Trusting a heat load calculation & downsizing…
I’m replacing my furnace & had a Manual J-based heat load calculation done by a reputable person using the Rhvac software. I paid a sizable amount of money for this heat load to be done – but I was tired of going to contractors who relied on rules of thumb & made educated guesses based on experience. I just want the concrete numbers to back up any claims.
It revealed that for my location, at a design wet bulb temp of -4 degrees and an indoor dry bulb temp of 70 degrees, my house needs 30kBTU/hr of heat. This is a 950sqft single story house; basement foundation, load-bearing double-wythe brick. Air infiltration measured 800cfm @ -50 Pascals.
My current furnace is a 75k output. It does short cycle & trip the limit switch (at 140 degrees). Rooms are not evenly heated. Proper sizing would be to my benefit but there is some hesitation on dropping from 75K output to a 35K output.
My concerns:
1) The duct work – these are old rectangular steel ducts under the floor joists. They are unsealed. My basement is somewhat finished with drywall ceilings which hides a lot of the duct work. I was told the ducts are within the envelope so there was no need to measure leakage to the outside. I’m afraid there might be some leakage to the outside but I can’t see it. Since this is a brick house, the floor joists sit into slots in the brick so there’s no rim joist. But there could be some outside infiltration there that I can’t get to because of the drywall basement ceiling.
==>>> Can you simply equate duct leakage to the outside to the same leakage caused by say an open window? I would think duct leakage infiltration would be more significant since it could be directly causing heat loss at the heat source.
2) Though the heat load included the basement in the heat loss, I don’t heat the basement (there’s one small supply). So which of these are true: a) heat main floor & not basement = less heat loss from building as a whole. b) heat main floor & basement = less heat loss from building as a whole. There is no insulation between basement & main floor, though I try to reduce air leakage from basement to main floor as much as possible since it’s usually cold down there.
==>>> If the Manual J was done to include the basement as living space, does this mean I am forced to heat the basement for the heat load to be considered accurate?
3) My greatest fear is that hardly any heat will come out of the ducts with a 40K furnace. The 140 degree heat from my 75K furnace is nice.
==>>> I understand it will take longer to heat the house, but what can I truly expect temperature-wise?
So that brings me to the bottom line – are there any reasons why I should doubt the heat load computed for my house, assuming it was done correctly down to the T?
Thanks – Jeff
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Replies
Jeff,
Q. "Are there any reasons why I should doubt the heat load computed for my house, assuming it was done correctly down to the T?"
A. No.
Note to self: no more close-ended questions!
Can you address why my concerns are invalid then?
My 75k furnace has trouble making the house warm that it makes me question if downsizing is the solution or will exacerbate some problem that the Manual J method can't really foresee. The house always feels cold when the furnace is not running even if the stat reads over 68deg. We keep the stat on at least 70deg & our fingers still get ice cold. Does the a properly sized system with no other variables changing actually fix that?
Would it be in my benefit to tear down the drywall ceiling in the basement even though there's only one supply register down there?
There's another thread going in this forum about the subjective and objective qualities of different types of heat; it sounds like your house is a perfect storm of subjectively uncomfortable heat: uninsulated brick walls; short-cycling forced air; cold floors. You didn't mention your climate zone, but a design temp of -4 puts you far enough north to warrant the usual advice: spend money on envelope before equipment. Building Science Corp website has the most comprehensive prescription for properly insulating masonry walls, with details of how to handle joist pockets. Looks like you're at around 6 ACH50--a bit of air-sealing work would probably go a long way. Hopefully your house is small enough that meaningful upgrades aren't absurdly expensive.
Then you won't feel obligated to heat an unoccupied basement.
And then switch to minisplits.
Jeff,
It doesn't take much of a furnace to heat a 950-square-foot single-story house in a climate with a design temperature of -4 degrees F. A 30,000 Btuh furnace should be more than enough.
If you are uncomfortable now, the problem almost certainly arises from either a bad distribution system (undersized ducts, leaky ducts, unbalanced ducts, or lack of return-air ducts) rather than an undersized furnace.
All kinds of building envelope problems -- including single-glazed windows, air leaks, and poorly insulated masonry walls -- can give you a chill during the winter. These problems have nothing to do with the size of your furnace.
