Are Outdoor reset and nightime setback mutually exclusive?

Knucklehead

If I size convectors and boiler appropriately for the heat loss predicted by a Manual J and control the system with outdoor reset, will the system be able to recover in a timely fashion from night time setback under design conditions? Is night time setback even necessary with outdoor reset? Will it offer any additional energy savings?

Mike T., Swampeast MO

Reset can/will affect the recovery somewhat. Two possible strategies: 1) most residential electronic thermostats have smart logic that senses wall/room temp and "learns" how long it takes that system to change room temp. Thus if you set it to be 70 degrees at 7:00 a.m., it turns the system on at whatever time before then to have the room at the temp you desire at the time you desire. 2) some resets offer a "boost" function that could be used during the recovery period.

Setback almost always offers energy savings though it can get rather tricky to control with radiant--especially in a commercial environment when the occupancy load peaks up at the same time that the radiant peaks in output.

GW

well...

Is it a true reset system? Is it constant circ or not? If the curve is set right on the money, then of course it will take longer to recover the temp that was dropped during the night set back. I think it's silly to set back the tstats when you have outdoor reset, but that's just me. If the boiler control can set back the temp, that may be better than the thermosats doing it.

Gary

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Boilerpro

You raise good points

which are often ignored. I think it is inappropreiate to use outdoor reset and setbacks together unless some provision is made to provide boost to get a reasonable length recovery. I use tekmar controls myself and they now have a boost feature on their new model boiler controls. I have one in use that is set up for setback at the boiler control (reduced system water temps) and it has worked very well. The system is setup for full outdoor reset so the boost ferature is used all heating season. On the older tekmar controls with two reset curves... occuppied and unoccupied.... I would set the unoccupied curve higher than the occupied and signal it to use the unoccupppied curve when a space was recovering from setback. In my situations I used a Honeywell Chronotherm 2 stage thermostat with the first stage signalling a call for heat on the occupied curve and the second stage signalling the unoccuppied (higher) curve. Works well. Best solution, however is outdoor reset with indoor feedback, unless you are working with apartment or other rental properties where people leave windows open, then straight reset with boost (if wonddows are left open spaces will cold down, forcing occupants to close the windows).

Boilerpro