outdoor reset vs cycles/hr

J.C.A._3

If the room/area sensors....(thermostats) are satisfied, the only reason for the boiler to be running is for a hot water demand. The outdoor sensor only relays to the boiler the temp. it needs to maintain and satisfy the programmed heating curve. If the thermostats are satisfied, there's no reason for the boiler to be running. Unless I've missed something in my training, this is the way it goes. Chris

R. Kalia

outdoor reset vs cycles/hr

Suppose I have a system with outdoor reset, plus room thermostat to keep the house from overheating.

If the reset curve is fairly well-matched to the house, I figure the thermostat will call for heat for very long continous periods, never mind what the cycles/hr setting is. (I am assuming the thermostat won't cycle on/off unnecessarily, just to obey the cycles/hr setting, if the setpoint hasn't been reached.) But occasionally the thermostat will reach the setpoint and that will cut off the heat for a while.

Under these circumstances, does it still make sense for the cycles/hr setting to be 2 or 3 for hot water? Maybe I could set it to 6 or even 12? My logic being, the greater the number of cycles/hr, the smaller the temperature drop before the thermostat calls for heat again, so I'll get more even heat. But I'm not going to get short-cycling because each on period will still be long due to the low water temperature.

Am I confused about how this works?

Mike Kraft_2

Not ............

always true JCA.the Ecomatic control keeps the boiler at design temp based on the degree day.This will cease when WWSD temp is acheived.

cheese

Wayco Wayne_2

I'm a little lost here

Is it a tankless coil were talking about, or an indirect. If it is a tankless there should be a limit on the aquastat that keeps the boiler warm enough to heat domestic hot water. If it is an indirect there should be a control set up to bring the boiler on from an aquastat. Hot water should not be relying on the reset curve at all. WW

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Joe_13

WW's right

A Tekmar will shoot the boiler to 180 degrees on a DHW call from the indirect. Between this and the reset curve why would you even care about cycles? I would leave the boiler in cold start or at least a low warm start to lengthen the burner on/off cycles. The post DHW call will leave the boiler way over the target curve in warmer weather, so it should not restart soon. I would also think the differential would be kept to at least 10 to 20 degrees to help prevent short cycling from the boiler controls.

R. Kalia

huh?

I don't understand the responses or maybe the responders didn't understand me. I didn't say anything about DHW.

There is no DHW, just a boiler with outdoor reset. My question is, do I have to set the thermostat at 2-3 cycles/hr. (the canonical setting for hydronic heat), or can I set it to 6 or 12 to get more precise temperature control without causing any problems? For fuller explanation see original post. Thanks for any suggestions,

Mike T., Swampeast MO

Problem in your premise

> Suppose I have a system with outdoor reset, plus

> room thermostat to keep the house from

> overheating.

>

> If the reset curve is fairly

> well-matched to the house, I figure the

> thermostat will call for heat for very long

> continous periods, never mind what the cycles/hr

> setting is. (I am assuming the thermostat won't

> cycle on/off unnecessarily, just to obey the

> cycles/hr setting, if the setpoint hasn't been

> reached.) But occasionally the thermostat will

> reach the setpoint and that will cut off the heat

> for a while.

>

> Under these circumstances, does

> it still make sense for the cycles/hr setting to

> be 2 or 3 for hot water? Maybe I could set it to

> 6 or even 12? My logic being, the greater the

> number of cycles/hr, the smaller the temperature

> drop before the thermostat calls for heat again,

> so I'll get more even heat. But I'm not going to

> get short-cycling because each on period will

> still be long due to the low water temperature.

> Am I confused about how this works?



"Suppose I have a system with outdoor reset, plus room thermostat to keep the house from overheating.

If the reset curve is fairly well-matched to the house, I figure the thermostat will call for heat for very long continous periods, never mind what the cycles/hr setting is.

Mike T., Swampeast MO

Problem in your premise

"Suppose I have a system with outdoor reset, plus room thermostat to keep the house from overheating.

If the reset curve is fairly well-matched to the house, I figure the thermostat will call for heat for very long continous periods, never mind what the cycles/hr setting is."

