The AM Forum

I am very seriously thinking of upgrading our existing gas-fired furnace to a latest-generation very high-efficiency unit. Our current unit provides about 80% efficiency, and the unit I'm thinking of installing is rated at a minimum of 95% AFUE.

However, this very high efficiency furnace is quite sophisticated on a technical basis and it contains a lot of electronics for the control and staging of the burner and blower, etc., and I am concerned about its susceptability to RFI from the HF part of the spectrum. The source of this RF would be my 2x 4-400A HF transmitter, with the house located within the near-field of the antenna. My existing furnace is about 25 years old, and although it does contain some solid-state electronics, it has never exhibited any RFI issues when I had the big rig on the air.

The manufacturer of the unit I am considering, Lennox, offers no information to me in this regard; the dealer, understandably, has zero idea as well.

This high-efficiency furnace also would require the replacement of our existing thermostat; the current thermostat is an older but faithful and reliable Honeywell programmable set-back type, and it has never had any issues with RFI. The Lennox thermostat required for use with the new Lennox furnace is more sophisticated than the Honeywell; the Honeywell thermostat provides a simple contact closure to the furnace when calling for heat or air conditioning; I think the Lennox unit has some type of electronic interface, making it possibly more prone to RFI issues.

I can run shielded multiconductor wire between the thermostat and the furnace; I would do this as a matter of course during the installation.

Does anyone have any experience with these latest-generation high performance gas furnaces where the AFUE efficiency is >95%, and their susceptability to RFI in the HF part of the spectrum?

The last thing I want to do is to attempt to RFI-proof the electronics in the furnace and/or thermostat in the event I should have an RFI issue.

Thanks & 73,

Bruce

I upgraded to a 95+ two years ago. The furnace is in the basement of the house. I have not noticed any problems with it or the air conditioning at FLL on 80 meters. On 160 or above 75 I use considerably less power. The antenna for 75 is an inverted V, peak at 40', with one end about 25 feet from the house. 
Come to think of it ('cause it just came on), I have a high efficency ceiling hung unit in the shack building and it is under the V, 4' fron the 4-400 transmitter and 3 feet from the coax heading outside. I have never had any trouble with it either related to RF.

Of course, "Your mileage may vary". ;D

Bill KA8WTK

We have two separate central heating/cooling systems, one for the master suite (which includes a lot of the radio gear in the basement below) and another for the other parts of the house (which includes the ground floor radio room).  The master suite has a Luxaire high efficiency (5 years old now) and last year we had a 20 year old Amana high efficiency replaced with a Coleman high efficiency as part of some other upgrades.  Both are fairly similar units with dual heat exchangers, forced induction draft, etc and use honeywell electronic programmable thermostats.

I have experienced no RFI to or from any of the systems.  I run the legal limit on all modes with both vintage and modern gear.  The antenna most used with the vintage gear is a full wave 80 meter horizontal loop one end of which is attached to the house chimney.

I have read of problems from and two systems so a bit of google search would be in order.

I have an Armstorng Air heat pump that is new on the market and does the stuff the Lennox you are looking at does.  It ramps up the compressor, turns it off and on then does the same with the air handler.  I have absolutly no interference problems as opposed to anything International Comfort Products made. 

Armstrong Air is a Division of Allied Air and they own Lennox and Armstrong Air plus another.  I found the Armstrong as reliable, efficient and costs less than the name brand.  Your mileage may vary. 

Internation Comfort is Heil and several other brands.  None will give you much in the way of customer support so I hope your dealer is very astute.  The factory tech is not savvy on interference.  Make the dealer of what brand you choose take you to an installation and take a radio that tunes the frequencies of question.  Listen around and see.

I have the exact furnace and air conditioner that you are talking about the 95 from lenox and I do not have any problems at all.

John W9BFO

sounds like a lot of money to save a few bucks. Many people I know who swapped to high efficiency units had to detune then due to water build up in the exhaust.
Mr. HVAC will set us straight

The furnace manufacturers know that their products produce rfi hash.   Rather than incorporate good design principles they wait for the complaints then provide rfi cleanup kits at no cost.   My dad had to do that about 3 years ago with a new furnace.  Other hams have mentioned going thru the same process.

Thanks for all of the input, guys.

To clarify my question: I am not so much concerned about possible RFI that the new furnace may produce. Rather, I am concerned about the susceptibility of the furnace and it's accompanying application-specific thermostat to the RFI from my high-power HF transmitter.

