One of the problems with wood heat is that it's tough to match the output of the boiler with the heat demands of the house. Without storage, there are three choices: Build lots of small fires, let the house temperature fluctuate a lot, or let the boiler idle a lot.
This gets at the heart of a major issue with wood heat. Building fires is a lot of work, and nobody likes to be cold. Many users take the third option and keep their fires going 24/7, running them at a small percentage of the boiler's rated capacity. This is convenient and helps maintain an even temperature, but idling is not ideal. This type of operation typically produces smoke, creosote, and reduced efficiency. Outdoor wood boilers represent perhaps the extreme of this approach.
Heat storage provides a way to capture the excess heat produced by the boiler and use it when the boiler is not burning. This allows a series of short, hot, efficient fires with time between fires where the heat comes from storage.
Storage Pros: More efficient fires, and flexibility about when to build fires. Reduced temperature fluctuations in the house. Near-infinite domestic hot water.
Storage Cons: Storage involves additional cost and complexity. Planning and designing storage is a big task, and qualified third parties may be hard to find. Both plumbing and controls become more complex. Storage requires space.
Our choice: We started without storage and added it after the first season. The big benefit to us is the ability to space out fires. We can often skip one or more days, especially early and late in the heating season. The house temperature remains constant, and we never run out of hot water.
Storage tank systems can be broadly categorized into two classes: pressurized and unpressurized. Pressurized systems are closed tanks where the water in the tank circulates through the boiler. In unpressurized systems the water in the tank is isolated from the water in the boiler, and heat transfer is accomplished via heat exchangers either in or out of the tank. 500 gallon propane tanks are often used as pressurized storage tanks. Unpressurized systems are very often open top containers, and may take many forms, including cisterns, 'above ground pool' type construction, and metal tanks of various configurations.
Pressurized pros: Because the water in the tank is being used directly, there is no temperature drop effect. If the water at the top of the tank is 180 degrees, then the water going to the heat zones is 180 degrees. With proper attention to plumbing, a higher degree of stratification can be maintained. There is no need for and no expense involved in a heat exchanger for the tank.
Pressurized cons: Pressurized tanks have to be designed and constructed to withstand the highest possible system pressures (typically 30 psi). The increased volume of water means that a much larger expansion tank is required - often in the 60 to 80 gallon range. The additional volume of water means that there is more dissolved oxygen to cause corrosion of metal components. It's difficult or impossible to add heat exchangers in the tank for things like domestic hot water preheat or solar panels.
Our choice: We went with unpressurized storage, mostly because we need heat exchangers for DHW and solar. We also found an unpressurized stainless tank that fit our available space perfectly.
There are a near-infinite variety of plumbing approaches, but one useful distinction is pump-based vs. valve-based zone control. In a pump based system, each heating zone has its own circulator pump which runs when there is a demand from that zone. In a valve based system, there is one circulator pump at the boiler and each heating zone has a zone valve which opens when there is demand from that zone. If there's more than one heat source, each source typically has its own circulator.
In a pump based system, each zone thermostat goes to a controller which controls the pumps.
In a valve based system, each thermostat is wired directly to the corresponding zone valve. Contacts in the zone valves provide a demand signal to the boiler when the valve is open.
Circulator pumps and zone valves cost about the same, so there is no significant cost difference. Both are reliable and easily repaired.
New pump technologies are producing pumps with much lower energy consumption as well as variable speed pumps with smart controllers. At the present, these pumps are not widely available in the USA.
Some pump-based systems are quite complex - see the section below on primary / secondary loops.
|Valve Based Zone Control||Pump Based Zone Control|
Pump pros: Pumps can be sized to the flow requirements of each zone. Flow rates can be better controlled. Newer technology pumps offer the possibility of very precise matching of flow to heat loads in the future.
Pump cons: Pumps consume more power than valves. Pumps do not provide positive isolation - water will flow forward through a pump even if it has a check valve. It can be more difficult to understand what's happening in a system with multiple pumps.
Our choice: We went with a valve based system, in part because that's what our plumber had recommended for our initial oil boiler installation. The lower power consumption is also a factor, and our installation is designed so that there is only one active heat source and one pump running at any time. That make it easy to understand what's happening with the system. We are upgrading to a multiple speed pump for the wood boiler to provide better control of flow rates.
If you're adding a wood boiler as a heat source, there's likely a gas or oil boiler in place that will act as a backup heat source. In some cases, the oil or gas boiler has an internal coil that provides hot water. There are two basic options for plumbing the new boiler: It can be plumbed in series with the existing boiler, or it can be plumbed in parallel.
A series installation routes the outlet of the wood boiler to the inlet of the oil/gas boiler, often using only a single circulator. A parallel installation has each boiler drawing from a common return manifold and pumping to a common supply manifold. Each boiler has its own circulator with a check valve to prevent reverse flow when the other boiler is operating.
Parallel Pros: The idle boiler is not heated, so there is no standby loss to the room or up the chimney from the idle boiler. Since the idle boiler does not have to be heated, the system comes up to temperature more quickly. Either boiler can be isolated and repaired or replaced without losing the ability to heat the house.
Cons: If the oil/gas boiler has an internal coil that provides domestic hot water, a parallel installation will require additional work and expense to provide hot water. Parallel installations involve a bit more plumbing and require an additional circulator.
Our choice: We chose parallel, and that's probably the best solution in most cases. Losing heat up the oil boiler chimney 24/7 was not appealing.
Ideally, every heat load and heat source has a flow rate that's well matched to its capacity. Ideally, the plumbing is simple and straightforward with a minimum of components and low power consumption. These objectives are in conflict to some degree. For larger or more complex systems, the primary/secondary loop approach is often used.
In this technique, there is a primary loop that all heat sources and heat loads connect to. The primary loop has a relatively large circulator that runs whenever there is any active heat transfer. Each heat source and heat load has its own loop with its own circulator that runs whenever that loop is active. Connections are made using closely spaced tees so that there is no flow in any loop if that loop circulator is not running.
In supply/return systems, there is a supply (hot) manifold and a return (cold) manifold. Each heat source and heat load is connected between the two manifolds. Heat loads may be managed with either circulators or zone valves (see section on circulator vs. zone valve control above). Each heat source typically has its own circulator.
It's also important to realize that there are many possible variations on these approaches. For instance, you could have two heat sources in parallel as part of a single secondary loop, or the heat sources and storage could form the primary loop as shown below.
Advantages of Primary / Secondary: It's possible to closely match the flow in each loop to the needs of the heat load or source. Multiple heat sources can be active at the same time. Adding sources and loads causes minimal disruption.
Advantages of Supply / Return: Conceptually simpler and easier to understand with fewer components. Typically only one pump is running, so power consumption is lower.
Our choice: We went with a supply / return configuration. My sense is that if pressurized storage is an option, the hybrid system shown above would be a very good solution for many residential applications.