As Andy explained, investments in envelope improvements always make more sense than the impulse to "just buy a bigger furnace." If you fix your envelope, you will be more comfortable.
Not sure I made it clear...but my current furnace outputs 75K. The Manual J says I only need a 30K furnace - I am downsizing & not buying a bigger furnace. And this downsize comes after a lot of envelope improvements were made.
About needing more air sealing - I was told 800cfm @ -50Pascal was pretty tight for my 950 sqft home of 8 feet ceilings & basement. It used to be 1500cfm @ -50Pascal. I already have a CO2/ventilation issue, and in fact the heat load for ventilation is higher than for infiltration. My heat load prior to envelope improvements I made was 50kBTU/hr...so I dropped it by 20kBTU.
Therefore my goal is to seek improvement in comfort by dropping the 75kBTU/hr furnace down to a 30kBTU/hr furnace that the post-improvement heat load says I need.
"All kinds of building envelope problems -- including single-glazed windows, air leaks, and poorly insulated masonry walls -- can give you a chill during the winter. These problems have nothing to do with the size of your furnace."
- but isn't that what the Manual J seeks to identify to determine your heat load, which in turn determines the size of the furnace?
The primary heat load components were walls @ 50% and windows @ 25%. But no where does it talk about accounting for duct issues unless that's balled into infiltration, which is accounted for.
Jeff, yeah, Martin didn't read back in the thread to read your original post about a SMALLER furnace. His point, though, is that if you're uncomfortably cold with a way-oversized furnace, then either something's quite wrong with your equipment and ductwork; or the Manual J is inaccurate. (Or both. In your original post, you said you paid a "sizable amount of money" for it--was the technician experienced in load analysis, or just generally reputable? The tool is only as good as the person wielding it, regardless of the cost. Did they do an infrared survey with the blower door? This always points to trouble spots you'd never expect.) In any case, you're not likely to get more comfort by putting in a smaller furnace without making other changes.
Are you 950 sq.ft. on first floor AND 950 sq.ft. in basement? Then your ACH50 number is closer to 3, not 6, which sounds reasonably tight for an older house. Do you already have the HRV in place? If that thing's not yet running, imagine adding its influence to your current setup. Brrrrr! Man, just sounds like something's wrong with the numbers.
If you added a tafiant heat source near your sitting area... Nice
I think your brick walls are radiating cold. Which really is your body warm losing heat to a cold surface...
Other than that your home may need way to much work to change your situation. You could add a pellet stove.... A gas wood stove... Or even a mini split... The mini split is low cost and would run most the time and adjust to lower speed automatically. I would add a gas or propane fired wood stove.
Wont the situation get better once a properly sized furnace is installed?
I would think the simple fact that the 2x oversized furnace is blasting his house with too much heat for a couple of minutes and then not running for a long time would be the cause of the problem. A properly sized furnace should have longer run times leading to a more even heat distribution.
Our house is dealing with the same situation right now. We still need to figure out just what size furnace we need rather then the current 60k btu unit which is short cycling. In my research I have found that a modulating furnace may be the answer for us... They can run as low as 40% of their rated output and keep the air continually moving around the house.
So to sum up I would think a smaller furnace that runs more often will make you more comfortable. Maybe ask about modulating furnaces as well.
Jeff,
I'm sorry if my answer wasn't clear when it comes to my advice about a smaller furnace vs. a bigger furnace. Here's what I was thinking: your Manual J calls for a 30,000 Btuh furnace, which you are (in theory) about to install. But you're worried about comfort issues -- so you are tempted to go for a 75,000 Btuh furnace (to match what you have always had). My advice was: if you have comfort issues, choosing an oversized furnace is not a solution.
Manual J doesn't address comfort, just proper system sizing. The problem you have is comfort, not inadequate heat. So that's what you need to address. One of the solutions to greater comfort is a properly-sized heating unit, which will make you more comfortable by running more frequently, causing more air mixing and less temperature stratification (a common problem with oversized furnaces; I have the same problem in my house with a 3x oversized furnace). As others have indicated, you should also focus on the building envelope, especially since your heat load calcs indicate that the walls and windows are 75% of the problem. AJ is right; those (presumably uninsulated) masonry walls are radiating cold and making you feel colder than the temperature might suggest.
