About 30 years ago, very few hydronic systems in Canada operated with outdoor reset control. Today, practically every system does, or at the very least, the controls being provided always give the user the option. So, what changed to drive such a massive shift in the way we control one of the world’s oldest heating technologies?
Besides the cost of digital controls generally plummeting, and the integration of boiler controls into boilers slowly becoming standard design policy, the big driver was the advent of the condensing boiler, which benefits greatly from reducing the boiler water temperature. Outdoor reset delivers precisely this outcome, but its benefits actually go well beyond improvements to boiler efficiency.
What is it?
First, a couple of things to keep in mind. For one, every structure has a total amount of heat energy contained within it, known as thermal mass. A typical hospital will have an enormous thermal mass, while a typical school can be expected to have less. What any heating system is trying to do is to add heat back into this thermal mass at the same rate that it is losing it to the outdoors. This can vary widely for many reasons, including how tight the building envelope is. In theory, the hospital and the school can have the same rate of heat loss, even though the hospital has more mass.
Indoor temperature targets typically don’t change, usually 22 C or 72 F, but a structure is going to lose heat much faster when it’s -40 C or -40 F outside than when it’s 10 C or 50 F. Think of a building like a water balloon with a pinhole leak. If you squeeze the balloon with your hands, the water will jet out faster, but if you hook the balloon up to a garden hose, you can keep it inflated anyway and replace the lost water to maintain the size of the balloon. A drop in outdoor temperature is like thermal pressure, trying to extract heat from a structure with increased speed, and the heating system must make up this rate of loss to maintain the structure’s thermal mass and maintain occupant comfort.
Stopping heat
Traditionally, the only way to stop a hydronic system from delivering heat into the space was to interrupt it with a thermostat, which is there to stop a heating system designed to keep a space warm even when it’s extremely cold outside from overheating that space whenever it’s warmer. Most hydronic heat terminal units (the technical name for where the heat leaves the water and enters the space) have been designed to supply water temperatures for the coldest day of the year, but still work to deliver some heat at cooler temperatures. Fin-tube baseboards may deliver the required design day heat load at 80 C or 176 F, for example, but while cooler water reduces its efficiency, it will still deliver about half as much heat at 60 C or 140 F.
So, if on a given day, you only need about half as much heat as you do on the design day, why not just supply the heat terminals with cooler water? That’s the reset part, automatically supplying cooler water to the system because the weather is warmer.
The rate of heat loss has dropped, and less heat is needed. Then, when a thermostat closes to allow flow into a zone, the water that arrives from the system is cool enough to replace only the heat the room is actually losing, and the room temperature stabilizes. In single zone systems, an outdoor reset controller working like this can actually replace the need for a thermostat entirely if it’s controlled well enough.
In multi-zone systems, outdoor reset will add value in that it will tend to cause more thermostats to be closed and more zones open at any given time. This reduces cycling on all of the equipment in the building, on top of increasing comfort. If the heat is coming from a single central source, it would typically just be programmed to supply water hot enough for whatever zone is the coldest and needs the highest temperature. For this zone, you may never stop supplying heat, while in every other zone where this supply temperature is too high, thermostats will step in to limit its overheating.
Understanding the system
There is a way for supply temperature reset to be based on indoor temperatures; the problem is just that you only know your supply water was too hot when it gets back to the mechanical room without losing as much heat as you expected. Outdoor reset is proactive instead, anticipating increases and decreases in the rate of heat loss based on what’s happening in the environment. The best systems will use both, however, because the ratio between the rate of loss and thermal mass will determine how long it will take for a change in outdoor temperature to cause a change in the indoor temperature, known as the structure’s thermal inertia. If the hospital and school have the same rate of loss but the hospital has more mass, it will take longer to start cooling perceptibly, meaning it has more inertia.
Once you determine what supply water reset temperature you want to deliver, there’s a couple of options for producing it. One is mixing reset, where a valve or circulator is used to blend the supply and return water together to knock down the supply to the level needed. Another option to consider is what used to be called boiler reset, where the boiler supply temperature itself drops down. This will tend to increase the boiler’s thermal efficiency, especially if the return temperatures can get into the condensing range, typically below 50 C or 122 F. This is where boiler efficiencies start to climb into the above 90 per cent range.
In multi-zone systems it may be necessary to do both mixing and boiler reset. For example, if more than one supply temperature is needed because of the use of different heat terminals, the hottest supply requirement could be met with boiler reset, while cooler zones drop further down with mixing reset. This is more common in commercial design where the systems are more complicated.
In residential systems, typically one temperature is needed for heating, but the system may also have an indirect water heater, which typically requires very hot water to allow a quick recovery.