In a Nutshell
For an apartment building with a common furnace, the management chooses a date in the spring to turn off the furnace completely for the summer and chooses a date in the fall to turn on the furnace for the winter. The problem is that there may be alternating very warm and very cold days around the chosen dates. Nowadays there are inexpensive methods of automatically shutting down the furnace completely for a warm day and activating it again the very next day if that is a cold day..
Different Kinds of Systems
A good control system for any heating system is a way to turn off the system automatically when the outside temperature is warm enough and turn on the system again when the outside temperature drops.
Such systems come with different features and varying levels of complexity. My involvement with such systems started over 20 years ago when another complex where I owned a unit evaluated a proposal by a company that provided such systems.
I separated the controller components for a FHW (forced hot water) system as described into parts:
Part 1. An outdoor thermostatic sensor shuts off the circulator pumps when the outdoor temperature is warm enough. If the furnace did not also supply domestic hot water the entire furnace could also have been shut off.
Part 2. A second (or maybe also a third) aquastat together with sensor(s) and/or timer allows the furnace temperature to drop to a lower level on warmer days. "Wasted" heat is less when the difference between the furnace temperature and the basement or furnace room temperature is less.
Part 3. A motorized tempering valve is used to mix water returning from the radiators with freshly heated water from the furnace and thus exercise control over the temperature of the water going to the radiators.
I estimated the cost of Part 1 to be less than one fifth of the cost of the entire system presented and I convinced that condominium's directors to start with only Part 1 as a home grown project. Also I estimated that the majority (perhaps as much as 3/4) of the fuel savings would be achieved by Part 1 only. They decided to install it only in a few buildings for the first season and analyze the results before continuing.
The sales pitch for the "complete system" claimed that the residents' comfort is improved when the FHW temperature is varied depending on the outside temperature and when the various units call for heat fewer times but for a longer period of time each time. Room thermostats and zone valves work in the same way as before, with results subject to the limits imposed on the furnace by the system.
In the more modern systems a microprocessor controls the operation of the above components and there is a lot of programmable flexibility. It is expected that most of the benefits of the system will be realized in spring and fall when outside temperatures vary widely from day to day, with the furnace operating pretty much continuously on "high" during the winter. All of the systems I read about include a Part 1.
Varying the FHW temperature has its limitations.
1. The various condominium units may be in a sequence along the FHW trunk lines and the incoming water temperature for the last unit may be somewhat less than for the first unit.
2. When the system selects "low" for the FHW temperature, it takes longer to heat up the apartment unit of the resident who had turned down the thermostat for the day and who just got home from work.
Overcoming these limitations would impact the potential additional savings that Parts 2 and 3 achieve over Part 1.
One way of evaluating Part 1 (or the entire system if chosen) is to compare the percentage fuel savings over the course of an entire winter with the saleman's claim of percentage savings. At the complex I studied, there were several buildings so they had or were able to gather figures for:
1. Test building fuel costs before.
2. Test building fuel costs with system.
3. Another building fuel costs before testing season.
4. Another building fuel costs during testing season.
One of the condominium directors, prior to selecting Part 1 only, commented (correctly) that it alone might not completely stop the flow of FHW to the radiators. But after the cost savings were analyzed they decided to go with a home grown Part 1 only for all buildings.
An important consideration is ease of override should a system malfunction occur. The motorized tempering valve in Part 3 could be an item difficult to override without a service call if it failed.
Insulating the FHW trunk lines passing through walls and ceilings is a separate topic.
Last updated 8/7/12
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