boo coo tracks
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- Jan 13, 2007
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- 134
Is there chart for cycle time, for a furnace to complete based on inside temp versus outside temp.
Tracks
Tracks
Cycle time, Nat gas furnace
- Thread starter boo coo tracks
- Start date
Not that I know of. I doubt it could be accurate, too many variables. T-stat cycle rate or heat anticipator setting, actual btu's vs load, actual insulation values, etc, .....
Alchymist
Well-known member
Add wind chill factor, doors opening/closing frequency, heat from baking/cooking, number of people present,etc.
This is where the benefits of a modulating or at least two stage furnace come into play. The way a furnace or any central heating device is selected is, first to match the furnace to the manual J derived heat loss calc. If they are accurately matched your furnace, when weather conditions are at design temp (lowest temp you are likely to see in your area based upon accumulated weather statistics), will operate continuously but maintain 68-70f in the house.
There are a couple problems with this method. First you are sizing your furnace based upon a condition which is very seldom seen. You will typically be at design condition less than .005% of the heating season so you have typical gross oversizing. The next problem is that a contractor will typically look at a heat loss number and think, "no way, it can't be that low". He doesn't want a call from you on a cold Xmas Eve, so rather than pick a 50kbtu unit it goes to 75 or even 100...regularly. Recognition of this is why more regulation in energy calc are being proposed and instituted. Generally we are oversized then by design and selection.
Given that the vast majority of heating equipment is of the very lowest technology, meaning single stage, every time the stat calls, although it could satisfy your heating needs with 10or 15kbtu, you get 100% of the rated input. This, leads to short cycling of the equipment, causes stress on the equipment, inefficiency and shortens the life of the equipment and costs you more to operate. This is why, if you want better comfort & better economy you want at least 2 stage and preferrably a modulating piece of equipment. When selecting that device what is most important to you is not how high it will fire, but how low it will fire. I have seen manuf advertise two-stage equipment that would go 80 and 100%. It may as well be single stage. What gives you the best comfort is long run times at low fire and low fan. As manuf develop these modulating warm air furnaces with low flow rates they end up with what is really their biggest problem and that is the duct work itself. The duct system is sized at a particular static pressure and when they run their fan speeds down to correlate with the modulating burner there are air circulation issues. In my area (New England) we have pretty stringent codes. All the houses are failing their Energy Star Certifications on the leaky duct work. The gov't suggests that can be 18-42% of duct leakage. So I have a new 95% condensing single stage furnace with 30% duct leakage or a new 66% efficient system...day one.
To get an idea of the weather statistics for your area go to Miniheat,com and run the numbers. I do not care about equipment selection of savings. Look at the table that has the weather statistics for your area and check the definitions section.
If I was building a new house, I'd never do geothermal. I know they are all the rage but I just don't think the numbers work. Without the tax credit these things stink and at some point we have to get out of me and everyone else paying for your ground source heat pump. If I wanted a big house to have warm air (personally, I'd never do it) I'd put in a modulating condensing boiler for better control and a series of small modulating air handlers (basically a box with a blower and heating and possibly cooling coil). That way I'd have small zones with small duct systems I could turn off/down as I chose. I have control.
What I have done to my new, to me anyway, home is to pull the lp furnace out completely. It was a 4 yr old system and the most unsafe system I've seen in 46 yrs of doing this, and that is saying something. All the ductwork which was also the worst possible installation was removed and I put in two minisplit systems. One upstairs, one down. As well, I added a couple Rinnai Energysavers and a tankless water heater and I have the ultimate in zone control, everything modulates and is net to the space. No duct or piping losses. That won't work for everyone, but I'm running 20 & 26 seer and real comfortable with it, economically
.
There are a couple problems with this method. First you are sizing your furnace based upon a condition which is very seldom seen. You will typically be at design condition less than .005% of the heating season so you have typical gross oversizing. The next problem is that a contractor will typically look at a heat loss number and think, "no way, it can't be that low". He doesn't want a call from you on a cold Xmas Eve, so rather than pick a 50kbtu unit it goes to 75 or even 100...regularly. Recognition of this is why more regulation in energy calc are being proposed and instituted. Generally we are oversized then by design and selection.