If you wrap the masonry in mineral wool or foam, then put stucco or siding over it, and replace single-paned or aluminum-framed windows with vinyl or fiberglass double or triple pane windows, you will probably find that the comfort goes up a lot even with the absurdly oversized furnace.
Andy - I have no reason to believe that the Manual J calculation was inaccurate. They're in the building science/testing field & not an HVAC company. And I've previously had energy audits which inspired the air sealing & insulation improvements I made which dropped my heat load by 20kBtu; these did involve infrared & blower door testing. The 800cfm @ -50Pascal infiltration was measured by blower during Manual J data collection. I only have my exhaust fans for ventilation. I have a basic rectangular building so 950sqft on the main floor matches the same area that the basement covers.
AJ - So it sounds like you're saying add another heat source? If Manual J says I only need 30kBtuh, why would I add another heat source which would exceed the prescribed heat generation?
William/Nathaniel - that's exactly what I am hoping would happen - comfort improves with a properly sized furnace as it gives the walls time to warm up. With the big furnace, the air feels nice & warm when it's on, but after it turns off, 5 minutes later the air feels chilly. A single-stage single-speed furnace is all I'm confident on going with since this is a small house & I don't need all the bells & whistles.
Martin - I don't plan to drop in a 75kBtuh furnace. In fact there's nothing wrong with my current furnace...my only reason for the swap is based on a Manual J after having comfort issues where 70 degrees on the stat is still "cold".
You guys keep hinting that I should do more to drop my heat load...I've already dropped it by 20kBtu post-energy-audit. Isn't the point of the heat load to calculate how much heat is needed to offset the losses? How much more am I supposed to go down to? 0? So, I'm hoping the furnace size mismatch is the missing link to comfort, but I just don't want to regret that move.
Also, my brick walls are insulated - R11 faced fiberglass batts. Foundation has R11 unfaced with some type of plastic on one side. Vinyl double-pane windows.
Jeff,
You've already received a lot of good advice in the thread. There's value in every response.
From my perspective, you've got a mess. (A 'mess' is a bunch of problems.) There may be a linkage among these problems. The PROBLEMS you mentioned:
1) Need to replace the furnace. (No mention of why).
2) Hands are cold in winter even when room temp is 70F. (No info on relative humidity).
3) "I try to reduce air leakage from the basement to main floor as much as possible since IT'S USUALLY COLD DOWN THERE.
4) "The house always feels cold WHEN THE FURNACE IS NOT RUNNING even if the stat reads over 68F. We keep the stat on 70F and our fingers still get ice cold."
5) "I already have a CO2 ventilation problem ..." (No numbers supplied).
6) (I'm getting envelop advice but I've already reduced infiltration from 1500cfm to 800cfm .... )
7) "I paid a sizeable amount of $ for this heat load to be done ..... "
Where to start? I tended to start where NG did (see Post #10). You have a comfort problem, not a sizing problem, and (see Post #4) it doesn't take much of a furnace to heat a 950 ft2 house ... .
However, I now think (after some reflection) I'll go back to the 'CO2 thing (5)' and the 'need to replace the furnace thing (1)'. If you have a known or suspected furnace integrity issue leading to CO2 or CO concerns, you've got to get that resolved first. You haven't told us but I'm guessing your furnace is natural gas and in the basement and it gets its combustion air from the surrounding living space in the basement and it discharges to a traditional flue.
If you determine it's appropriate to get the new furnace, there are several options. A high efficiency unit with outside air for combustion air and condensing flue gas should be attractive and that will lead to a less porous building and improved comfort. Then, the additional work on sealing leaks and insulation around the building will create the additional comfort you're looking for. You shouldn't require a continuing blast of 140F just to feel comfortably warm in your modestly sized home.
W D - I agree, I appreciate all responses & input.
It's very disconcerting to hear I've got a mess after all the work I've done into getting this house more energy efficient, sealed up, and insulated. I'm not a random homeowner who just decided to spring for a new furnace because we had a couple of cold days, but I've been researching energy efficiency improvements since acquiring this house a year ago.
My CO2 issue is that the numbers are high, where "high" means over 1000ppm. I have a CO2 meter, purchased after I had a PE assess air quality & revealed my levels are high. It's not a furnace integrity issue but an air quality/air tightness thing. My RH is higher than what I read it should be on the net. Looking at my indoor hygrometer now, I'm 70deg @ 50%RH, with outside temp @ 30deg, 74% humidity (we had some snow flurries earlier).
