Ground-source heat pumps for radiant, DHW

IN ORDER TO USE the relatively low-temperature water available from ground-source heat pump systems, the designer must consider all the parameters required to deliver comfort when it is needed. The designer must pay attention to floor finishes for radiant floors. He should explain to the consumer that, in some cases, no carpeting can be installed if the heat load is to be carried by the radiant floors

IN ORDER TO USE the relatively low-temperature water available from ground-source heat pump systems, the designer must consider all the parameters required to deliver comfort when it is needed.

The designer must pay attention to floor finishes for radiant floors. He should explain to the consumer that, in some cases, no carpeting can be installed if the heat load is to be carried by the radiant floors only. If the customer insists on having carpet, then the designer will need to consider ways to add heat by way of fan coil units.

In the case of non-radiant systems, fan coil distribution really fits the bill well. With the advent of variable-speed blower motors and continuous-run blowers, a decent comfort level can be delivered to the occupants. It will never be as comfortable as radiant floors, but radiant floors are not in everyone's budget.

When sizing the radiant floor heating system, pay particular attention to the tubing density patterns. Due to the low temperatures' availability, you will most probably need to install the tubing at a higher density than you would typically use with a boiler-based system. Follow the tubing manufacturers guidelines. As with any hydronic radiant floor system, you must have good insulation below the floor in order to guarantee excellent thermal performance.

As I discussed last month, the economics of GSHP installation currently dictate carrying roughly 90% of the space heating loads ("Ground-source heat pumps," October, pg. 36). The designer has to factor in supplemental heat for that 2% of the time that the outside temperatures really take a nosedive.


"Make sure that your stored DHWvolume can fill that bathtub."


Supplemental heat can take the form of an electric element in the buffer tank coming on as a second stage, or it can be provided from any other fuel source. The key in doing this is to make sure that the heat pump is used as much as possible, and that the supplemental heat only comes on when really necessary. This may require your replacing the inaccurate surfacemounted thermostats that usually serve electric water heaters. You should select a more reliable, more accurate means of sensing and controlling fluid temperature.

Follow the GSHP manufacturer's recommendations as they pertain to the sizing and controlling of the buffer tank. Unless you're using the GSHP system for doing a major heavy load such as snow melt only, the buffer tank is not an option.

Many GSHP manufacturers also have a way for you to pre-heat domestic hot water during air conditioning season. In air conditioning terms, you're "desuperheating" the condenser. That heat is rejected back into the ground in summer, except in this case youre using some of the heat for DHW before it's pumped into the ground.

You will not be able to carry 100% of the load. It is, however, entirely possible to carry a major portion of the DHW needs of a home with the GSHP, but it requires a departure from the normal North American method of heating DHW.

Instead of using a large tank with a relatively small immersion coil being heated by 180°F boiler water, use of the low-temp GSHP water requires a European-style tank that is loaded with copper tube inside. These tanks allow a transfer of about 98% of their stored energy into the DHW stream.

Even with this technology, you may still need an auxiliary backup heater to meet peak hot water demands. Backup heating can be any conventional water heating method including electric water heaters, tankless water heaters (gas or electric), stand-alone gas or oil-fired tank-type water heaters, or boiler/ storage tank combinations.

As with any DHW system, dont forget to take into consideration the largest individual dump load and make sure that your stored volume can fill that bathtub. All the highest efficiency systems in the world are virtually useless if they cant completely fill the Missus soaking tub in one fell swoop.

For air conditioning, you can use a conventional DX coil and air handler, or the GSHP can provide you with chilled water that you can distribute using conventional piping methods and remote fan coil units.

Don't forget to consider the potential for condensation in locating the fan coil unit and in your choice of insulation to cover the chilled water piping. Make sure the insulation you are using is rated for chilled water service.

Generally speaking, the heating load will exceed the air conditioning load, and the system sized to handle the 90% of the space-heating load will obviously be capable of carrying 100% of the air conditioning load.

If youve not yet done so, I would strongly suggest that you join an organization like the International Ground Source Heat Pump Association, which is based at Oklahoma State in Stillwater, Okla. You can get more information on this nonprofit organization by going to http://www.igshpa. okstate.edu/membership/home.html

Next month well begin looking at hydrogen fuel cell generators. Until then, Happy IGSHPA Hydronicing!

Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at [email protected] or by phone at 303/778-7772.