thermostat settings for forced air

[alfio]

alright thinkpaders, i know someone on this forum knows how to solve this...

we have a fairly new forced air unit in our home. it works ok most of the time but i am annoyed by the inefficiency of the constant starting and stopping. is there a way to adjust this thing to maximize efficiency by say starting to heat at a point below 65 degrees and going all the way to 70, only re-start again when below 65 and all the way back up to 70?

can the forced air unit be adjusted for this (is there some sort of delay?)? or do can we get a new thermostat to do it? right now we have a very basic carrier thermostat which has no setting for something like this.

i'd appreciate any ideas. thanks as always

alfio

[Neil]

Yes, a thermostat with automatic settings could probably do what you want. But, I would think you would find it uncomfortable to have the temp go down too low before the heat kicks in, and then you would likely feel too hot before it shut off, with wide settings. It's most efficient to hold a nearly constant temperature.

What we do is set the fan to run continually. That way we don't really notice when the heat comes on or when it goes off. And the constant hum of the fan makes for a nice sleeping environment, for us anyway.

[GomJabbar]

Some thermostats have an adjustable differential, but AFAIK most do not.

Your furnace cycling too much indicates that you have too large a furnace for your home (too high a BTU rating).

Like Neil, I generally prefer to leave the fan on to keep the air temperature more uniform. However I just upgraded my furnace earlier this year to a top of the line York, forced-air, natural gas furnace. The computer in the furnace adjusts the amount of flame and the speed of the fan and this works quite well.

[alfio]

you would likely feel too hot before it shut off, with wide settings. It's most efficient to hold a nearly constant temperature.

What we do is set the fan to run continually. That way we don't really notice when the heat comes on or when it goes off. And the constant hum of the fan makes for a nice sleeping environment, for us anyway.

i forgot to mention that we are grad students getting by with just enough so savings trumps comfort any day right now. running the fan would eliminate the starting and stopping of the fan but not the inefficiency of the burner being on for a few minutes at a time. forced air works best when it is on for a while so as to minimize the 'warm-up time' of the burner.

i am on the lookout for said thermostat, point me in the right direction if you see anything. thanks

[alfio]

Some thermostats have an adjustable differential, but AFAIK most do not.

Your furnace cycling too much indicates that you have too large a furnace for your home (too high a BTU rating).

great, yet another impossible item to track down - why am i always going after the stuff nobody makes?

can you explain this a bit further?

thanks

[GomJabbar]

A differential setting might be from 1 to 10 degrees. The thermostat cuts in at say 65 degrees and needs a 5 degree differential to cut out at 70 degrees. On some thermostats this differential is adjustable. You'll have to see which ones have this feature. Here are a couple of links I found with Google.

http://www.about-i-series.com/ifea.asp

http://www.prothermostats.com/product.p ... uct=172265

[alfio]

thanks both for the term 'temperature differential' (i knew there was a name for what i was looking for) and the links. now that i know what i'm looking for it's easier. (robert shaw seems to have a couple of good options)

Your furnace cycling too much indicates that you have too large a furnace for your home (too high a BTU rating).

i was asking for an explanation on this comment, though are probably right i don't get why an oversized furnace would cause this. thanks

[PhilD]

... constant starting and stopping. is there a way to adjust this thing to maximize efficiency by say starting to heat at a point below 65 degrees and going all the way to 70, only re-start again when below 65 and all the way back up to 70?

is the temperature uniform throughout your house? if not, try closing the vents nearest the thermostat, this will reduce the cycling (premature shutting off due to thermostat reacting to heat from nearby vent) and may result in a more unifrom temperature through out.

[GomJabbar]

i was asking for an explanation on this comment, though are probably right i don't get why an oversized furnace would cause this. thanks

The following probably provides as good an explanation as anything.

http://www.patentstorm.us/patents/70727 ... ption.html

PhilD brings up a good point as well. The thermostat should not be too close to a heat register (vent).

[Tõnis]

There is a heat anticipator in some thermostats to help control cycling by compensating for the mass of the thermostat. It's a small lever under the cover plate of the thermostat, and it can usually be adjusted "longer" or "shorter." There's also a precise formula for calculating where it should be set using some readings taken at the controls of the heater itself. I'm an electrician, so I should know the formula, but I have forgotten it and would have to ask one of my colleagues. I suppose you could play around with it.

Tony

[Microspeed]

Most Honeywell programmable T-stats have adjustable differential. However, I don't think all of your problems will be taken care buy just changing the differential. More than likely you have several things going on. Other members have mentioned the vents too close and such. There could be cold air getting to the T-stat from the attic. There are a number of things to check, but I would start at the unit itself. Is the filter clean?
When the unit was installed was the "rate of rise" properly set? What this means, is the air coming out of the vent within the manufactures specs? Air too hot or too cold will cause cycling problems you describe. Did someone change the fan speed? It is likely you need someone to do a fresh start-up on the system. If it wasn't done when it was put in it wouldn't surprise me. If you live in a track home the installations are typically done buy people just getting into the business. They don't have the experience needed to correctly "Start-Up" a new unit. They are paid on how many the get put in, that’s all they care about.
If you just had it replaced recently and it is out of warranty. They might have used the same approach. Slap it in and if no one complains it must work fine. Forced air furnaces have come a long way in the last 20 years. PM me I would be happy to help you troubleshoot your system.