Is this what bad heating efficiency looks like? (interpreting datalogger graph)

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Old 04-12-13, 09:56 AM
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Is this what bad heating efficiency looks like? (interpreting datalogger graph)

I installed a battery-operated datalogger in my cottage so I could get a good idea what happens there during the 99% of the time I'm not there. I programmed it to take a temperature & humidity reading once every 5 minutes and with its onboard memory that gave me nearly 4 months of data. I placed the unit in the living room near the thermostat just after Christmas and retrieved it early April.

I thought I would see what happens when the power goes out (as it frequently does there) but during this spell no trees fell on any of the power lines in the neighborhood. That's a first
What I find interesting is the crazy on/off cycling early in the data. you can clearly see each time I visited. When I went to the cottage near the end of January I replaced the old mechanical round Honeywell thermostat with a new digital non-programmable round Honeywell. I'm not real sure I understand what I'm seeing in January. Does the old thermo have such loose control of the temp vs. the new one that seems to hold within a degree? This is a simple thermo that has no adjustment for hysteresis but it does have a "cycles per hour" dipswitch that I set at "3" for "hot water heating" per instructions (even though that didn't seem like enough ON cycles for a drafty cabin). Sure looks like a huge improvement over the mechanical one. I have no idea if the anticipator was set correctly on the old one.

Any HVAC guys cover this in school that can help me understand?
 
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Old 04-12-13, 10:21 AM
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Did your fuel consumption/energy use go down measurably after the thermostat replacement ?
 
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Old 04-12-13, 10:47 AM
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No, that data has nothing to do with heating system efficiency. What it shows is the difference between an old mechanical thermostat and a new electronic thermostat. The old thermostat probably had a "dead band" of two degrees meaning that if it were set to 50 degrees it could not come on until the temperature dropped to 49 degrees and it would stay on until the temperature was up to 51 degrees. Modern electronic thermostats can operate within a 1/2 degree dead band.

The new thermostat is definitely keeping a "tighter" control on the inside temperature but that could under certain circumstances actually lead to a lesser efficiency for the heating system as a whole. Previously you may have had burn times of 10, 15 or 20 minutes where you now may have a burn time of 3 minutes. This "short cycling" is less efficient than a longer burn time. There are many other factors to keep in mind when figuring efficiency than just the room temperature.

Of course it takes more sophisticated data logging but I would like to see inside temperature, outside temperature, boiler run time, outlet temperature to the heating elements and return temperature from the heating elements. Stack temperature would be a plus.
 
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Old 04-12-13, 03:40 PM
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Furd doesn't the "3 cycles per hour" setting eliminate short cycling? With only 3 ON/OFF cycles in an hour a 3 minute burn would never hit the setpoint.

I would like to see inside temperature, outside temperature, boiler run time, outlet temperature to the heating elements and return temperature from the heating elements. Stack temperature would be a plus.
Sorry but that is WAY beyond the scope of my little experiment
Of course I do need more insulation but I'm just content to see short rate of rise & coast down times in hours rather than minutes.
 
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Old 04-12-13, 09:09 PM
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Honestly, I do not understand how thermostats that limit "cycles per hour" work and I do not use or recommend them.

Taking a "snapshot" every five minutes may be too large an interval to really see what is happening. Taking a temperature snapshot every minute, or every half-minute will give some interesting data on rise and fall times. With the five minute interval a lot could be happening in between logging points. It appears that you have some pretty fast rise times and equally fast falls. Fast falls are obviously due to a lack of weatherization but the fast rise times lead me to thinking you have an oversized system, at least way more heat emitter surface than necessary under the conditions when the data was logged. Or it could just be due to the snapshot interval.

Without further data all your chart means is that your system, including the new thermostat, is doing a good job at maintaining the set temperature. Whether or not it is doing such in an efficient manner simply cannot be ascertained from this limited data.
 
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Old 04-12-13, 09:28 PM
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Oversized? Probably--but that's what's needed when you show up in the evening after a long drive and want it comfortable in a short time. In a vacation home that's empty 99% of the time the money saved by being well insulated won't even pay for the insulation.
 
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Old 04-12-13, 09:33 PM
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Then what is your question? If you are happy with the way the heating system functions and are not concerned with the cost of operation, then it looks like everything is just fine.
 
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Old 04-13-13, 03:37 PM
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Originally Posted by Furd
No, that data has nothing to do with heating system efficiency. What it shows is the difference between an old mechanical thermostat and a new electronic thermostat.
Originally Posted by Furd
Then what is your question?
Which costs less: 5 temperature cycling or tight control? Does "cycles per hour" eliminate short cycling, or does it have some other purpose?
 

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Old 04-14-13, 12:37 PM
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Which costs less: 5 temperature cycling or tight control?
I would bet that if you went ahead and installed a bunch of extra sensors, specifically one that clocked fuel use, and were able to measure fuel vs outdoor temp vs thermostat behavior, that you would find the difference in fuel used would be nil... small enough that you wouldn't be able to say for certain one way or the other... below the 'noise floor' of the data.

Does "cycles per hour" eliminate short cycling, or does it have some other purpose?
Over the years I've tried to discover what the engineers that design these things are thinking...

I still don't understand it because it seems that there are multiple schools of thought on this.

My 'feeling' is that at say 3 CPH, the boiler would only cycle 3 times an hour... makes LOGICAL sense, right?

So one would expect that a boiler might fire for say 10 min, and then be off for 10 min. Or on for 5 min and off for 15 min...

I don't think it works that way though. There's something in the 'spec' about a 50% duty cycle, but I don't understand what it all means.

What if conditions only call for a 20% duty cycle? Will the boiler be allowed to fire MORE than 3 CPH? Or if a 70% duty cycle... less than 3 CPH?

I believe that the CPH is supposed to replace the 'anticipator' setting of the last century.

Someday I will find a definitive explanation!
 
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Old 04-14-13, 01:20 PM
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Hi Troop.
If curiosity gets the better of me I could take one to the test bench and see how it reacts to several temp swings over an hour...both short duration & long. This digital thermo (Honeywell T8775A1009) seems too cheap and simple to REALLY control cycles per hour.

So perhaps the only things I've learned from my experiment is:
The power didn't go out this winter. But I knew that from the digital clock.
I like the constant temperature for comfort sake.
My baseboard heat warms up the house much faster than I thought it could.

--All good for the learning curve.

The thermostat swap also alerted me to a high limit that was wired wrong AND set wrong.

I still don't like hot water heat but with this better understanding I think I can live with it.
 
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Old 04-14-13, 05:31 PM
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Thanks, Trooper, I'm glad I'm not the only one who doesn't understand the "cycles per hour" thing. I don't know if the cph is the new heat anticipator or if it is a differential setting or maybe a combination of both. I also agree that if guy could track fuel used vs. degree days the difference between the two thermostats would be rather small with any given inside temperature setting held constant. The biggest difference between the two thermostats is how close the newer unit will hold the setpoint.


I still don't like hot water heat but with this better understanding I think I can live with it.
Most people who have lived with both warm air and hot water heating systems prefer hot water. I would dearly love to have a hot water radiant floor but the cost is simply prohibitive in my case. I will agree that with a cabin/vacation home that mostly sits empty a hot water system is probably not a good choice in that they are usually much slower to recover from a deep setback temperature.


Guy, if you really want to improve that heating system then a form of outside temperature reset would be the easiest to install and get you the most bang for your buck.
 
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