Tight Thermostat

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ShawnTRD
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Post by ShawnTRD » Mon. Oct. 30, 2017 8:12 am

My old thermostat had a 0-3 degree differential adjustment setting. My new Honeywell RTH8580WF WiFi thermostat does not provide temperature differential adjustments. It attempts to keep the room temperature within 0.5 degrees of the set value. Do you think this is good or bad for a baseboard system? I'm thinking it's better to keep it tight anyways.

 
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Post by lsayre » Mon. Oct. 30, 2017 11:46 am

ShawnTRD wrote:
Mon. Oct. 30, 2017 8:12 am
My old thermostat had a 0-3 degree differential adjustment setting. My new Honeywell RTH8580WF WiFi thermostat does not provide temperature differential adjustments. It attempts to keep the room temperature within 0.5 degrees of the set value. Do you think this is good or bad for a baseboard system? I'm thinking it's better to keep it tight anyways.
The 1/2 degree T-Stat will certainly fire the boiler more frequently. I'm not sure if that's good or bad. Rooms should feel more comfortable.

 
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Post by Lightning » Mon. Oct. 30, 2017 1:53 pm

Tight settings aren't as tight as they seem. I have mine adjusted to + or - half a degree. You'll feel the difference before the thermostat will. There seems to be some lag time before the thermostat notices a temperature change.

 
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Post by McGiever » Mon. Oct. 30, 2017 5:58 pm

ShawnTRD wrote:
Mon. Oct. 30, 2017 8:12 am
My old thermostat had a 0-3 degree differential adjustment setting. My new Honeywell RTH8580WF WiFi thermostat does not provide temperature differential adjustments. It attempts to keep the room temperature within 0.5 degrees of the set value. Do you think this is good or bad for a baseboard system? I'm thinking it's better to keep it tight anyways.
For a flat-bed stoker it will keep you off the "hold-fire timer" more often.

Other stoker types may not burn so well with losing the fewer but longer cycles of the greater spread.

Thermostats are generally a under-appreciated and misunderstood component in a heating system.
This is especially true when it comes to burning solid fuels. :annoyed:


 
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Post by coaledsweat » Mon. Oct. 30, 2017 6:37 pm

It's only going to run the pump more. Shouldn't change the total load much. I may be wrong but its my 2 cents.

 
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Post by Rob R. » Mon. Oct. 30, 2017 6:46 pm

coaledsweat wrote:
Mon. Oct. 30, 2017 6:37 pm
It's only going to run the pump more. Shouldn't change the total load much. I may be wrong but its my 2 cents.
I agree.

Also - most thermostats have a setting for the # of cycles per hour.

 
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Post by franco b » Mon. Oct. 30, 2017 8:09 pm

With the honeywell triple aquastats a call for heat starts the pump, but also bypasses the low limit with the high limit becoming the controller. The combustion motors will certainly become active then. Short cycling with an oil burner is detrimental to efficiency, but I don't know how that effects a stoker fire, but in general a prolonged firing is more efficient in terms of combustion.

A tight differential is an effort to simulate a gravity system where some radiant heat is almost always present. Discomfort is felt when that stops, even though air temperature might be at set point.

 
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Post by ShawnTRD » Mon. Oct. 30, 2017 9:00 pm

I'm thinking that if it calls for heat with say a 10 minute break before calling again. It will be easier for the boilers to get to the high temp setting and quicker for the thermostat to return to set temp. I hope it doesn't turn into using more coal.


 
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Post by coaledsweat » Mon. Oct. 30, 2017 9:18 pm

The BTU load isn't going to change. You'll deliver more frequent bursts of less water. The boiler should still be running off it's setpoints to meet the very same total demand. No?

 
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Post by ziggy87 » Tue. Oct. 31, 2017 1:33 pm

Short cycling would be my only concern. That can cause other issues...

 
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Post by franco b » Tue. Oct. 31, 2017 3:49 pm

ziggy87 wrote:
Tue. Oct. 31, 2017 1:33 pm
Short cycling would be my only concern. That can cause other issues...
A tighter differential has to cause shorter more frequent run cycles. How that effects longevity is another questio.

There is also the difference in efficiency of the idle fire opposed to the full fire. The difference might be small.

With no experience in stoker fires, the pictures I have seen, especially of flat bed stokers seem to have a great amount of excess air. CO2 percentage of flue gas would confirm or refute that. Nobody seems to do that, probably because CO should also me measured to get useful information.

A gas flame is easy because it starts to go yellow if air starved.
The oil flame gets smoky which is easy to measure.
The coal flame generates CO which is not east to see or measure.

 
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Post by Rob R. » Tue. Oct. 31, 2017 4:02 pm

As I said earlier, if you don't like the way it is behaving, change the number of cycles per hour.

Also, there is a lot more than just the thermostat sensitivity that determines the cycle time. The feed rate of the stoker, mass of the radiation, boiler, and piping, and the amount of radiation in the living space all play a part.

One of the reasons I used a Hydrostat 3250 aquastat on my EFM is that it purges the heat in the boiler before starting the stoker - this totally eliminates short cycling from small zones. You could also just hold the boiler at a setpoint with no regard to heat calls.

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