Frank/GFZ: These very high performance gas furnace units cannot be ducted to the chimney; the exhaust is largely condensate, and that is dumped out of the house along with any combustion by-products through a PVC pipe. The temperature of the exhaust is too low to duct into a conventional chimney, as it would condense within the chimney, creating excessive H2O build-up within the flue. My existing Lennox induced draft furnace is 80% AFUE, and that is about the highest efficiency unit that can be ducted into a chimney, but it still requires a stainless steel liner within the chimney to prevent condensation from forming. And the liner is insulated within the flue.

Here is the link to the Lennox data sheet for the furnace I am considering. The data sheet is not technical (it is oriented to the typical consumer), but the cut-away dwg. will provide a good idea as to the complexity of the unit, at least when compared to the average 80% AFUE rated run-of-the-mill gas-fired forced hot air furnace.

http://www.lennox.com/pdfs/brochures/Lennox_SLP98V_Gas_Furnace.pdf

I hope that Jared, W1ATR will weigh in here with his comments and thoughts.

Thanks & 73,

Bruce

Bruce,

The high efficiency furnaces continue to get more sophisticated but I had our first high efficiency unit installed in 1991 (I thought it was later but I still had the old paperwork) to replace a Moncreif furnace that sent about as much heat up the exhaust stack as it did into the house.  The furnace was still in perfect working order but it was inefficient, over sized, and a couple of times I came home to a cold house after the excellent draft provided by the very tall chimney (originally used with a coal fired boiler back in 1901) sucked out the standing pilot on occasion.  The blower assembly from this furnace still lives on as part of an ambient dust collection system I built for my wood working shop in the barn.

Since 1991 I have replaced one igniter and one gas control valve.  The furnaces are more complex but at least I have been fortunate with reliability.  A few people in the area have run into problems with a high efficiency unit failing to light when high winds create enough pressure that the purge blower safety sensor doesn't signal sufficient flow but that is probably an installation issue and a problem I have not experienced.  One thing I noticed is the lubrication ports have been removed from the purge blower on the newer furnaces.   Our HVAC guy remarked that it was because nobody ever bothered to lubricate them and I had some difficulty finding the special high temperature oil originally.  He warned me not to expect the purge blowers on my new furnaces to last 20 years.   I remember about 10 years ago there were a lot of reports of defective control boards (used across multiple brands) and a couple of my colleagues got bitten by those problems.

Jared should be able to provide the final word on this and give you the best recommendation.  I get a little nervous about modulated burner flames and continuously variable blowers given the increased susceptibility to RFI ingress.  If you have any remaining concerns, it would be worth having your HVAC installer use a good line filter at the furnace power connection and I would also use a shielded cable for the thermostat.  It probably isn't necessary but it would be easier to install this at the time of installation to forestall problems.

Hi Bruce.
30 some years working on HVAC systems and I have never heard of RF causing any problems with any furnaces or thermostats. The biggest thing is proper grounding for your hvac equipment and the radio station.
The modern pc boards are housed in a metal case so it should be a good rf block.
Bob.

Thanks, Bob. That is somewhat reassuring.

Being that you are in the HVAC trade, my selection of the replacement furnace is coming down to two possible choices: the Lennox modulating (in 1% increments) 95% AFUE unit (this is the unit where I attached the link to the data sheet), or a multi-stage unit with a variable speed blower. The multi-stage unit will be cheaper to purchase and is somewhat less efficient, but I'm concerned about the reliability of the more efficient Lennox modulating unit, due to it's increased electrical and mechanical complexity. Our home is very energy efficient; I have recently completely reinsulated the interior and exterior walls, basement, and the attic was reinsulated by me years ago, and I also performed a manual-J calculation to determine the heat loss and heat gain of the reinsulated structure. Infiltration losses are very low, too, as I completely wrapped the exterior sheathing in Tyvek Drain-Wrap, with all of the seams, etc., sealed with Tyvek tape. The sill plate interface was sealed as well. Per the calculation, the predicted heat loss is 19.8K Btu/Hour at an outdoor ambient of 15 degrees F, and an interior ambient of 70 degrees F, so we need a very small BTU output furnace to minimize short-cycling, and regardless, our gas usage will be quite low as well. The calculation also assumes the heat loss from a heat recovery ventilator unit, which I plan on installing at the same time as the replacement furnace.

Your professional guidance here would be appreciated. Most of the HVAC dealers I have spoken with are "salespeople", and I need honest and informed answers to my questions.

