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Old 10-21-2008, 08:51 AM   #1
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How much swing to set on digital tstat


Hi;
I just installed a digital-programmable thermostat for my hot water heating system (gas, 24V).
I was wondering whether the furnace would operate more efficiently if I have the swing set to 2 deg instead of 3.
My thinking is that the smaller swing will not allow the water in the radiators/pipes and the furnace to get as cool between runs, thus requiring less energy to bring back to temp.
The smaller swing should also result in less overshoot, which might further reduce energy cost.

What do you think?

Thanks

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Old 10-21-2008, 11:51 AM   #2
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How much swing to set on digital tstat


Leave it where it is. The most energy is lost in starting up and shutting down. The more cycles you have the more energy you will lose.

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Old 10-21-2008, 01:42 PM   #3
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How much swing to set on digital tstat


When you start to short cycle heating equipment you do 3 things.
1. The AFUE efficiency goes down so it is less efficienct
2. You increase the wear on all the mechanical parts. They would rather run than start and stop.
3. Increase maintenance costs.
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Old 10-21-2008, 02:08 PM   #4
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How much swing to set on digital tstat


Thanks for the advice. I'll leave it set to 3 deg F, the default.
Of course, because it's a hot water system, there is going to be a few deg overshoot, so I'll have to tweak the temp settings that work best.
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Old 10-21-2008, 03:33 PM   #5
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How much swing to set on digital tstat


Quote:
Originally Posted by KE2KB View Post
Thanks for the advice. I'll leave it set to 3 deg F, the default.
Of course, because it's a hot water system, there is going to be a few deg overshoot, so I'll have to tweak the temp settings that work best.
What brand thermostat is it?
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Old 10-22-2008, 08:54 AM   #6
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How much swing to set on digital tstat


It's a RiteTemp. Bought it at HD a few years ago. I originally used it to control my window A/C, using a home-made hookup using the AC's remote.

The thermostat appears to be working properly. I have no complaints, except that with a hot water system, there's no way to avoid the overshoot in temp after the furnace shuts off.

I think the old analog thermostat used a "heat anticipator" to trick the t-stat into shutting off early.
I'm not sure if I can do that with this digital one. It is really designed to be used on forced air systems.
Perhaps I'm just using the wrong thermostat. I'm sure there are ones made specifically for hot water and steam systems that have a lot of hysteresis, unlike forced air systems.
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Old 10-22-2008, 09:46 AM   #7
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How much swing to set on digital tstat


Quote:
Originally Posted by KE2KB View Post
It's a RiteTemp. Bought it at HD a few years ago. I originally used it to control my window A/C, using a home-made hookup using the AC's remote.

The thermostat appears to be working properly. I have no complaints, except that with a hot water system, there's no way to avoid the overshoot in temp after the furnace shuts off.

I think the old analog thermostat used a "heat anticipator" to trick the t-stat into shutting off early.
I'm not sure if I can do that with this digital one. It is really designed to be used on forced air systems.
Perhaps I'm just using the wrong thermostat. I'm sure there are ones made specifically for hot water and steam systems that have a lot of hysteresis, unlike forced air systems.
Latency is the big issue with water based heating systems.

It takes a long time relative to forced air systems to get going and once it gets there there it is hard to stop.

I have a Honeywell 8000 and it learns how long it takes to get to a certain temperature. If it is set to 66 at 7AM and the temperature is now 58, it knows that it will take 30 minutes to get there and will start heating at 6:30.

My house is run by a computer. I can see where my remote temperature sensors and some simple programming could fix this problem inherent in water based systems.

It would be nice to have a stat that was based on some logic for high latency systems like hydronics. It would be easy to do. Just have it shut down when the temperature reaches (set temperature -2). This would allow the system to stop heating 2 degrees below the set level so there is less over shoot.
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Old 10-22-2008, 09:57 AM   #8
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How much swing to set on digital tstat


Quote:
Originally Posted by Marvin Gardens View Post
Latency is the big issue with water based heating systems.

It takes a long time relative to forced air systems to get going and once it gets there there it is hard to stop.

