Heating System's Cycle Rate

109A_5

Since the "T87 thermostat" post is doing so well, let's also explore Heating System's Cycle Rate. What determines the Cycle Rate and why? Lets start back in the coal days before the T87 thermostat to the present tech that is way more sophisticated than the tech that took man to the Moon.

EdTheHeaterMan

Hand fired systems did not have a cycle rate. Unless you count how many times the house wife went to the basement to add coal to the firebox. That might be 3 cycles or 4 cycles per day. Dan would have some interesting books on how to operate such a system with a series of pulleys and dampers to control the draft thru the gravity boiler of furnace.

There were no high limits or fan controls because there were no electrical moving parts to control. No thermostat on the wall in the living room. If you felt cold, you would go to the heater and add a shovel full of coal. It was very basic.

Steam systems today are still using gravity to operate. No pumps or valves to regulate the flow of the heat to the radiators. The only thing that changed from hand fired to automatic burners is the controls that measure the heat can shut off the burner. The pulleys and dampers are gone in favor of the gas valve or oil burner motor.

I found this article interesting
https://www.endesa.com/en/blogs/endesa-s-blog/air-conditioning/illustrated-history-of-the-thermostat

109A_5

I would include the human factor, since without it the fire would go out and stay out. And they are the stimulus to fuel the fire (like a thermostat). So 3 cycles or 4 cycles per day, I'm good with that.

Jamie Hall

Let's make sure we're all on the same page here, since I see plenty of weeds to get into...

And talk, quite strictly, about the overall cycle rate. Not the off and on firing of a burner to match the burner output to the system load-- that's a lively topic, but not the same.

So... really we are concerned, I think, with how well the system power output matches the heating load demand, and the cycling (if any) needed to make that match.

So then, that rate will depend on two things: what the heating load at the moment vs. the system power output.

At one extreme we have a beautifully calibrated modulating system with an accurate measure of system load (mostly, but not entirely, determined by the temperature difference from inside to outside)(outdoor reset). If one gets such a system properly calibrated, it shouldn't cycle at all -- it never turns off. The burner (or heat ump or resistors) operate at exactly the power level needed to match the system load, and the pumps or fans circulate that power at just the right rate to the right places and everyone is very happy.

Obviously things happen to change the system load, so that happy state can only be approximated most of the time -- but one can get very close in a well insulated house with only moderate glass and stable interior loads. We strive for it.

In many cases, though either the available power is greater than is needed or can't be modulated -- or both. In those situations, it is necessary to turn the whole system off and on so that the average power output, taken over time, matches the system load. Now you get into two major factors: what amount of error is tolerable in maintaining the desired set point, and what is the thermal inertia of the building -- how fast does it heat up or cool down.

The second of these is fixed by the building structure. A lightly constructed building with minimum internal mass will heat and cool rapidly. A heavily constructed building -- say concrete or stone -- with a lot of internal mass will heat and cool very slowly. Another factor in the second is the type of heating system. Some systems, such as forced air, can respond (at least so far as air temperature is concerned) to a demand for heat within literally seconds. Others may take longer -- minutes to hours or even, in the case of some radiant systems, days.

The first of these is determined by occupant preference almost entirely. Some applications -- or some occupants! -- may demand a very tight control of temperature. Others may be perfectly happy with the temperature rising and falling several degrees.

These two interact with each other in such a way that there really can be no one set cycle time given. That said, though, there are useful generalisations or rules of thumb for the cycle time -- the period -- often though of as cycles per hour (the ratio of on time to off time, on the other hand, within each cycle is determined by the match between current heating load and system power and is a completely separate topic). To look at two extremes, a building with very little thermal inertia, but a rapid responding heating system, will do well with a very short period -- perhaps only 10 minutes between cycles. A building with larger thermal inertia and a slow response time may be quite satisfactory with a very long cycle time -- one per hour or even longer. An occupancy with very tight temperature control demands will need a shorter cycle time; one with more relaxed demands will do well with a longer cycle time, all else being equal.

Not sure all that really is in line with what you are wondering, @109A_5 , but is only a brief look at some of the variables involved.

109A_5

Thanks @Jamie Hall, @EdTheHeaterMan and to any future posts. Since the "T87 thermostat" took a sidebar into heat anticipators and I have seen heat anticipators closely correlated to Cycle Rate, or Cycles Per Hour (CPH), I thought it was a good topic for a different post.

To me old coal systems seem to be very analog except for the human factor (sensing it is time to shovel coal, and then shoveling coal), so Cycles Per Day. New high tech systems try to approach that analog portion to heat delivery similar to coal.

It seems to me when the thermostat went electronic (or no Mercury) the heat anticipator was lost and a software engineer replaced it with Cycle Rate, or CPH. Which got me thinking did coal systems have a Cycle Rate? The hardware maybe not, but for the overall system to work, it does.

I noticed all the confusion with thermostat settings from the heat anticipator to the CPH. Did CPH crudely replaced the heat anticipator? If so, I guess that is all they had at that point in time. Could they have done it better, could they do it better now with low end thermostats? Maybe the resolution should be in minutes instead of CPH. Where the cycle length is in minutes instead of large fractions of an hour. So instead of 3 CPH a closer fitting settings for a given system could be say 22 minutes or 17 minutes. Maybe more comfort and economy would be the result.

It seems to me the actual CPH of a system using a T87 is infinity variable (within reason) and is based on the characteristics Jamie mentioned and not the thermostat. The basic T87 is oblivious to time.

Ed I'll look at that article, thanks.

Jamie Hall

The basic problem with the electric anticipator on the T87 (and earlier electric thermostats) is that it is fiddley to get it right, and most installers -- then and now -- slap it on the wall and walk away. If you are really lucky, they set it to the shop standard setting. And, granted, it can be way off if you do that. There is a similar problem with the digital thermostats and the cycles per hour or system setting -- most installers just slap it on the wall and walk away. And it doesn't work all that well either, if you do that! That really has nothing to do with cycles per hour.

EdTheHeaterMan

Very well put Jamie. Your last post reminds me of the Taco Zone Valve that draws almost one full amp. I remember setting the heat anticipator to .9 when installing a new thermostat. Some thermostats had an anticipators that were more robust than others. If you left the replacement thermostat at the factory setting, one of two things would happen.
1. The thermostat would cycle on and off at a rate of 40 to 70 times an hour, or about one cycle per minute. At this alarming rate many home owners did not realize the heater was cycling until the outdoor temperature got into the lower 20s. That is when the average output of the burner being off for half of the time would affect the system output enough to be noticed.
2. The second is more immediate. The heat anticipator would burn out like a fuse rendering the new thermostat defective.

An experienced tech that made this rookie mistake could cover it up by adjusting the heat anticipator to a higher amp rating where the circuit would be completed with less resistor in the circuit.
This only works once for the higher current control. If you try to repurpose this thermostat on a lower current control, the needed setting will be past the failure, and therefore the thermostat will no longer be able top be adjusted to the proper setting for comfort.