If you've recently installed a new condensing boiler -- Baxi, Viessmann, Buderus, MZ, Munchkin, etc. -- and are experiencing problems, we can help. As a "new" technology, condensing boilers require a special knowledge and new set of rules when it comes to piping and controls.
Unfortunately, many "old school" plumbers and heating guys don't fully understand or appreciate the new piping and control methods that condensing boilers require in order to work effectively, efficiently and trouble-free. As a result, many new condensing boiler installations are rife with problems -- frequent lock-outs and failures, excessive heating bills, over-heating and under-heating of rooms throughout the home, etc..
While the list of symptoms are long and varied, most fall within one of three categories: 1) piping problems, 2) under-control, and 3) poor commissioning.
Let's start with the first category -- piping problems. Conventional non-condensing boilers are a relatively simple affair. When the thermostat calls for heat, the boiler turns on and gets hot (180 degrees +), hot water leaves the boiler, goes through the radiators, and comes back at about 160 degrees. When the room is sufficiently heated, the boiler turns off. The piping to accomplish this is also simple -- supply-piping out, return-piping in. Done deal.
Unlike their bigger, meatier non-condensing cousins, condensing boilers, by and large, are designed with smaller, narrower heat-exchangers. As a result, there is a substantial "pressure-drop" through the boiler. In condensing boilers, the heat-exchanger acts as a bottle-neck that gives standard circulator pumps a heck of a time pushing sufficient flow through both the boiler and the radiators it's connected to.
Many installers fail to take this high pressure-drop into consideration when piping a new condensing boiler into an existing system. The result of this oversight is often a house that becomes under-heated, particularly as the weather gets colder.
The fix is simple -- read the boiler's installation manual! Most, if not all, manufacturers provide detailed piping diagrams for their boilers. And most, if not all, involve some form of "primary-secondary" piping and/or hydraulic separation. The theory and practice of hydraulic separation is too much to get into hear, but to read more visit here.
As an aside, homes with small heating loads, say, less than 70,000 btus, depending on the boiler, may not need hydraulic separation. Because the load is small, the boiler pump can move enough btus, despite the bottle-neck, to heat the home sufficiently. Also, homes with large diameter pipes (originally designed for an old, gravity-type system), can also often be heated without hydraulic separation, since the flow-rate requirements and pressure-drop through the pipes are usually very low. For everyone else, hydraulic separation is the way you need to do it.
The next category of problems results from under-control. Condensing boilers are typically designed with modulating burners, which (theoretically) ramp up or down depending on heating conditions. Most boilers have one or more mechanisms to tell the boiler when to step on or let off the gas, but the usual starting point is feedback from temperature-sensors within the boilers supply and return pipes. When the temperature difference between supply and return increases, the boiler assumes more heat is need to the house and steps on the gas. As the temperature difference decreases, it steps down.
This is fine for many applications, but also has some shortcomings that, under certain conditions, can cause over-firing and/or short-cycling. Some cheaper condensing boiler rely solely on boiler supply/return temperature-sensors to modulate the burner. Higher quality, more sophisticated models, however, also utilize one or more forms of external feedback in the form of an outdoor-reset control and/or indoor temperature-feedback. Such controls often convert outdoor or indoor temperature feedback into a 0-10vdc electrical signal that ramps the burner up and down depending on the weather outside or the indoor ambient temperature. Such controls do a superior job at matching the boiler's output to the actual load of the home, and maximizes the boiler's efficiency.
Some boilers, like the Viessmann Vitodens 200, provide both outdoor-reset and indoor temperature feedback as a standard feature. Other boiler manufacturers sometimes offer such features as an optional upgrade. In these cases, unfortunately, many installers fail to inform homeowners of the importance of such controls and fail to integrate them into the home.
Why are such controls so important? They are often the main means by which the boiler actually condenses its flue-gases and provides combustion-efficiencies in the 90 percent + range. In order for a boiler to actually condense, its flue-gases must drop below the magic number of 137 degrees fahrenheit (in natural gas-fired boilers). In order to meet this target temperature, the boiler must produce lower than normal water temperatures, which, in turn, means it must lower its burner-output. If, on the other hand, the boiler fails to maintain sub-137 degrees, its efficiency is only in the 85 percent ballpark at best. Or, if it maintains low temperatures, but must run short cycles to do so, you add significant wear and tear on the boiler and in some cases may not be actually achieving high combustion efficiencies.
The third category of problems often associated with condensing boilers is often due to poor commissioning practices by the installer. Some condensing boilers are equipped with sophisticated computers. That said, the computers are only as smart as the user and require some basic inputs by the installer that affect the modulation and control of the boiler. Some installers fail to provide such inputs and allow the boiler to run off its default settings. That result is a boiler that fails to operate at maximum efficiency.
In other cases, some boilers require an initial adjustment to the boiler's modulating gas valve. Failing to match the incoming gas pressure to the appropriate outputs (high and low fire) can result in over or under-firing and, short-cycling, or even outright malfunction and breakdown. Proper commissioning is crucial to owning a happy condensing boiler.
Finally, another often-seen problem with condensing boilers is the failure to properly match the boiler with the right distribution system (i.e. radiators, baseboard, etc.). As efficient as condensing boilers can be, they're not always a good candidate for some homes, and aren't worth spending the premium to install. Some homes, particularly those with finned-tube baseboard, require high boiler supply water temperatures to remain heated comfortably. Since condensing boilers can't condense above 137 degrees, it doesn't make sense to employ one if the home's baseboards require higher temperatures.
Now, if you're in a position where you've had a condensing boiler installed on a baseboard system and you have't noticed a significant drop in fuel bills, you can always upgrade your standard-output baseboard to a "high-capacity" model and/or increase the length of it whereever feasible.
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