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Old 11-10-2008, 06:23 AM   #1
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Proportional cycling for gas furnace


Hi;
I recently installed a Aube TH140-28 programmable thermostat for my gas fired hot water system.
The default setting has the thermostat set to proportional control, which means that it will adjust the heating cycle based on the previous cycles, resulting in a more comfortable environment.

My concern is that the additional cycling the proportional mode causes will result in shorter life of some of the furnace components.
Last month when I inquired about a smaller swing temp, I was advised (by members of this forum) that I should not reduce the swing because it will reduce lifespan of components, and result in lower efficiency of the furnace. This proportional control seems that it would have the same effect.

The Aube thermostat instruction manual says that the default cycle in proportional mode is 15mins, but the furnace could run for as short a time as 1/10 that time, or 1.5mins. That seems like much too short cycle time.

I am probably going to change the setting to conventional with anticipation, since my main objective is efficiency and life of the furnace.

I would appreciate any advice on this.

Thanks

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Old 11-10-2008, 09:38 AM   #2
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Proportional cycling for gas furnace


1.5 minutes???? That is not only bad for the components it is extremely inefficient. To fire up for less than 2 minutes is totally out or line.

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Old 11-10-2008, 06:06 PM   #3
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Proportional cycling for gas furnace


Quote:
Originally Posted by Marvin Gardens View Post
1.5 minutes???? That is not only bad for the components it is extremely inefficient. To fire up for less than 2 minutes is totally out or line.
Here is the extract from the user manual:

"1.4.1 Proportional Adaptive Mode (switch #5)
This mode analyzes previous cycles to define the next duty cycle.
This control mode guarantees optimal temperature control based on the system's capacity. To extend the life of the system, a minimum On/Off time of 10% of the cycle has been implemented. For example with a 15-minute cycle, the system would not start or stop for less than 1.5 minutes.

Ideal for:
>Radiant or convection electrical heating system
>Circulator control in a hot water system
>Electrical hot-air furnace

Not recommended for:
>Gas or fuel furnace or boiler with a wall chimney having a 30-second or higher combustion gas purging cycle. To figure out the gas purging cycle of your system, count the time between the heating command sent by the thermostat and the moment when the burner actually goes on.
>Multi-zone systems, where several thermostats command a single heating unit.
Note: In these two cases, conventional mode with anticipation is recommended."

I reset the thermostat for the conventional mode with anticipation.
The cycle is still set to 15 minutes, but is now limited to that time, and not to 10% of it.

I suppose that if the furnace were running constantly, and the thermostat were activating only the water pump, or opening a zone valve, the proportional mode would be useful.
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Old 11-10-2008, 06:33 PM   #4
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Proportional cycling for gas furnace


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Originally Posted by KE2KB View Post
I suppose that if the furnace were running constantly, and the thermostat were activating only the water pump, or opening a zone valve, the proportional mode would be useful.
Agreed.

And another scenario is if it were a tankless hot water heater then it would be worth it.
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Old 11-10-2008, 07:17 PM   #5
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Proportional cycling for gas furnace


Quote:
Originally Posted by KE2KB View Post
To extend the life of the system, a minimum On/Off time of 10% of the cycle has been implemented. For example with a 15-minute cycle, the system would not start or stop for less than 1.5 minutes.
So startups significiantly shorten the life but continuous running or proportional mode not so much, and this 10% restriction is built in.

The proportional mode cycling is incremental changes, not cold starts.

I don't think you are interpreting this correctly.
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Old 11-10-2008, 09:53 PM   #6
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Proportional cycling for gas furnace


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Originally Posted by Yoyizit View Post
So startups significiantly shorten the life but continuous running or proportional mode not so much, and this 10% restriction is built in.

The proportional mode cycling is incremental changes, not cold starts.

I don't think you are interpreting this correctly.
Whether cold start or incremental changes, it will still result in the furnace turning on and off a lot more than it needs to, which leads to reduced life.
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Old 11-10-2008, 10:15 PM   #7
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Proportional cycling for gas furnace


Is your system, cast iron rads or copper baseboard?

Proportional will run the heat more often.

You would probably never have that 1.5 minute run time.
It takes longer then that to warm up copper baseboard, let alone cast iron rads.

If you want efficiency, then proportional is not what you want.
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Old 11-11-2008, 07:56 AM   #8
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Proportional cycling for gas furnace


This still doesn't sit right with me.

The question seems to be
what effect duty cycle has on furnace lifetime,
whether it's worse to have many small corrections or several large ones, and the effect of cold starts.

Will you post the nameplate data from your furnace? I'd like to e-mail the manu. and see what they say.

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