Thanks & 73,

Bruce

Thanks for all of this input, Rodger.

With regard to your comments regarding the  reliability of these latest generation gas furnaces, please see my questions above to Bob, KE7KPB.

73,

Bruce

Most furnaces are made by a few company's like BDP is Carrier, Payne, or Day and Night. Every furnace manufacture has there differences in how they are controlled. The more bells and whistles are installed well you know where I'm going. It is a mater of your pocket book and how fancy you are willing to go. I installed a new 95% in my house and it is made by Goodman. The biggest thing I would look at is Who is doing the installation. There are many install company's out there but you need to do your research on the subject.
The system is only as good as the installer. I live in Montana and we depend on our heating systems as it can get very cold here. The other thing is the amount of insulation in the house and how tight the construction is.
You run into another problem, the house needs to be able to breath. If the house is to tight the condensation will build up and your house will rot. You can help that problem with a air to air exchanger.  This will bring in fresh air and exhaust air so you won't have rain in your house.
I hope this helps a little.

Yup, my furnace guy told me 80% is about all you can get with the typical chimney if everything else is right.
I'm waiting for the light bill for the week we spent at the beach after the storm to compare cost. I noticed the heat didn't run all that much after the house warmed to operating temp. R35 in the walls R67 over head. Still need more insulation in the basement with only R25 in stick walls.

Bruce,
It makes no sense to worry about a problem you don't even have.  If you want a new high efficiency furnace, then get one and deal with problems only if they appear.    :)    Bill

big problem with new homes and such people adding so much insulation, and create a super air-tight space. No air exchange with fresh dry air outside.  My house is so drafty it doesn't matter.  But I've seen new homes with lousy interior air quality, and some older ones that were build up for insulation that had problems afterwards DIYers ending up with mold and stuff after a poorly executed insulation job.  

A bunch of years ago we switch to 95+ efficiency Nat-gas furnace, knock wood no complaints since.  Now I'm waiting for the Electric HWH and Dryer to die so we can switch them to gas as well.

The nice thing about 90+ furnaces is they use outside air to burn. They are vented with PVC pipe. One for the combustion air and one for the air inlet to the combustion chamber but you can vent it with a single pipe.
A word of WARNING, never vent the 90+ pipe into a unlined brick or clay chimney. You should run it all the way to the roof or do a horizontal pipe with a slight crack the bubble back to the furnace so the condensation will flow back to the furnace drain. Also you can add some limestone to the drain of the furnace to neutralize  the water to the furnace drain. Sorry for the rant.

Hi

We have a 15 yr old Trane XL90 high efficiency that breathes out of a PVC. Parts always seem available. And never a thing done to it, except inspections, cleaning, and every 2 years check the refrigerant in the condenser unit outside for the A/C. It is also a Trane. "Hard to stop a Trane"
There is a module in the furnace to 'run the show' and never a trouble with me operating HRO AM. The OWL is 10 feet from the furnace. I installed a Honeywell wireless thermostat to control A/C/ and heating in our tri-level house and no RFI from the furnace or to the furnace or wireless thermostat.
Fred

Thanks to everyone for all of the informative comments.

It looks like I can install this type of furnace unit here at the QTH, and the probability of RFI issues to the furnace and thermostat from the rig should be minimal.

73,

Bruce

QTH here is so tight we need to leave bedroom windows open a crack or you get a head ache. Controlled leak works fine to keep fresh air in the house.

I thought I was the only one.  

Even with older house construction (1910) I still need to let some air in at night for breathing. I don't know how some can sleep in a room with the windows shut and walls air tight.  Yaz uses his share of oxygen too... ;D

Plus, the coal stove uses its share of air. I installed an outside air inlet for the stove. You can feel it sucking the air in when operating.

Good luck with your new installation, Bruce. Interesting topic.  At the moment we go thru 1.5 tons of coal each year and fill the oil tank maybe once a year for hot water use and back up. This has saved a lot of $.  I do like the feeling of a coal stove in the living room when it's cold out.  

I'll bet these 90%+ high-efficiency units will become mandatory for furnaces at some point in the future.

T

I have a 3 year-old NG gas Lennox (AFUE Rating of 92.1) with the electronic controls and have had no problems.

At one time I had my inverted-L running along the gutters and the only problem I had was RF getting into one old analog televison set and the LED Christmas lights out front. I was running 325 Watts carrier on AM.

Inverted L now runs north and south out into the yard away from house.

