HPAC Magazine

Don’t Toss It Out

May 7, 2024 | By John Siegenthaler

John Siegenthaler shares methods for adding an air-to-water heat pump to an existing gas-fired boiler system.

Adding an electric heat pump to a gas-fueled boiler hydronics system.

It’s hard to go through a workday in the HVAC industry and not read about some organization or government agency promoting the need to electrify everything that provides heating or cooling to buildings.

The “flames” associated with electrification and its accompanying phase-down of fossil fuels have been fanned by politics — a place where reality is a very fluid concept.

As has happened with many other issues, deep divides have been created between people depending on their worldview, profession, willingness to accept technical realities, and opportunities to profit from a stance on either side of the issue.

Like most people, I have beliefs about our current energy sources, and specifically how they play into the future of hydronic heating and cooling technology. I believe that our industry can and should increase its use of renewably sourced energy. However, I don’t believe that such a progression should be immediate or exclusive of fossil fuels.

With this in mind, let’s consider the merits of pairing an electrically operated air-to-water heat pump with a fossil fuel fired boiler.

Systems using both heat sources can be designed and installed in either new or retrofit applications. Given the vast number of fossil fuel boilers already supplying hydronic systems, it’s likely, in the near term, that retrofit applications where an air-to-water heat pump is added to a system already supplied by a boiler will be common.

Keep It

I’ve interacted with people who think fossil fuels are the scourge of the earth, and as such any equipment operating on these fuels should be scrapped.

I suspect that such mindsets are based on socio-political leanings rather than practical and technically accurate assessments of alternatives.

Sure, there are boilers out there that get patched up to hobble along “one more season” while operating at conditions that are far from optimal. Like any device, all boilers eventually succumb to a life of hot, wet, and chemically reactive conditions, often with minimal maintenance.

Good technicians can assess the status of an existing boiler, and when they conclude the boiler is at the end of it service life recommend it be removed. They can also determine when an existing boiler has remaining service life and judge its suitability to remain in the system as an occasionally used “copilot” to a new air-to-water heat pump.

Consider the following seven advantages of keeping an existing boiler in the system.

  1. Booster: The boiler can provide supplemental heat for the heat pump on very cold days when the heat pump’s capacity cannot meet the load. Without it, many owners would resort to portable electric space heaters, the operation of which, when accounted for back to the electrical generation level, likely contributes more carbon to the atmosphere than burning the fossil fuel needed to supplement the heat pump.
  2. Backup: Assuming the boiler was adequately sized, it can provide full backup heating when the heat pump is down for service. Modern heat pumps are complex devices. It’s unrealistic to think they will operate for 15 to 20 years without any issues that would take them offline for a time. Depending on the location of the system, who is responsible for servicing it, and the availability of replacement components, it’s also unrealistic to think the repair will always take place within hours of the problem’s occurrence. It might be several days. Most owners who understand this also understand they need a “plan B” for heat should the problem occur in cold weather. They already know the existing boiler can heat the building. Encourage them to incorporate this known entity into any system makeover.
  3. Major Contributor: Simulations of air-to-water heat pumps in cold climates have shown that the vast majority of seasonal space heating energy required in a typical home can come from the heat pump. The boiler may only contribute 5 to 15% of that total seasonal energy depending on the supply water temperature required by the distribution system (lower is always better), the location and the capacity of the heat pump relative to the building’s design load.
    In my opinion that’s a very respectable shift away from fossil fuels without sacrificing reliability, comfort, or scrapping equipment that still has several years of remaining service life.
    Furthermore, the cost to remove and dispose of a boiler that still has service life, along with its fuel and venting systems, and replacing it all with an electric boiler for auxiliary/backup only adds to the installation cost of the heat pump, and thus discourages potential customers.
  4. Service Entrance Upgrades: Another necessary consideration for full electrification is the ampacity of the building’s electrical service. Older homes with fossil fuel boilers may only have 100- or 150-amp services. Adding a heat pump may be possible, depending on its capacity, and the existing loads on the service. However, adding a heat pump and an electric boiler sized to meet the building’s design heating load will inevitably require a minimum 200-amp service.
    Include an electric vehicle charger in addition to the heat pump and electric boiler, and the service entrance ampacity could be even higher than 200 amps. Given the current cost of electrical hardware, upgrading a service entrance could easily cost several thousand dollars. If the existing fossil fuel boiler is retained as the backup heat source service upgrades may not be required.
  5. Futureproofing: It’s easy to plan a new or retrofit installation so that a fossil fuel boiler could eventually be replaced by an electric boiler if that becomes economically viable. This can be done with minimal piping alterations, and without draining the system. Just include isolation and purging valves when connecting the fossil fuel boiler to the system.
  6. Resiliency: Leaving an existing fossil fuel boiler in the system provides the ability to keep the building’s heating system running, perhaps for several days, without utility power. During a power outage a typical residential fossil-fuel boiler and two or three circulators can be powered from a small portable generator that costs less than $1,000. Provisions for connecting such a generator to the heating system are relatively simple and could be included in the installation. The fossil fuel mode of the system could also be powered from modern customer-side battery storage systems such as the Tesla Powerwall or Generac PWRcell.
    A nominal 5-ton (60,000 Btu/h) heat pump and its associated distribution system can also be power by a generator, but likely one with a minimum output of 10kW. Add in 9kW of electric resistance back up or supplemental heating and you’re up to a nominal 20kW generator as a minimum. It can be done, but it’s expensive to install and to operate.
  7. Adaptability: Many electric utilities have, or soon will have, time of use electric rates. In essence these rates offer a discount on each kilowatt-hour (kWh) of electricity purchased during specific times of the day, as well as on weekends and most holidays. Utilities do this to help level their demand curve. Some on-peak versus off-peak differentials are very attractive. The high on-peak rate creates a strong disincentive to operate high demand electrical devices whenever it’s in effect.
    If the system includes a fuel boiler it could supply heat when necessary to minimize use of on-peak electricity. The air-to-water heat pump could be operated on the low cost off-peak rates. Thermal storage might also be included.

