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Simplest Pressurized Storage

This was originally posted on Hearth.com. It's the result of many discussions with people who were daunted by the prospect of designing heat storage into their system.

This topic has come up many times, and I don't think I've been able to explain this well enough. Here's an attempt to explain storage plumbing and control for the simplest and most bulletproof design that I've been able to come up with. Bottom line: Storage does not have to be complicated.

This example is based on a simple parallel wood/oil system with zone valves. This is a very common approach, and is typical of wood/oil systems. The same basic approach would work with other system designs such as systems that use a circulator for each zone rather than a zone valve.

Baseline System: Plumbing

In this system, each boiler has a circulator that runs whenever that boiler is hot. These circulators need to have check valves to prevent reverse flow. Whenever there's a deamnd for heat, the associated thermostat or aquastat's contacts will close, applying power to the zone valve motor, The zone valve will open, allowing flow through that zone.

In all cases, flow is clockwise - upward through the boiler out to the zones, then back to the boiler.

Baseline System: Controls

Each heating zone has a zone valve that's controlled by the zone thermostat (or aquastat in the case of the indirect DHW that I've shown). This is the normal configuration and no changes are necessary.

Zone valves have built-in limit switches with contacts that close when the zone valve is open. Normally those contacts are wired in parallel and connected to the oil boiler demand input. Here, we'll have to make some changes.

Storage System: Plumbing

One or more pressurized storage tanks (used LP tanks are a good option) are installed in parallel with the boilers. One goal of the system is to maximize thermal stratification. In order to do that, we want flow in storage to be top-to-bottom when charging the tanks, and bottom-to-top when extracting heat.

The plumbing connections to storage are important. Tees must be oriented so that flow to and from the heat loads is directed preferentially through storage. This helps avoid 'ghost flow' through the boilers that could occur when the load circulator is running and the boilers are shut down.

In addition to the storage tanks themselves, an additional circulator is added to the plumbing, This is called the Load Circulator and its job is to ensure that the appropriate flow is maintained through the heat loads if and when heat is demanded.

Storage System: Controls

There are a few changes to the control wiring. First, the zone valve limit switches are removed from the oil boiler demand input and connected to the 24vac thermostat transformer (see schematic).

Second, we need a 24VAC relay that is energized when any of the zone valve limit switches are closed. This relay will turn on the Load Circulator.

Third, we need a 24VAC relay for the oil boiler demand input - when this relay is energized, it creates a closed circuit that turns on the oil boiler. This takes the place of the zone valve limit switches.

Finally, we need an aquastat at the top of our storage with contacts that close only when the storage temperature drops below the minimum usable temperature - we'll say 140 degrees for this example. The signal from the zone valve limit switches passes through this aquastat before going to the oil boiler demand relay. In that way, if the storage is above 140, the oil boiler will not come on when the zones call for heat.

Theory of Operation

1) Heating from storage

If there is any heat demand, then the corresponding zone valve(s) are open and their limit switch(es) are closed. This energizes the Load Circulator. If the storage is above 140, and the wood boiler is cold, then the load circulator will draw hot water from the top of the storage and return cool water to the bottom of the storage. At this point, there is a clockwise loop from storage through the zones and back. There is no flow through either boiler.

2) Heating from Wood Boiler

If the wood boiler is fired up, then the wood circulator will come on. If there's still demand, then the load circulator is still running and the bulk of the flow will be from the wood boiler through the zones and back. Depending on the relative flow rates of the two circulators, there may be some flow into or out of storage.

3) Heating storage from the wood boiler

If the wood boiler is running and there's no demand, then the load circulator is off and there is no flow through the heating zones. In this case, the flow is from the wood boiler clockwise to the top of the storage and back from the bottom of the storage to the wood boiler.

4) Heating with oil

If there's demand from one or more zones and the storage is below 140, then the storage aquastat contacts are closed. This passes the zone demand signal to the oil boiler demand relay, which will turn on the oil boiler. At this point, the primary flow is from the oil boiler through the zones and back to the oil boiler. As in the case with the wood boiler, there may be some small amount of flow into or out of storage based on the difference in flow rates between the oil circulator and the load circulator. This should be minimal and not be a big concern.

With this approach, storage adds one circulator, one aquastat, and two relays. The system does exactly what you would want - heat from the wood boiler supplies zone demand first, and heats the storage whenever there's excess heat produced. Heat demands are satisfied by wood if it's running, then by storage if it's available, then by oil as a last resort. No additional controller is needed.


There are no components that are particularly specialized. The relays, aquastat, and circulator can be obtained almost anywhere. If you have the time and inclination. eBay is a great source.

Any 24vac relay that can switch a couple of amps will work fine - Omron's LY2-AC24 (sold by Digi-Key as Z-788-ND) for example - they're about $8 each, and you can get plug-in sockets for them that simplify connections.

The aquastat needs to be a break-on-rise model. For example, the Honeywell L6006A1145 would work well, and it has an adjustable differential temperature. It's about $80 from PexSupply and other sources. Installing a well for it is an exercise left to the reader.

The load circulator should be similar in size to the existing circulator(s), or smaller if the heat loads are much smaller than the output of the boiler.