The Glitch

Increasing fuel costs, in combination with the desire to be carbon-neutral, has prompted a homeowner to purchase a wood gasification boiler to act as the primary heat source for a new home with low-temperature radiant heating. The backup heat source will be a propane-fired mod/con boiler.  

The owner has read that a generously sized thermal storage tank is necessary to allow a gasification boiler to operate at consistently high efficiency. He finds a local source for a 500-gallon, pressure-rated tank and buys it.  He then hires an installer to put these components, along with the balance of the system, together. The resulting installation is shown here (right).

A setpoint controller monitors the temperature at the top of the storage tank and turns on circulator (P1) whenever there is a call for heat and the tank’s temperature is above 110 degrees F.   

The mod/con boiler has a system supply sensor that continually measures the water temperature supplied to the manifold stations. If this temperature drops below the minimum acceptable water temperature determined by the boiler’s control, the boiler and boiler circulator are turned on to provide backup heat.  

Challenge: There are a lot of things incorrectly installed or missing in this situation.  Can you find at least six things that should be changed?

The Fix

Here’s a listing of some things that need changing, in no particular order. Changes are shown in the figure shown here (right).

1. All pressurized wood-fired boilers should have a heat-dump provision in the event of a power outage when the boiler is firing. A relatively simple way to do this is to install a passive convection loop of copper fin-tube element mounted above the boiler, in combination with a normally open zone valve with relatively low flow resistance. If the power goes out, the zone valve opens and hot water thermosiphons through the fin-tube to dissipate heat.  

2. Wood-fired boilers also should be protected against low-temperature operation. There are different ways to do this using mixing devices, or using a boiler with a built-in “circulator toggle” that measures boiler water temperature and decides if the bypass circulator or tank circulator should be running. Only one of these circulators runs at any given time.

The internal control toggles between these two circulators as necessary to keep the entering water temperature high enough to minimize creosote formation in the boiler. Notice that both these circulators are shown with internal check valves to prevent reverse flow.  

3. The 5-gallon expansion tank shown in the Glitch drawing, while often large enough for a typical residential hydronic system, is definitely too small to handle the expansion of 500 gallons of water, plus the water in the remainder of the system. The 44-gallon tank shown is representative of the size necessary for the system assumed in this schematic. However, the expansion tank size for a given system should always be verified through proper sizing methods.  

4. In the Glitch drawing, there is no means of regulating the water temperature supplied from the storage tank to the low-temperature distribution system. It’s very possible the storage tank could reach some very high temperatures (180 to 200 degrees F). Such temperatures are much too high to send directly to the distribution system.

The correction is to convert circulator (P1) into a variable-speed injection pump. The controller driving this circulator monitors the temperature supplied to the zones, and regulates the injection pump speed to maintain either a setpoint or a supply temperature based on outdoor reset control. The latter option requires the outside sensor.  

5. The distribution system shown in the Glitch drawing is not a primary/secondary system. The zone circuits are connected across the upper and lower horizontal piping; thus, these pipes are headers.

In the Fix drawing, the left ends of these pipes have been capped. The “primary circulator” has been eliminated and the piping is labeled as “generously sized headers.”  Such sizing allows the zone circulators to provide flow through the two sets of closely spaced tees that connect to each heat source.  

6. Allowing the mod/con boiler to come on whenever the tank temperature drops below a specified value will ultimately use the mod/con boiler to keep the storage tank warm. This is unnecessary and undesirable. Instead, the injection-mixing controller must be disabled whenever the mod/con boiler is operating. The latter detail prevents water heated by the mod/con boiler from circulating into the storage tank.

By the way, the boiler piping is also incorrectly reversed at the closely spaced tees.  

7. The injection pump (P1) and the tank circulator (P2) have been positioned so that their internal check valves discourage thermosiphon flow from the storage tank.  

8. A separate setpoint (or reset) controller is shown in the Fix drawing.  It monitors storage tank temperature and determines whether the storage tank or mod/con boiler serves as the heat source upon a demand for heat.  

9. Purging valves have been added at key locations.  

10. A second air separator has been added to the distribution side of the system.

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