Cycle times, what's better

[pbibm]

For a high effeciency heater, generally whats better for the unit:

1) More cycles with shorter runtimes, and shorter space between cycles

-or-

2) Fewer cycles with longer runtimes, and longer space between cycles

Is there any impact to the unit if its one vs the other, good or bad?

Paul

[coolwhip]

Short cycling= bad, long run time= bad.

This is why it is important to have the system sized and installed by a professional. Thats why manual J and manual D were created.

[james mo]

What is the concern relating to? Are you getting a new system or are you trying to modify an existing one?

[hoffa]

Its a question of comfort and cost.

Short cylcing usually equates to higher cost (not always) but more comfort (its near the set temperture more ofter). Long cycling the other way around.

As to which is "better" for the system, my belief is that experts would disgaree with reams of technical data. Not profesional is going to help you on that.

In general if the thermostat lets you do it its probably OK. Then its largley a user decision.

YOu may need to clarify what you mean by better: better reliablity or beter efficiany, etc

[pbibm]

Thanks. My problem is that I when bought the house, it came with 2 furnaces in it. They had been there for 10 years. It passed inspection, and I didnt think to do a deep dive. The inspector looked at the furnaces, said they looked ok. He thought the front one looked oversized but he wasnt alarmed by it. So I decided I would at least keep an eye on it.

I observed the furncaces for a several months, and noticed the front furnance has cycles as low as 6 minutes, and the back furnace as long as 40. I am now montioring the ontime and cycle time of each. They are pretty consistent with the numbers I gave above, changing based on outside temp.

I have a yearly inspection by an HAC pro that I trust, and he agrees this is not right. He beleives that how the ducting is laid out in the house and addition is right for 2 heaters, but that one furnace is too big, and one is too small. As of our last talk, he doesnt know how to help other than ripping out and installing new furnaces...

So I am looking for options other the replacing them, since worst caset thats what I will have to do. Other than being told its wrong, I wouldnt know, they work. However if I can avoid replacing them, I want to explore that.

Paul

[pbibm]

Originally posted by hoffa
Its a question of comfort and cost.

Short cylcing usually equates to higher cost (not always) but more comfort (its near the set temperture more ofter). Long cycling the other way around.

As to which is "better" for the system, my belief is that experts would disgaree with reams of technical data. Not profesional is going to help you on that.

In general if the thermostat lets you do it its probably OK. Then its largley a user decision.

YOu may need to clarify what you mean by better: better reliablity or beter efficiany, etc

I want to try to modify what I have, replacing 2 heaters is a going to be a lot$$ and I want the best mix of reliabilty and effiiency.

I have thermosats that I can spread the set vs trigger point, and thats my current activity. The results thus far are addressing both. Comfort is off a "tad", but cycle times improved for both, and time beween cycles is tremonous. The actual ontime for the units per day has dropped.

I wanted to be sure that by heading down the efficiency path, I wasnt making a bad reliability problem even worse. Sounds like I am heading in the right direction.

I still welcome input on this and thanks to all who have commented so far

[jacob perkins]

Originally posted by pbibm
I am now montioring the ontime and cycle time of each.

Check your temperture rise.They should be about mid-range.
(They should be printed somewhere on the furnace)

Also,the location of your thermostats is critical.

[pbibm]

Originally posted by jacob perkins

Originally posted by pbibm
I am now montioring the ontime and cycle time of each.

Check your temperture rise.They should be about mid-range.
(They should be printed somewhere on the furnace)

Also,the location of your thermostats is critical.

Jacob... thanks. I will look at the temperature rise. I'll get my HVAC pro if its not in the middle.

I do know that the stats are placed right, at least I have two different professional opinions on that.

Paul

[beenthere]

A longer run time is better for eff.

How long do you plan to live in this house.

If they are over sized, their using more fuel then they should.

[docholiday]

beenthere, dont make me pull out the ice cream again!

Longer run times are for comfort more than efficiency, it costs more money to run the fan longer, technically. Short cycling is harder on the equipment and leads to uneven temps and large swings and if extreme can reduce efficiency since the time between firing and fan are fairly inefficient but its minimal.

An oversized unit may waste some money in the efficiency losses up the chimney but the btuh output is the same over the course of an hour from either furnace. On just takes more of the hour to use it than the other.

[pbibm]

I will be here a long time... This is the house I want.

Its really coming down to what I can controll, and what I can't. By spreading the setpoint vs trigger point on the stat, I can get the runtime longer on the big furnance. I actually have it go 1 degree above desired as the high point and two degrees below before triggering. Run times have gone from 6 minutes to 14 - 20, but....... Its a long time inbetween cycles now, up to 50 minutes.... thats a lot better.. And, because the space being heated overlaps heat from the front furnace moves into the back of the house. At least that looks like whats happening.

The back furnance is harder. I can spread the setpoint vs trigger there too. Cycle times are still long, but it stays off for a long time in between.

Overall the overall runtime per day of the units is down. The house is cooler in different spots in short spells as it cools off. So I need to work those setpoints to balance comfort and efficiency. And, for the big furnace, its running longer cycles.