This is what makes me pull my hair out because I come here & I hear that I need to do more sealing yet I've cut infiltration in half already! I've sealed up more stuff than I can count, which includes time in a dusty, low-pitch attic & basement, and there isn't much "left" to seal! The air quality guy even told me to stop sealing.
Also, my current 75k furnace is a sealed combustion high efficiency unit with intake & exhaust pvc pipes. I mentioned my reason for downsizing is to improve comfort and not kill this furnace from the constant short-cycling that it does.
Jeff I agree with you.
It would seem that everyone is looking for a problem with the house itself when there really doesn't seem to be a problem other then the size of your furnace.
You have addressed the air sealing issues. You have posted elsewhere that you have R60 in your attic. Your windows are double pane units. What else can you do short of major renovation work...
Unless there is something that is being missed I think you should feel confident that an investment in a properly sized mechanical system will be the final piece of the puzzle.
We have all at one time been in houses that are average in terms of insulation and glass that have a properly sized system and been comfortable simply because the system operates at a rate needed to make the house comfortable. There doesn't always need to be a problem with the enclosure that has to be solved. So many houses do have those problems though that we tend to focus on them first.
If you do decide to swap the furnace I hope you come back and post an update letting us know if it has solved your problems. Good luck!
Results are all that matters. If you're not comfortable yet, then the work isn't finished.
Since your furnace is already sealed combustion, then I'm guessing that unless the furnace is dangerously broken, the CO2 is coming from a natural draft gas water heater, or a gas range. Use your CO2 meter to see if the concentrations are waaaay higher around those appliances. If so, you've found the problem. If you don't already have one, you may need a "low level" meter to get accurate instantaneous readings. If these appliances are problematic, you could replace the water heater with a sealed combustion gas tankless or storage water heater. For the range, make sure you're always using your range hood.
If the house always feels colder than the thermostat reading suggests it should, you need to identify the source. Are there any interior surfaces that are very cold to be near? Any drafts? Is your thermostat reading the temperature correctly? Does it match a reading taken with another device? Is there a lot of temperature stratification such that when the air around your thermostat reads 70, the rest of the house is actually 65? Are the floors uncomfortably cold? Humans tend to be more uncomfortable with cold feet.
The cold basement is probably a big source of the problem. I'd work on putting even more insulation up on the walls. Make sure the rim joists are properly insulated too. They're a common source of heat loss in houses on basements or crawlspaces. Insulating the floor space between the basement and the ground floor may also help quite a bit if you're able to do that.
Nathaniel
He is talking about CO2 (carbon dioxide) not CO (carbon monoxide) like you would find from a faulty furnace or water heater.
Oh, whoops, silly me.
Well a carbon dioxide problem would be caused by a well-sealed house without any provision for constant ventilation and make-up air. All the CO2 you're exhaling can't escape now that you've tightened it up. Time to install an HRV or some other provision for constant air exchange.
Wool socks and sweaters will help round out the comfort issue but nothing for the pollution humans create (CO2, toxins, etc). I vote for the smaller furnace.
Nathaniel -
"Are the floors uncomfortably cold? Humans tend to be more uncomfortable with cold feet."
"The cold basement is probably a big source of the problem. [...] Insulating the floor space between the basement and the ground floor may also help quite a bit if you're able to do that."
I've gone crazy with the amount of thermometers & hygrometers in my house. They're calibrated & at least consistent & I check on them almost religiously. They'll read 70 degrees just like the stat, but it just "feels" colder. It's made me question if I really know what 70 is supposed to feel like.
But yes, floors are cold & I know I am losing heat to the basement since it's not heated. You suggest insulation between floor/basement & I was wondering about the opposite - just to heat the basement. The Manual J spit out 30kBtu/hr for the whole house (main floor + basement).
So my main goal with posting this thread was if I would regret trusting a heat load and downsizing my furnace if an oversized furnace doesn't already make me comfortable. Though I can always do more in the way of envelope improvement, what I didn't hear is: "if you've got comfort problems with a big furnace, you'll have MORE comfort problems with a properly-sized furnace."
The heat load proved 75% of heat loss is due to walls & windows, and unfortunately there's no escaping that based on the design of the house. I think I have my answer that downsizing should only help with that.
