I’m often asked by clients, why do we need a buffer tank (with a hydronic – water-to-water – geothermal system)? After all, they add cost and take up space.
The reason for a buffer tank is to enable reasonably long run times once the heat pump is energized to (1) minimize compressor wear (and extend its life) and (2) improve overall energy efficiency. Like most powered devices – think cars and computers, a far-above-average amount of energy is consumed and wear occurs during the instant the device is turned on; once on, the device runs much more smoothly and at a lower rate of power consumption.
In a hydronic geothermal system, the heat pump is directly heating water (as opposed to air) which is eventually conveyed throughout the building to distribute heat where it’s needed. Without a buffer tank, single- and dual-stage water-to-water heat pumps would be energized every time a zone within the building calls for heat. In most cases, the heat pump would raise the temperature in that zone a few degrees in a matter of minutes, the thermostat setting would be reached, and the heat would shut off. Depending on the rate of heat loss from that zone, it might be only 5 or 10 minutes before the temperature in that zone drops a few degrees and calls for heat again – the heat pump is energized and the cycle repeats.
If, instead, the heat pump’s job is to maintain a tank of water within a specified temperature range – 10 to 15 degrees F is typical, when a zone calls for heat, it draws it directly from the tank; the heat pump is not energized until the tank temperature drops below it. Often, the zone’s heat requirement will be satisfied by the buffer tank without having to energize the heat pump. For example, let’s say the buffer tank temperature range is set at 100 to 115F. When zone #1 calls for heat, the tank is at 114F. As the heated water is circulated to the zone’s heat emitters (e.g. radiant floor, baseboard, and/or hydronic air handler) then back to the tank, the buffer tank temperature drops. By the time the zone call is satisfied, the tank temperature has dropped to 108F. Soon after, zone #2 calls for heat. Again, the buffer tank temperature drops as it addresses the zone call. Let’s assume the heat loss in zone #2 is significantly greater than that for zone #1, so the tank temperature drops below 100F before its call is satisfied; the heat pump is energized to replenish the heat in the tank. Before the heat requirement for zone #2 is satisfied, the heat pump raises the buffer tank temperature to 115F and shuts off. In this simple example, the heat pump was on for one cycle and two non-overlapping zone calls were satisfied; without a buffer tank, the heat pump would have run for two shorter cycles.
In order to minimize stand-by losses – heat in the tank that is lost to its surroundings through the walls of the tank, we set up the controls to shut off the heat pump if no zones are calling for heat, even if the tank temperature has not yet reached its upper set point. In the previous example, if the call from zone #2 had been satisfied before the heat pump raised the tank temperature to 115F, the heat pump would have been shut off. By doing this, we’re reducing the stand-by losses – the higher the tank temperature, the higher the rate of stand-by loss.
The presence of a buffer tank also enables another efficiency improvement – outdoor reset. Outdoor reset refers to a control scheme whereby the buffer tank temperature range setting is automatically adjusted based on the outside temperature – the lower the outside temperature, the greater the building heat loss rate, the higher we need the tank temperature range to be to satisfy it. For example, when the outdoor temperature is 15F, the buffer tank temperature range setting might be 98 to 113F, but when it’s 55F outside, the range is lowered to 92 to 107F. There are two key principles that we’re taking advantage of here. First, it’s more efficient to maintain our heated water at a lower temperature. (Extending this principle to boilers, helps to explain why older boilers, which required water temperatures to be greater than 140F to avoid flue condensation that risks destructive corrosion of the boiler and its components, are much less efficient than newer, so-called condensing boilers that can safely operate at temperatures well below 140F.) The second is that by lowering the tank water temperature range when it’s warmer outside, we’re better matching our system’s heat output to the building’s heat loss, thereby enabling longer – more efficient – cycle runtimes for the heat pump.
Now for the exceptions: There are two circumstances for which a buffer tank may not be necessary.
If the system is only heating a thick concrete slab (radiantly, of course), the amount of heat required to raise the temperature of the high-thermal-mass slab a few degrees (to satisfy the zone thermostat) will require sufficiently long heat pump runtime to avoid detrimental short cycles. (Similarly, the slab stores so much heat that it will be able to satisfy the zone’s heat requirements for much long periods without heat pump input than low-mass heat emitters like baseboard and thin-slab radiant floors.)
The second exception has only recently emerged… as a few models of water-to-water heat pumps now incorporate variable-speed compressors. A variable-speed compressor enables the heat pump to modulate its output to better match the building or zone’s heat loss, thus enabling longer heat pump runtimes and minimizing short cycles. Enertech, the manufacture of the Hydron Module line of heat pumps (which we handle, by the way), introduced its first variable-speed water-to-water heat pump a few months ago. It’s sure to be a hit with multi-zone single-family dwellings.