Utility rate offerings are rapidly evolving to keep up with projected demand while minimizing the need for significantly greater generation and transmission capital costs. A dual fuel system provides the flexibility to leverage these rate structures.

Parallel Piping

In Figure 1 (below) I show a possible piping configuration where a split-system air-to-water heat pump is piped in parallel with an existing fossil-fuel boiler.This configuration allows either heat source to operate independently of the other.

Figure 1. A split-system air-to-water heat pump piped in parallel with a fossil-fuel burning boiler.

A typical control sequence would call the heat pump to supply the required water temperature to the distribution system whenever possible, and bring on the boiler only when the heat pump cannot maintain the target supply temperature.

Notice that the existing (conventional) boiler has a thermostatically operated anti-condensation valve added to its near boiler piping. It’s there to prevent sustained flue gas condensation, assuming that there will be times when the return water temperature from the distribution system is below the dewpoint of the flue gases. Existing boilers and their venting pipes that have operated at higher temperatures, and lasted for years under these conditions, can be destroyed in short order if “reassigned” to produce low water temperatures, and not protected from sustained flue gas condensation.

Figure 2 (below) shows a similar arrangement – only using a monobloc air-to-water heat pump. In most applications this system would operate with an antifreeze solution to protect the heat pump from freezing.

Figure 2. A monobloc air-to-water heat pump piped with a boiler, both supplying a buffer tank.

The diagram shows both heat sources supplying a buffer tank.

This is appropriate when the distribution is divided into many independently controlled zones.

When fewer zones are present, and the heat pump has a variable-speed compressor, the buffer tank could be replaced by a hydraulic separator.

In applications where the heat pump and boiler have high turndown ratios, it may even be possible to pipe the heat source headers directly to the distribution system.

These same concepts hold true in applications using split system air-to-water heat pumps.

Strength through diversity

Although there are those who advocate that all energy needs can be met through electrification, the cold climate peak demand situation remains an inevitable hurdle, one that hasn’t been thoroughly and honestly addressed to date, especially within the political realm.

As mandates for electrification progress this issue will repeatedly separate aspiration from realization.

As this all plays out, the versatility of modern hydronics technology provides a flexible and adaptive framework upon which to build future heating (and cooling) systems. It allows realistic and beneficial combinations of electrical and fossil fuel energy for the present as well as the years ahead. <>

John Siegenthaler, P.E., has over 40 years of experience designing modern hydronic heating systems and is the author of Modern Hydronic Heating (4th edition) and Heating with Renewable Energy (visit hydronicpros.com).



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