I beleive gas is more expensive then electricity now, so the fan run time should be made up by the furnace running less time. I'll continue to monitor that.

Paul

[hvac-tech-lane]

Originally posted by coolwhip
Short cycling= bad, long run time= bad.

This is why it is important to have the system sized and installed by a professional. Thats why manual J and manual D were created.

long run time= bad

HUH???????? wouldn't the best system for comfort and eff. be the one that would run constantly and maintain set point, thats what I was taught.

[hvac-tech-lane]

Does the furnace that is suposedly "too small" keep up or does it run continously and loose control (space temp drops) if not it is not too small.

[contactor]

your "trigger" point is the heat anticipator setting. Have a tech check this, that and other settings . A good one can do this in 1/2 hr or less & check on sizing. High eff. units can be set as much as 25% over antic reading.

[pbibm]

Originally posted by hvac-tech-lane
Does the furnace that is suposedly "too small" keep up or does it run continously and loose control (space temp drops) if not it is not too small.

It has what I consider long run times, 40 - 50 minutes, can go off for 5 - 10 minutes, and then go right back at it again. So I would charactarize it as "barely keeping its head above water". I hear you, I am making a comparison to the front big furnace, and may be assuming that the "small" back furnace is not configured right. I did get opinions on this one too. I have been able to settle it down by having anticipation be longer, 2 - 3 degrees. I can get almost 40 minutes between cycles with not much longer runtime. I track the actual on time of the unit when it draws power, and this technique has the furnace running about 5 hours less a day with my family not noticing. so far...

Its march now, I dont think this will work on the coldest days. Since the space open to the front big furance, I think some heat is coming across from that side of the house. Overall though, my runtime for both furnaces is down per day. I am translating that into less gas being used, the bill will prove that..

The other challenge for the "small" back furnace is that almost all the ducts go over an unheated crawlspace, or up outside walls. So the furnace spends the first 5 minutes warming the ducts back up. It takes almost that long to feel that the air is warmer than room temp.

Can ducts in crawl spaces be insulated? Its that a good idea?

Paul

[hvac-tech-lane]

"Can ducts in crawl spaces be insulated? Its that a good idea? "
Absolutely as a mater of fact its not only the proper thing to do but its code where i'm at and I would believe everywhere now. If you do insulate your ducts you will need to use insulation with a vapor barrier made for insulating duct. Also sealing your duct work at all the joints is a good idea using duct sealing mastic.One caution about tightening up a crawl system be alert to the potential that exposed pipes could now freeze.

[PullaVac]

you might look at the location of the T-stats. I found one that was 6'3" above the floor. Are they in the proximity of their respective units. Check for air flow through the t-stat wire hole in the wall behind the stat, plug it if there is one. I had one that would never reach set point because of the hot air being pulled through the wire hole. The center of the room would read below set point,(in cooling mode). Are they located on the interior walls or an exterior walls. You might check the blower speeds on the units. And, just out of the wild, clear, blue yonder, is there any chance that the units are connected to the wrong or oppsite areas that they were intended for. I have found that to be on a multi-install systems. Had several units in one mechanical room, with two of them going to the wrong area. The big unit was suppling to the smaller area and vise a versa. Just asking questinons,picking your brain with a bunch of what ifs. Sometimes it the simplest and dumbest things that make you pull you hair out.

[pbibm]

HVAC-TECH-LANE- OK. Good input and that looks like a project. I understand about the freezing, i have run into that at another house closing off the crawlspace to the house space. Water pipes froze right up, but putting the foam around them fixed that.

Originally posted by rudeman
you might look at the location of the T-stats. I found one that was 6'3" above the floor. Are they in the proximity of their respective units. Check for air flow through the t-stat wire hole in the wall behind the stat, plug it if there is one. I had one that would never reach set point because of the hot air being pulled through the wire hole. The center of the room would read below set point,(in cooling mode). Are they located on the interior walls or an exterior walls. You might check the blower speeds on the units. And, just out of the wild, clear, blue yonder, is there any chance that the units are connected to the wrong or oppsite areas that they were intended for. I have found that to be on a multi-install systems. Had several units in one mechanical room, with two of them going to the wrong area. The big unit was suppling to the smaller area and vise a versa. Just asking questinons,picking your brain with a bunch of what ifs. Sometimes it the simplest and dumbest things that make you pull you hair out.

Those are all good points. I am pretty shure about the stat placement, I know they are in the areas they service. They are both on inside walls. They are clean (brand new). Before I put the new ones up, I hung them on the wall by the old ones (12 year old honeywells) and discovered that the honeywells tripped at 1 degree anticpipation. I can change that on the new ones, and they have relays so I could watch how they behave before I switched over.

A lot of other things for me to investigate and review with my local tech. Thank you!

Paul

[hvac-tech-lane]

Originally posted by pbibm
HVAC-TECH-LANE- OK. Good input and that looks like a project. I understand about the freezing, i have run into that at another house closing off the crawlspace to the house space. Water pipes froze right up, but putting the foam around them fixed that.



Paul

Finer point on pipe insulation: Insulation slows thermal transfer, thus an insulated pipe will merely take longer to freeze " Howcome my pipes froze an broke while I was on vacation I insulated all of them."