I have a Honeywell 8000 and it learns how long it takes to get to a certain temperature. If it is set to 66 at 7AM and the temperature is now 58, it knows that it will take 30 minutes to get there and will start heating at 6:30.

My house is run by a computer. I can see where my remote temperature sensors and some simple programming could fix this problem inherent in water based systems.

It would be nice to have a stat that was based on some logic for high latency systems like hydronics. It would be easy to do. Just have it shut down when the temperature reaches (set temperature -2). This would allow the system to stop heating 2 degrees below the set level so there is less over shoot.
I have a hydronic system and there usually isn't much overshoot. I think I have a LUX TX500. The differential is set to four (I'm not sure if that's degrees, it can be set from 1 to 10). Best thing to do is see how long the system runs(once it's at operating temp) on an average cold day. Most literature out there recommends 20 minute on/off cycles. Set the swing to achieve this.
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Old 10-22-2008, 10:13 AM   #9
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How much swing to set on digital tstat


Quote:
Originally Posted by jerryh3 View Post
I have a hydronic system and there usually isn't much overshoot. I think I have a LUX TX500. The differential is set to four (I'm not sure if that's degrees, it can be set from 1 to 10). Best thing to do is see how long the system runs(once it's at operating temp) on an average cold day. Most literature out there recommends 20 minute on/off cycles. Set the swing to achieve this.
I think that water temperature of the system plays a big role in over shooting.

A system that is set for 160 versus a system that is set for 120 will have more over shoot.

Once the temperature gets to the desired setting there will still be a lot of 160 degree water under the floors. If that water were 120 it would have less of an impact.

This might be worth looking at.

My last hydronic system was a Rinnai tankless and it could be set to any temperature from 180 to 98.
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Old 10-22-2008, 10:31 AM   #10
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How much swing to set on digital tstat


I'm just using a conventional boiler with baseboards. My high limit cutoff is somewhere around 180.
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Old 10-23-2008, 09:51 AM   #11
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How much swing to set on digital tstat


The t-stat I ordered has two features that will make it work much better than my current one:

1) A heat anticipator program
2) A Proportional control program.

In the proportional mode, it adjusts the cycle time according to the temp swings, etc. That should provide the most comfortable setting.
The caveat is that in the instructions (which I have already downloaded), there is a statement about using the proportional mode with gas furnaces, and making sure that the burner light delay isn't too long to prevent the gas from being purged from the chimney.

From what I understand, there is a certain amount of gas that escapes up the chimney when the burner lights, and this is drawn up and out of the chimney during the run cycle of the furnace.
If the run cycle is too short, the gas may not be completely purged from the chimney, and thus could settle back down (gas is heavier than air) and cause a blowback when the burner starts again.
They talk about burner light times of less than 30 secs. I am sure that from the time the gas valve opens, to the time the burner ignites is less than 5 seconds on my furnace, but I suppose there are some systems where it may be longer.

Aside from what I have said above, I can't imagine the furnace short cycling less than 20 minutes in my home.

FW
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Old 10-23-2008, 10:12 AM   #12
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How much swing to set on digital tstat


Quote:
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(gas is heavier than air)
NG is lighter than air. It also needs a concentration of 5-15% and a flame of more than 1000 degrees to ignite.

When the gas shuts off there is no gas left as it has shut off and burned any excess gas. So there is no left over gas in the chimney. And it there were it would rise and dissipate, the concentration would be less than 5% and there wouldn't be a 1000 degree ignition source.

Propane is heavier than air.
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Old 10-23-2008, 04:29 PM   #13
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How much swing to set on digital tstat


Quote:
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NG is lighter than air. It also needs a concentration of 5-15% and a flame of more than 1000 degrees to ignite.

When the gas shuts off there is no gas left as it has shut off and burned any excess gas. So there is no left over gas in the chimney. And it there were it would rise and dissipate, the concentration would be less than 5% and there wouldn't be a 1000 degree ignition source.

Propane is heavier than air.
OK. I got that wrong. I'll have to read that statement again, but I don't think it's going to be something I need to worry about.

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