Given that the vast majority of heating equipment is of the very lowest technology, meaning single stage, every time the stat calls, although it could satisfy your heating needs with 10or 15kbtu, you get 100% of the rated input. This, leads to short cycling of the equipment, causes stress on the equipment, inefficiency and shortens the life of the equipment and costs you more to operate. This is why, if you want better comfort & better economy you want at least 2 stage and preferrably a modulating piece of equipment. When selecting that device what is most important to you is not how high it will fire, but how low it will fire. I have seen manuf advertise two-stage equipment that would go 80 and 100%. It may as well be single stage. What gives you the best comfort is long run times at low fire and low fan. As manuf develop these modulating warm air furnaces with low flow rates they end up with what is really their biggest problem and that is the duct work itself. The duct system is sized at a particular static pressure and when they run their fan speeds down to correlate with the modulating burner there are air circulation issues. In my area (New England) we have pretty stringent codes. All the houses are failing their Energy Star Certifications on the leaky duct work. The gov't suggests that can be 18-42% of duct leakage. So I have a new 95% condensing single stage furnace with 30% duct leakage or a new 66% efficient system...day one.
To get an idea of the weather statistics for your area go to Miniheat,com and run the numbers. I do not care about equipment selection of savings. Look at the table that has the weather statistics for your area and check the definitions section.
If I was building a new house, I'd never do geothermal. I know they are all the rage but I just don't think the numbers work. Without the tax credit these things stink and at some point we have to get out of me and everyone else paying for your ground source heat pump. If I wanted a big house to have warm air (personally, I'd never do it) I'd put in a modulating condensing boiler for better control and a series of small modulating air handlers (basically a box with a blower and heating and possibly cooling coil). That way I'd have small zones with small duct systems I could turn off/down as I chose. I have control.
What I have done to my new, to me anyway, home is to pull the lp furnace out completely. It was a 4 yr old system and the most unsafe system I've seen in 46 yrs of doing this, and that is saying something. All the ductwork which was also the worst possible installation was removed and I put in two minisplit systems. One upstairs, one down. As well, I added a couple Rinnai Energysavers and a tankless water heater and I have the ultimate in zone control, everything modulates and is net to the space. No duct or piping losses. That won't work for everyone, but I'm running 20 & 26 seer and real comfortable with it, economically
.Please explain. 80% 2 stage furnaces commonly run on low fire with efficiency lower then their advertised efficiency. I don't believe that to be true of 90%+ furnaces. It would seem, assuming a single stage and modulating or 2 stage are running at the same efficiency, the non-single stage would use less fuel under most circumstances by reducing the amount of times the furnaces starts and stops. Obviously with every start there is an exchanger warm up period in which the burners are firing before the fan actually comes on. A given amount of fuel will be vented and not be introduced into the home as heat. This is the same reason an oversized furnace will use more fuel then a properly sized furnace. No?
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boo coo tracks
Well-known member
- Joined
- Jan 13, 2007
- Messages
- 134
Furnace at my son's house, probably 65% eff. While working in his basement while temp difference about 20 degrees, the furnace would kick on & off every few minutes. So I am assumeing that Btu rating too high. Not an old mecury therm. that used tension of bi-metlic spring to call for heat. My home has 95% eff, uses 2 speed on heat not on a/c which uses high speed only.
Greatbear
Well-known member
Most times when a furnace cycles on and off during a call for heat it's not always from overcapacity, but a lack of heat transfer. In the case of forced air, there is probably insufficient airflow to carry the heat from the exchanger, which will overheat and shut off the flame until the airbox cools sufficiently. This cycle repeats until the call for heat at the t-stat is satisfied. Buildup of dust and crud in the ducting and the heat exchanger can cause this too.