Edit: just thought I'd add - I've had all the CAZ testing & such, there's no bad exhaust gas issues.
Jeff,
Just to state the obvious: not everyone is comfortable at 70 degrees F. Lots of people prefer 74 degrees F, especially as they get older. My guess is that you are probably at least one year older than you were a year ago.
For heaven's sake, if you are cold at 70 degrees F, the first step (assuming you don't like sweaters, which is your privilege) is to set the thermostat at 74.
Even an uninsulated 950' house with only 800cfm/50 of leakage wouldn't have a heat load as high as 75K.BTU/hr @ -4F outside/70F inside. A load of 30 K sounds like there is no foundation insulation, and probably a bunch of U0.5 window area (?), or an overestimate of ventilation rates.
At 800cfm/50 there is a strong risk of backdrafting, if you have any atmospheric drafted hot water heaters, etc, which could explain a high CO2 reading.
What is the temperature in your basement? I live in a house that has no floor insulation, but with ~R17-R18 insulation on the foundation walls on the unheated basement, which stays at or above 65F at -5F outdoor temps (and colder) when the floor above is kept at 70F. The only locations where floors are cold is in rooms with excessive window area causing a convective loop cooling the floor. If you have ANY single pane windows (even modestly sized), that can create a localized cold floor issue.
Ducted air systems tend to be lower comfort than hydronic radiator solutions, due to both wind-chill issues while operating, and the fact that the heating system does not affect the average radiation temperature in the room. While it's a fairly expensive "fix", low temperature panel radiators underneath windows raises the average radiation temperature coming from that direction. The interior surface temperature of a U0.5 window when it's 0F is nearly freezing, but if the space adjacent to it is a panel radiator with a surface temp of 100F or 120F, it nulls out that radiant-loss issue that would be so apparent to exposed skin. More about average radiant temperature & window effects on comfort here:
http://www.cbe.berkeley.edu/research/pdf_files/SR_NFRC2006_FinalReport.pdf
In other words... the oldsters who put radiators under windows had it right! :D
Just for comparison I have a 75,000 BTU furnace in my 1400 square foot, 2 storey brick house with no insulation in the walls and it gets the job done. According to manual J, I need 68,000.
Seal the basement from the upper floors with a well installed air vapor barrier, taped and caulked and all seams overlapped. Wrap the poly up the rims and tape or caulk it to the top of the rim joists. Seal the basement door like it is an outside door.
CO2 levels world wide peaking this year.
http://www.theweathernetwork.com/news/articles/watch-the-northern-hemisphere-disappear-under-a-swirling-blanket-of-carbon-dioxide/40170/
Nathaniel G: "In other words... the oldsters who put radiators under windows had it right! :D "
That's right- crappy leaky single pane windows are horrific from a comfort point of view, but with that 200F steam radiator under the slightly open window (to limit the mortality risk from carbon monoxide leaking out of the coal-fired asbestos covered beast in the basement ) it really wasn't all that bad.
Of course having a 200F radiator next to a U1.0 window raises the as-used heat load skyward too, with a column of 150F air streaming over the single-panes rather than 70F room air. Heating with radiators next to cold windows they got right, but there has been just a wee bit of building-envelope efficiency improvements since 1860, eh? (Space heating radiators were invented in St. Peterburg, Russia, about that time.)
Better windows = higher interior surface & radiation temps, needs less offsetting with radiators for comfort, but a radiator solution would still be more comfortable than a hot-air furnace.
Even if the new right-sized furnace is only delivering 110F air at the furthest register (possible, with a condensing furnace and uninsulated ducts in an uninsulated basement), it's still pretty comfortable while running. With longer run times you won't have an overshoot blast of heat followed by a long slow drafty chill either. But it's still a far cry from the comfort of radiators.
The fact that the rooms aren't heated evenly is an issue. If the ducts are designed to Manual-D and the furnace is right sized for the load that should improve, but with uninsulated ducts of a substandard layout & unknown leakage it's really hard to say for sure how much improvement you'd get out of just right sizing the furnace.
Jeff,
First, to your original questions. 1) The duct work shouldn't be a problem, most of the heat loss will be to the basement, which you don't heat. This would only be a problem if you have a major loss of heat from the basement. 2) a) for the smallest heat loss, but not for the most comfort. 3) The temperature of the heat coming from the furnace will matter more on the type of heat source then the size of the unit. I'm sure that you would be fine with the heat coming from a 40K gas furnace.
