with even shorter re-start times after brief off cycles.
Modern ultra-low-flow fixtures and WaterSense-listed faucets present another challenge. Rinnai responded by dropping the low-flow activation threshold from 0.7 gallons per minute to 0.4 gpm for cold starts and 0.2 gpm for re-starts. However, flows below 0.3 gpm still trigger shutdown, which means tempering tanks remain an important tool for maintaining consistent delivery temperatures at very low flow rates.
| Intake Water Temperature | Efficiency | Notes |
|---|---|---|
| 100°F | 91% | Optimal condensing operation |
| 110°F | 89% | Reduced condensing benefit |
| 120°F | 88% | Marginal condensing |
| 130°F | 86% | Non-condensing operation |
Setting the water temperature too cold creates its own low-flame issues. A 140°F setpoint was identified as the sweet spot for the cleanest burn in most condensing tankless units, balancing efficiency and combustion quality for tankless water heaters used in combined hydronic heat and hot water applications.
Best Practices for System Design and Component Selection
Getting a combined hydronic heat and hot water system to perform reliably and efficiently requires attention to several critical design decisions, from pump sizing to anode rod selection.
Pump Sizing and Tank Turbulence
One of the most surprising takeaways from the expert session was the importance of proper pump sizing. Oversized recirculation pumps create turbulence inside the tempering tank , a phenomenon known as “rolling the tank” , where incoming water creates currents that push hot water to the bottom and cold water and sediment to the top. This turbulence accelerates anode rod wear and sends debris toward the tankless water heater’s intake filter, causing chronic clogging.
The solution, according to Bosch representatives at the session, is to use smaller pumps with shorter run-times. A Taco 006 circulator may be sufficient where a Taco 009 has been traditionally specified. Reducing flow velocity not only minimizes tank turbulence but also improves stratification, keeping the hottest water at the top of the tank where it belongs.
Anode Rods, Fittings, and Corrosion Management
Tank durability emerged as a major concern in combined hydronic heat and hot water applications. The increased cycling inherent in combo systems accelerates tank aging and anode rod consumption. Larry Weingarten from Water Heater Rescue recommended several strategies:
- Use powered anode rods instead of sacrificial magnesium or aluminum rods for longer service life
- Avoid soft aluminum anode rods, which degrade rapidly under increased cycling
- Use plastic-lined steel nipples at tank connections to minimize galvanic corrosion between glass-lined steel tanks and brass or copper fittings
- Replace the factory plastic drain port with a proper 3/4-inch ball valve for easier annual flushing
Clogged intake filters on tankless water heaters were linked to debris shed from degrading anode rods. Using a smaller recirculation pump (which reduces tank turbulence) combined with a J-shaped cold-water dip tube , a design that pushes debris toward the drain port , can significantly reduce filter maintenance. Some manufacturers suggested that after an initial two-month break-in period, the intake filter may be removed entirely, though this recommendation was not universal across brands.
The Case for Non-Modulating and Tank-Type Alternatives
One provocative discussion explored whether non-modulating, non-condensing tankless water heaters might outperform their advanced counterparts in some combo applications. Bosch representatives noted that their simpler, non-modulating units (many selling for around $600 at big-box stores) avoid the low-fire efficiency penalties that plague modulating units when maintaining a tempering tank. In some cases, pairing an inexpensive non-modulating unit with a properly sized hot water recirculation pump delivers better real-world performance than a premium condensing unit operating outside its sweet spot.
Condensing tank-type water heaters such as the A.O. Smith Vertex offer another compelling option. These units achieve condensing efficiencies even when supplying both space heating and domestic hot water, solving many of the problems associated with marrying a tankless unit to a tempering tank. While historically more expensive than tankless alternatives, the price gap has narrowed considerably.
Future Directions in Combined Heating Technology
The building science community continues to push for better solutions in these systems. Several promising developments are on the horizon.
Heat Pump Water Heaters and Split-System Designs
Tank-top heat pump water heaters have gained traction in recent years, but the next frontier is the mini-split style heat pump water heater, where the compressor is located outdoors and harvests heat from outside air rather than cooling the interior space. These systems promise dramatically higher efficiency for combined applications by avoiding the parasitic loss of indoor units that cool already-heated air. Multiple manufacturers are working on these designs, though challenges remain in optimizing coil heat transfer and managing refrigerant circuits.
Improved Testing Standards and Real-World Performance Data
One of the frustrations voiced at the expert session was the inadequacy of current testing standards. The ASHRAE 124-2007 standard for combined heat and hot water annual efficiency testing prescribes supply and return temperatures that may not match real-world conditions and requires that smart controls be disabled during testing. This makes it nearly impossible to compare products on an apples-to-apples basis for actual combo system applications. A new ASHRAE 206 standard for heat pump water heaters and the Canadian CSA P.9-2011 standard represent steps toward better performance characterization, but the industry still has work to do.
Integrated Controls and System Optimization
Dave Hammond from A.O. Smith highlighted fast-acting probe-type thermistors as an improvement over surface-mounted aquastats for controlling recharge pump activation in combo systems. Faster temperature sensing means the pump activates and deactivates more precisely, reducing energy waste and improving system efficiency. Simple innovations like wrapping thermistors in electrical tape to prevent metal-to-metal contact issues can resolve reliability problems that have plagued installations for years.
For projects that include radiant heating and hydronic systems, the choice between a tankless combo system and a dedicated boiler plus separate water heater depends on climate, load profile, and budget. In well-insulated passive solar homes with slab-on-grade construction, the simplicity of a single water heater serving both radiant floor tubing and domestic hot water remains appealing , provided the system is designed with the lessons learned from expert sessions like the one at Summer Camp.
As heat pump water heaters continue to evolve and converge with traditional hydronic technologies, the line between dedicated space heating equipment and domestic water heating will blur further. Building professionals who stay current on these developments will be best positioned to design systems that deliver comfort, efficiency, and durability in equal measure.
Whether retrofitting or designing new, the key takeaway is clear: these systems demand respect for the details. Proper pump sizing, thoughtful component selection, and attention to water chemistry and corrosion management can mean the difference between a system that performs beautifully for decades and one that generates frustration from the first winter.