Dual-fuel System: Pros and Cons

Many times, we see a design for a ground-source heat pump (geothermal) system that is sized just to cover the cooling load. The rest of the load is then picked up by a gas-fired heating appliance. While this works, net zero cannot be achieved. Yes, you can offset the gas consumed on-site, but you cannot offset the carbon directly.

We have a saying in the energy industry – folk are often “burning gas to save your ass”. But, in order to install the right system for your home or commercial building, it’s important to understand what systems offer in terms of safety and efficiency. In this article, we will investigate the pros and cons of deploying a dual-fuel system for a geothermal system.

What is a Dual-fuel System?

A dual-fuel system refers to a type of engine or heating system that can operate on two different types of fuels. The purpose of a dual-fuel system is to provide flexibility and efficiency by allowing the user to choose between two fuel sources based on availability, cost, or other considerations.

When to Use a Dual-fuel System

A dual-fuel heating system is a versatile option that provides flexibility and benefits to a heating/cooling system. Here are some points as to when to consider such a system.

• Needing to handle fuel cost fluctuations
• When fuel availability and reliability is uncertain
• When equipment technology cannot meet the full demand
• When equipment efficiency is greater under certain operating conditions, leading to a lower carbon footprint.

Is the dual-fuel approach to a ground-source heat pump (geothermal) system the best approach? When we talk about a dual-fuel approach using a ground-source heat pump, we typically see, here in Colorado, a system designed to cover the cooling load, with the remainder supported by a gas-fired heating appliance. Other times, we see a system using a gas-fired boiler for pool startup or snowmelt, coupled with a ground-source heat pump system that heats and cools the residence. This system provides 10-90% of the auxiliary heating with the help of gas-fired heating appliances.

What is a Ground-source Heat Pump System?

A ground-source heat pump (GSHP) system, also known as a geothermal heat pump, is a highly efficient heating and cooling technology that uses the earth’s constant temperature to exchange heat. Unlike air-source heat pumps that extract heat from the outdoor air, ground-source heat pumps utilize the relatively stable temperature of the ground or groundwater to provide heating, cooling, and, in some cases, hot water for residential and commercial buildings.

Just using the ground-source heat pump system to cover the cooling load takes advantage of the tax credit and can make a system more economical, but it is a less elegant solution to solve the problem of providing the best heating and cooling system.

In the Front Range of Colorado, cooling typically represents 75-80% of the heating load. However, the time we need cooling is only 13% of the time we need heating.
Using a representative system and comparing the two design scenarios, the system sized just for the ground-source heat pump system (geothermal) to cover the cooling load has the heat pump in cooling mode running around 481 hours. In the heating mode, the heat pump runs for 3,021 hours, for a total heat pump run time of 3,502 hours. The right-sized system runs for 384 hours in cooling and 2,357 hours in heating, for a total run time of 2,741 hours.

The difference in the run times of the equipment and the operating cost of the gas-fired appliance leads to an operating cost difference of $481 more to run the dual-fuel system. The upfront capital cost for this representative system would be greater by about $1,500.00 for the right-sized GSHP, but any capital cost savings are negated in the first two years by not taking advantage of the tax credit and the increased operating cost.

Final Considerations

When a dual-fuel system is being proposed, it is typically for reasons other than saving capital costs. It is more often than not a contractor who is covering their bases and not fully understanding the intricacies of a ground-source heat pump system, falling back on the old reliable gas-fired equipment to cover the lack of understanding.

Using a dual-fuel system makes the most sense when the auxiliary heating of the pool or snowmelt outweighs the heating of the residence by multiple factors. The only way to truly understand the cost of such a system is to run a Life Cycle Cost Analysis.