From what you have written, you have done a number of the things you can do to reduce your heat load, but it seem that your major problem is not with the heat load, but your comfort level.
What the cycle time of the furnace is when it zero outside. If it 30 to 40% of the time, then the manual J value of 30K would be about right.
What the temperature of the basement is when it zero outside.
Most important, when is the comfort level a problem. If it all the time, or only when it very cold outside.
I also have done things to reduce my heat load, reducing my heating bill by 30%, but it hasn't changed my comfort level in my living-room much. My townhouse is built with my living-room over an unheated two car garage, so I end up with cold floors and a draft from sliding glass doors, but my problem only happens when it very cold out. For most of the year, my comfort level is fine. From all my research, my problem is caused by the unheated room below the living-room and the poor insulation job that separate the two.
To fix your cold floor from what I have read, you have two major ways of doing it, heat the basement (or the floor), or insulated and air seal the space between the basement and the living-room. Both of these solutions might not fully fix the comfort problem, but should go a long way to fix it. In most houses with a basement, the easy solution is to heat the basement.
I would also check with your HVAC contractor to see if it possible to temporary disable half of your heat source. It wouldn't be as efficient as a new furnace, but should give you some ideas if the longer cycle time fixes the problem or not.
Carl
I repeat:
Seal the basement from the upper floors with a well installed air vapor barrier, taped and caulked and all seams overlapped. Wrap the poly up the rims and tape or caulk it to the top of the rim joists. Seal the basement door like it is an outside door.
Until you have sealed off the basement air flow (infiltration drive, stack and convection) into the main floor, you cannot determine what is failing. Same goes for the attic plane. This will cheaply hugely improve the comfort levels in the house. FG insulation will not seal the flow, an air barrier will. Dense packed cellulose will do both (seal and insulate). If your budget is thin, air-seal before insulating (this situation).
My Stelrad Euro style radiators are all under the windows. That location reduces the space loss if the rads were elsewhere. We run a 95F to 110F supply with our condensing boiler. Windows are thermal pane. The key to steady heat, no draft comfort is air-sealing and a modulating thermostat/boiler that keeps the temps even (no big swings).
Chemlink Duralink polyether caulk: and caulk the heck out everything.
http://www.chemlink.com/index.php/chemlink-products/duralink
You can also use this same caulk to seal every seam and joint on every envelope penetration indoors and outdoors.
[[ Martin ]]
I understand that people can have varying levels of comfort. However, even if I turn the stat up, once the air handler is off, my thermometer might say 74 but I still feel a cold sensation. My wife is a lot more sensitive though. We're in our 20s...I doubt we're getting closer to the stage where we'd need more heat for comfort.
[[ Dana ]]
A 30K load is with R11 unfaced fiberglass + plastic cover on the foundation walls, no insulation under the foundation, windows double pane u-0.55. According to CAZ testing, no spillage of my water heater exists. Basement temp tends to be about 10-15 degrees below main floor [in the 50s], outside temp is in the 20s. Thanks to this being a small house, with small rooms, it's hard to get away from the windows...every room has at least two. And of course the bottom 2-3 inches of all windows have condensation.
When I had an HVAC guy over, he could find no issue with the ducts as the 'cold' rooms still put out heat. The ducts are sealed in the sense that they are in the air space below the subfloor & above the basement ceiling drywall..which extends wall to wall. I was curious if this means the ducts are connected to the air space behind the (air permeable) fiberglass on the foundation walls which would classify it as being outside of the envelope. This is what makes me leary because I feel like this couples the ducts to the outside (and NOT the living space of the basement).
The layout of the ducts is very simple. Most registers only suffer a single curved bend: a straight shot from the plenum & a curve upwards to reach the register. Rooms on the side of the house opposite the furnace have 2 bends.
[[ Flitch Plate ]]
Isn't drywall a decent air barrier? The bottom of the floor joists have drywall already, so the rim joists & ducts aren't visible. The joists that are visible in the mech room are in pockets within the brick.
[[ Carl ]]
Thanks for answering my original questions. It's in the 20s outside right now, with the stat on 70, this thing comes on for 4 minutes, & shuts off for about 15; repeat.