Providing auxiliary heating in a solar house could have temperatures rising.

Hydronic professionals are sometimes asked to provide an "auxiliary" heating system for a passive solar house. You know the type: A wall full of south-facing glass, intended to soak up lots of free solar Btus; a live-in photon collector, where owners hand out Oakleyg shades while greeting visitors at the door.

Upon examining plans for such a house, the heating contractor quickly spots the insulated concrete slab floor specified for thermal mass, and envisions it filled with tubing. This will be perfect they think. Let the sun warm the house by day, and the boiler by night (and cloudy days).

Unfortunately, buildings with even modest solar heat gains will experience significant overheating when the non-solar heat is supplied from a high mass heat emitter such as a radiant floor slab.

The wider the day/night outdoor temperature swings, and the more the solar heat gains, the worse the overheating. Just ask hydronics pros from the states in the Rocky Mountains.

A Matter Of Timing

In a passive solar house, the thermal mass of the concrete slab floor is intended to soak up solar energy during daylight hours. The warmed slab then releases this heat during the night as the house cools off. However, if the slab is already warm in the morning (from operation of a radiant heating system at night), its ability to soak up solar energy during the day is essentially gone.

It's like trying to capture a deluge of water using an overflowing cistern. Without a place to be stored, the incoming sunlight quickly raises the air temperature in the space, often well into the upper 80s. The occupants either sweat it out (if they're passionate about free solar Btus), or open the windows to dilute the tropical conditions.

Installing thermostats in the solar heated spaces that operate valve actuators to stop flow through the floor circuits when overheating occurs does not help much. That's because the Btus being released from the floor surface during the overheating period were transferred from the tubing into the floor slab several hours before overheating began - when it was cold and dark outside.

To get a feel for this, think about what happens in a passive solar house with a radiant floor "auxiliary" heating system, when a cold clear night is followed by a sunny day.

Start the process at 5 a.m. on a February morning. The outdoor temperature is approaching its lowest value for the day; perhaps it's even below zero. The outdoor reset control regulating water temperature to the floor circuits is sensing this situation and adjusting the mixing device to deliver relatively high water temperature to the floor circuits.

Heat is being injected into the core of the concrete slab at or near design load rates. Little does that control know, in about 4 hours, bountiful quantities of sunlight will be streaming in through east- and southeast-facing windows.

To complicate things a bit more, assume the tubing is located relatively deep in a 4- to 6-inch thick slab. It will take several hours for the heat surge to drive up the surface temperature of the floor, but drive it up it will. Just as the solar heat gains are coming on strong, the heat wave injected during the early morning hours reaches the surface of the slab to worsen the already toasty situation. The rapidly wilting owner seriously contemplates a naked roll through the snowdrifts outside.

Low Mass To The Rescue

What's needed in such situations is a heating system that can attain its rated heat output rapidly when turned on, and, even more importantly, stop delivering heat within a few minutes of being turned off - a thermal drag racer as opposed to a Btu freight train.

Several types of hydronic heat emitters can meet this need. Examples include fin-tube baseboard, thin panel radiators, as well as low mass site-built radiant walls.

Here are some possible scenarios for integrating such hardware into a passive solar house.

Scenario No. 1: Install tubing in the slab, to handle a portion of the load, in combination with a low mass heat emitter to supplement floor heat output during high load conditions. Limit the water temperature to the slab circuit, so the floor surface doesn't exceed say 75 degrees F. Why such a low surface temperature limit? Because when solar gains heat the room air (and room surfaces) to 75 degrees F, the temperature differential between the floor and room air (as well as the room surface) is zero, and heat output from the floor ceases. If the floor were allowed to operate with surface temperatures up to, say, 85 degrees F, as is typical in many systems, it would still be cranking out heat - even with the room well on its way to sauna status.

Because a 75 degree F floor can only deliver about 10 to 12 Btu/hr./sq. ft., supplemental heating will likely be needed. Those large windows that heat things up during the day represent a sizable heat loss when the sun is not out. Fin-tube baseboard installed under the window areas could provide the supplemental heating as well as counteract downward drafts from the windows.

Panel radiators with convective fins on the back side are another option. Floor heating as well as the supplemental heating could be controlled using a single two-stage thermostat.

Scenario No. 2: Let the floor be used solely as the thermal mass for the passive solar gains, and use low water content panel radiators or a radiant wall system to deliver the total design load. Because both types of heat emitters have low thermal mass, they can respond quickly. Be sure to zone the system so heat emitters in rooms receiving solar gains can operate independently of rooms with little or no gain.

Scenario No. 3: Use a ducted air handler with a hot water coil for auxiliary heating. Although the comfort won't match that of a radiant panel, an air handler is perhaps the ultimate low mass heat emitter, and, as such, can respond very quickly.

There also will be times when just the blower of the air handler could be operated to redistribute surplus heat from the solar spaces to cooler rooms. Another plus of this approach is the potential to deliver central cooling in warm weather.

A Few Precautions

I've heard of schemes that place the outdoor sensor of a reset control either in the attic or on the sunny side of the building, in an attempt to respond to a combination of air temperature and solar heat gain. Although this concept may help with a low mass system, it's not effective for high mass systems.

Again, it is a matter of timing. If the heat is injected into the slab when an outdoor sensor is cold, there's no practical way to prevent that heat from migrating toward the surface of the slab in the hours that follow.

Besides, how do you accurately calibrate a temperature sensor placed in the sun or in an attic to the solar heat gain characteristics of the building? What color should the sensor enclosure be? How much should its resistance change at a given solar intensity, etc.

There's just too much guesswork involved to rely on such an approach.

I'm also hesitant to suggest radiant ceiling panels as supplemental heat emitter in rooms that count on a floor slab to absorb large solar heat gains. The reason is that a radiant ceiling will heat the objects it "sees" below it, including the floor slab. If the surface of the slab is warm due to night time operation of a radiant ceiling, it won't be able to absorb solar heat gains when necessary.

In Case You're Wondering

Do I think passive solar houses deliver comfort comparable to less fenestrated homes with well-planned hydronic radiant heating systems? No way! But as long as architects win awards based on how many windows they can put on a wall, as long as clients ogle over all that glass, and if the cost of conventional fuels increases as it did in the 1970s, we hydronic professionals will be asked to provide "auxiliary" comfort in place of, or perhaps in spite of, the solar heat that pours through all that glass.

Whatever hydronic heat emitter system you choose for such a job, be sure it can respond quickly to load changes.