RF never seemed to affect the Lennox.

Phil - AC0OB

Check this out for the Air to Air heat exchangers.
http://www.smarthome.com/3033A/HE100-Air-to-Air-Exchanger/p.aspx

The air-to-air unit is discontinued.....sounds like GFZd needs one.

Frank your house is too air tight and you are adding more insulation?
Ct. winters get cold but ya gotta breathe.
Fred

Sorry about that. Try this one.
http://residential.fantech.net/residential-products/indoor-air-quality/

This is just a way to get fresh air into the house with out costing you money in trying to reheat that cold air.
After all who wants to reheat -30 degrees.

There was a study done several years ago by the Univ. of Minnesota concerning the optimum level of humidity within a home, vs. the potential for moisture/condensation damage to the structure, such as water-soaked insulation, rotting exterior sheathing, mold within the home, etc. At outside ambient temperatures of something like 30 to 40 degrees F, the maximum relative humidity within the dwelling should not exceed 40%. As the outside temperature drops below 30 degrees F, the level of the humidity within the dwelling must drop accordingly, so that at some very cold outside air temperature (see the results of the study), the maximum humidity within the home should not exceed 20%. That's pretty dry.

Otherwise, with tightly sealed home construction, you very much run the risk of the so-called "sick home syndrome", including not only moisture damage, but possible health hazards associated with the various pollutents within the home, including formaldahyde (used in some building materials) and a slew of other toxins. One established remedy for the this, particularly with homes so tightly sealed that air infiltration losses are significantly minimized, is the use of a heat recovery ventilation system, or HERV. These are essentially very efficient heat exchanger units that bring in the outside air, pre-heat it from the furnace to reduce the delta T losses, and replace the stale air within the home.  I'm installing one of these at the same time that I replace the furnace unit.

Residential HVAC technology has come a long way in the past 15 years or so, largely motivated by the need to conserve energy, along with the way that modern homes are built. Take a look sometime at the Canadian R-2000 standard for new home construction. And to build a highly energy efficient home that is inexpensive to heat and cool does not materially add to the price of construction.

73,

Bruce

Hiya Bruce. Gotta keep it short, but the 98v is a good furnace. Zero problems with them in the field except for some igniter failures which were an easy fix. The expensive stuff (blower motor, board, etc) is covered by a 10yr warranty so make sure you use a decent lennox dealer that will be around when you need them. If your electric bills aren't too bad now, then consider installing a hybrid system (swap your condensor for a heat pump) and set the changeover to switch it out around 33deg. A manual D ductwork calc is important to make sure the 98 is going to run at it's rated efficiency. Too many of them are installed on pissweak ductwork and the gas bills actually go higher than a 90% would use. (Or even an 80% in some cases I've seen.) The heat exchanger has a lifetime warranty on it, BUT there can be problems with a claim if the ductwork isn't up to snuff and theres a failure. Stay away from pleated filters unless its one of the bigger 'air bears', otherwise just spring for an electrostatic. This furnace is all about proper airflow.

The thermostat should be what lennox offers for the system, but you can use any two stage stat and set the high low off settings with dip switches on the board. The lennox stat is better in a few different ways however. This is good info to know should it die and another isn't immediately available, (like at 2am in Feb) then a regular two stage stat can be slapped on the wall temporarily.

As far as the house being too tight, a lot of people don't quite grasp how severe it can really get with barely any signs until it's too late. There was a family around here, maybe Mass, I don't remember, but they gave themselves some sort of weird blood poisoning (I think a kid didn't make it) from indoor air pollution from cooking, candles, cloths drier, etc, in a house that was too tight. As Rob suggested, all you guys that like insulation and weather proofing just a little too much really REALLY need to invest in a fresh air exchanger. Frank, if your getting headaches during sleep (When your brain and body needs clean O2 the most) and you have to crack a window, the indoor O2 levels in your house are already way too low.

Good luck with the new install Bruce.       

Hi Jared!

Thanks for the great input here. If anyone knows this stuff, it's you, and I was hoping you would respond.

Very interesting about the ductwork, as none of the HVAC contractors I met with have brought that up............I'll have to look into this.

Can I call you at a convenient time to discuss this in greater detail? Plus I'd like to speak with you about another (unrelated) topic as well.

I may have misplaced your phone number. Can you advise via PM your phone number and a convenient time for me to call? My e-mail is triodes@optonline.net

Looking forward to your reply.

73,

Bruce