As heating professionals, we recognize dozens of things can cause such a situation. Aside from the obvious problems caused by poor design or installation, there are several less conspicuous snags lying in wait for unsuspecting hydronics specialists. Here are a few I’ve encountered:

Year-Round Cooling: Many hydronic heating systems are installed in homes that also have a central forced-air cooling system. Usually the air handler and ducting for the cooling system is installed in the attic above the ceiling insulation. Supply air diffusers are placed at ceiling level (which is usually ideal for a cooling only system).

A serious problem can develop when the ducted system supplies diffusers on two different levels of the building. Figure 1 shows a typical situation. During cold weather the ducting above the insulated ceiling cools off, even if it’s insulated. Air in the ducting cools and its density increases. Combine this with a return grill on the upper floor ceiling and you’ve got a perfect set-up for a convective loop.

Warm air inside the house goes up through the high ceiling return, cools within the ducting and pours down through the lower ceiling diffusers. The result is a self-powered cooling system that can pump lots of cool air into your hydronically heated spaces. Enough cool air prevents your system from sustaining comfortable temperatures. The colder the attic, the stronger the convective loop becomes. Any cold air leaking into the ducting only aggravates the problem. Ducting routed vertically in exterior walls is also a willing participant in convective loops.

The simplest solution is to cover the supply air diffusers and return grills during the heating season to block the air flow. Although some ceiling diffusers come with adjustable vanes for controlling air flow, many don’t close tightly. Thin magnetic sheeting is available that can be cut to fit the shape of steel registers and grills. It’s flexible enough to partially conform to the shape of the registers and grills, providing a reasonably tight air seal along the perimeter. Have the owner put them in place in the fall and remove them in the spring. By the way, those covers would probably be a great place to put your company logo extolling the comfort advantages of hydronic heating.

“Well obviously that floor heating system is the problem:” Here’s a synopsis of a phone call I got this past winter. The owner of a marina built a new steel-frame building for servicing boats. Because boats would be coming into the facility wet and snow-covered the appeal of warm and dry floors really clinched the deal offered by the local hydronic specialist.

The new hydronic system did just what it was supposed to. It warmed and dried the floor. The problem was the evaporated water had to go somewhere. The general contractor failed to seal the vinyl vapor barrier on the fiberglass batts draped over the roof framing, or provide building ventilation. Air leaking up into the ceiling carried vapor with it until the latter encountered some nice cold thermal imperfection (such as where the insulation was highly compressed by the steel roofing), and condensed back to liquid. The water droplets flowed down the top side of the vapor barrier until they found a suitable (unsealed) seam, from which they cascaded to the floor. The colder it got outside, and the better the floor dried itself, the faster the ceiling rained.

Anyway, when the owner brought the problem to his attention, the G.C. immediately went into blame-shifting mode.

The G.C. contended the problem was solely the result of the heating system drying the floor and putting all that water vapor in the air in the first place. Using his logic, I would have to conclude if the owner needed to use pressure washers in the building all day long the same problem couldn’t possibly occur.

What do you think? If this were your project, would you let the G.C. “spin” the story to the non-technical owner to get off the hook? I can probably guess what most of you hydronic heating contractors are thinking. But lest you conclude that the G.C. is a total slime ball, consider the fact that a better informed heating professional might have averted the entire situation by alerting both the owner and G.C. that vapor barrier detailing was critical, especially given the intended use of the building and the fact that heated floors do dry up wet floors and increase interior humidity.

Incidentally, an experienced “forensic” engineer estimated new metal buildings with severe moisture problems have about two years before corrosion of fasteners can become the overwhelming, and potentially catastrophic problem.

The Cable Guy Did It: Another recent experience was a floor heating job for a retail building with a showroom and several offices. Comfort was just fine in the building, except for a small break room. This room had its own relatively short circuit of 5/8–inch tubing 9 inch o.c. and thin carpeting. The balancing valve on the manifold was wide open, and water temperature and circuit temperature drop were all acceptable. As I walked in the room I could feel the radiant coolness of the ceiling. I popped up a ceiling tile and looked up at kraft-faced fiberglass batts with no ceiling board or air barrier. Although not a great detail, it was still the same detail used in the other (very comfortable) rooms.

After putting the ceiling tile back in place, I went outside and looked at the shadowed side of the roof. A nice layer of snow blanketed all areas except over that room. Hmmm, what’s going on here? Something is obviously letting heat escape at a higher rate over that room. After returning to the room, (this time with a flashlight), I popped another ceiling tile, and viola, there it was. A piece of fiberglass insulation 2 ft. x 4 ft. had simply been torn out of the framing and tossed up elsewhere in the attic. I was now looking directly at the underside of the roof sheathing. Cool air poured down on me like a waterfall. Up through the vast gap ran a 1/4–inch diameter coax cable for the room’s TV set. Hey, nobody ever explained convective loops to Sparky down at the local cable company, where the motto regarding insulation is “You seal it … we peel it.” The fiberglass batt was replaced and the problem was solved.

Check The Details: In addition to the deficiencies described above, other conditions can compromise the performance of your hydronic heating systems if ignored:

• Poor air sealing around recessed lights.
• Poor (or non-existent) dampers on ceiling exhaust fans.
• Non-continuous or punctured vapor barriers (Especially important in high moisture areas like bathrooms).
• Lack of band joist insulation and air sealing along foundation sills. This one’s especially important when installing underfloor radiant heating. Air leakage across the building through the heated joist cavities can purge heat straight outside.
• Any situation that allows air to move through or around fiberglass insulation. Don’t count on fiberglass to stop air flow. Remember, they make furnace filters out of this stuff.
• Missing insulation behind shower stalls on exterior walls.
• Air leaking into soffits over kitchen cupboards.
• Wall framing where top plates are above the ceiling insulation (which allows convective loops to be established within stud cavities).
• Air flow through mechanical chases that pass up through multistory buildings. Ideally these chases should be sealed at each floor level.
• Poor air sealing above suspended ceilings. The guiding principle of some insulation installers seems to be: If you can’t see it from below don’t worry about it. The classic case is fiberglass insulation (sometimes without facing) installed between roof framing with no sheetrock or other air barrier layer below it. Unfaced batts installed like this just make one giant air filter between room air and an unheated attic.
• A high water table under heated slabs. High water tables should be lowered by proper site drainage.
• Lack of adequate insulation details along the edge and under heated slabs. Few buildings not specifically planned with floor heating in mind will have adequate slab insulation. Make sure the building designer knows what is necessary when floor heating will be used.

As heating professionals, we can’t ignore the fact that our heating systems are really sub-systems to the building itself. Even the best hydronic heating system in a “thermally challenged” building will seldom give the desired results. We owe it to our customers to check out the energy detailing of the buildings for which we provide heat, and follow-up early by tactfully suggesting corrections while there’s still time to fix the problems. Prudent suggestions will often lower both the installation and operating cost of our systems, yield greater comfort and make you more competitive against those who simply decide to “overpower” lousy energy detailing with larger, more expensive heating systems. It’s one of those elusive win-win scenarios.