---------------------------------------------------------

Outdoor reset + room thermostat + DIGITAL boiler...

Boiler is ALWAYS producing heat at the MAXIMUM possible rate given the instantaneous supply temp/return temp/combustion air pressure & temp combination. This output has NOTHING to do with the reset curve other than how the reset curve affects the supply/return temperatures.

In other words, the boiler is trying to put its maximum output into a system with WILDLY varying requirement.

Reset is the attempt to make the temperature of the circulating water quite close to that required by the heat loss of the structure at some given point.

If your reset curve is perfect and provides just the amount of heat required to exactly maintain indoor conditions:

The cycles per hour of the boiler will increase the farther "down" the reset curve you travel...

The only way to maintain the same number of cycles per hour (again digital boiler driving a reset system) is to store the extra BTUs produced during the cycle that weren't needed by the system in some sort of buffer... (If there's some way around this that doesn't involve breaking some law of thermodynamics please let me know as I could REALLY use it!)

--------------------------------------------

With a boiler capable of PROPORTIONAL fire however you are stiving to produce either 0 or 1 cycles per hour (depending on how you look at it).

---------------------------------------------

Forgive my ignorance please but I haven't seen a boiler with "cycles per hour" control. Such WOULD though conflict with reset the farther the current outside temperature is from design outside temperature.

R. Kalia

cycles/hr setting is in room thermostat

> Forgive my ignorance please but I haven't

> seen a boiler with "cycles per hour" control.


Clearly my pot was not well worded. The point is, it isn't possible for the reset curve to be perfect under all conditions---there is internal heat generated at random times in the house (e.g. oven on, or sun shining in), or in the other direction there can be high heat losses on a very windy day., So setting water temp purely by measuring outside temp isn't going to work perfectly.

What one has to do, as I understand it, is set the curve a little conservatively, so the system will keep the house warm under all conditions, but also leave a traditional room thermostat connected in a policing role (so to speak), to stop the boiler if the house gets too warm on a calm day or when the sun is shining in.

My question was about how one should set this room thermostat. I'm of course assuming that it is an electronic thermostat, with a cycles/hr setting.

R. Kalia

...

> The cycles per hour of the boiler will increase

> the farther "down" the reset curve you travel...

>

> The only way to maintain the

> same number of cycles per hour (again digital

> boiler driving a reset system) is to store the

> extra BTUs produced in some sort of buffer...


The Tekmar reset controllers increase the temperature window (maybe not the right term, but you know what I mean) as the water temperature goes down, which is intended to address the problem you describe. Not related to my question, though.

Mike T., Swampeast MO

"My question was about how one should set this room thermostat. I'm of course assuming that it is an electronic thermostat, with a cycles/hr setting."

Personally--I would say that the "best" setting is "infinite."

Steve Eayrs

so...............

Are you talking about cycling the pump and/or zone valves, or the burner. ?????

You must be talking about the pump or zone valves , right?

Because short cycling the burner not only would not make sense but goes against teh whole idea of I/O controls.

As for the zones or pump, in my experience they are simply a call for heat, and the I/O control it operating and controling the boiler temp., via the burner operation.

Sorry I must be missing something.

Steve

Mike T., Swampeast MO

I thought that WAS your question

...cycles per hour as related to supply temperature reset...

leo g_73

invensys

is the e-stat we use. it works like the old mercury ones, just a switch, no "cycle" programming needed.

leo g

Dave Stroman

I understand what you mean. I am always playing with the cycles per hour. Tekmar makes controls with an indoor sensor. It will parallel shift the heat curve be based on what it sees inside and outside. I will put the sensor in the coldest part of the house and zone the other areas with a thermostat where I think I will get heat gain from other sourses. I always thought that if a thermostat is below the set point it will not cycle. But I have seen this not to be true. They seem to still cycle. Have not actually timed the cycles. And I am sure it depends on the brand or type of stat you have. Anyway, without an indoor sensor, a thermostat is needed. Just play with the cycles and see what works.