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Old 10-09-2009, 04:55 PM   #1
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Installing natural gas furnace in place of heat pump, please help


Hello everyone, I'm new on here.

I have a 30 year old heat pump that is failing in an area that has pretty harsh winters. The 70,000 BTU unit has always had enough heat. It has an analog thermostat inside the house to control it. But it is toast now. The property is a rental and it is 300 miles from my permanent residence.

I got a quote for a new Gaspack system (hybrid) at $6300 installed. They said a furnace outside would cost about the same. I found a 110,000 BTU furnace on sale online for $500 shipped, and I want to hear some ideas and get a few answers from all you experts. I think it is worth my time and money to do it myself (I know I will lose A/C, I don't mind).

Here is my plan for the install:
  • Remove old unit
  • Cinder block foundation for new unit
  • Run 110/220 electrical if it isn't already there
  • Plumb in intake to furnace
  • Plumb in output to house
  • Build "doghouse" for furnace with flu/chimney vent for waste heat and vents
  • Hook up old controls to new unit
  • Plumb natural gas to new unit
some other information I am going to look up is weather or not this can meet local code, and what my limitations are for the outdoor cabinet.

My questions:
Is this a realistic project for a DIYer with plumbing and electrical and home structural experience? I have a lot of experience in automotive and other fields as well. I am a mechanical engineer. I haven't done much HVAC before though.

How likely is it that the old thermostat can control the new unit? It is a simple one with just a temperature slider for heat, a temp slider for AC, and a selector (off, fan, heat, AC, em. heat). It is of course the old style (mercury contact).

I want to have the structure mostly built before I get on site (it will be small), so I can spend less time on the site. I plan on doing this whole install in two or three days. Is that possible? I do work fast and work long days.

I'm sure I will have more questions later. but this should get a good discussion started. Thanks in advance for the help everyone!

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Old 10-09-2009, 06:03 PM   #2
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Installing natural gas furnace in place of heat pump, please help


Building permit for structure.
Electrical inspection.
Chimney flue must be 2' higher then any wall or roof within 10' horizontal of the top of the chimney.
Building code may require poured concrete foundation for furnace room.

Sounds like your looking to buy a (over sized) furnace that requires more air flow then your current duct work can support. So you may need to redo the duct work already in the house.

Old stat may be a heat pump only stat.
If the lease says A/C included. Most states will require you to install an A/C.

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Old 10-12-2009, 09:34 AM   #3
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Installing natural gas furnace in place of heat pump, please help


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Originally Posted by beenthere View Post
Building permit for structure.
Electrical inspection.
Chimney flue must be 2' higher then any wall or roof within 10' horizontal of the top of the chimney.
Building code may require poured concrete foundation for furnace room.

Sounds like your looking to buy a (over sized) furnace that requires more air flow then your current duct work can support. So you may need to redo the duct work already in the house.

Old stat may be a heat pump only stat.
If the lease says A/C included. Most states will require you to install an A/C.
I can pour a foundation, I hoe the code doesn't require it though...

Good to know on the flu, I'll make sure and verify what the code is on that.

The old ducting was to support a 2.5 ton unit, this 3 ton will run intermittently, so It should be okay. I'll be sure to check the MFR recommendations first.

I have learned that honeywell makes a wireless stat, so that is a quick and easy solution. I will probably go that route so I don't have to worry about wires. Time saver.

I will be ordering the equipment, building permits, and probably having a contractor put a "T" in the gas line today. I am going to have a contractor do it to save me a little more time.

Thanks for the help so far.

Lucky me, the lease I wrote doesn't mention A/C.
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Old 10-12-2009, 11:28 AM   #4
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Installing natural gas furnace in place of heat pump, please help


Doubtful the old duct for the 2.5 ton was proper size.
So your 3 ton will probably never deliver 3 tons of cooling. And may be damaged by flood backs.

At 110,000BTU input 80% efficient. You need to move 1160CFM just to stay down to 70 temp rise of the heat exchanger.

Supposedly. A 2.5 ton duct system would be designed for only 1000 CFM. 1000 CFM on that same furnace would be a yemp rise of over 81F. his causes lots of troubles. Such as failed limits, damaged heat exchangers. Lots of down time with no heat.

A furnace that size should be moving around 1300 CFM.

When its installed on a duct system that is only sized for 1000CFM. It will be a gas hog.
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Old 10-12-2009, 03:35 PM   #5
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Installing natural gas furnace in place of heat pump, please help


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Originally Posted by beenthere View Post
Doubtful the old duct for the 2.5 ton was proper size.
So your 3 ton will probably never deliver 3 tons of cooling. And may be damaged by flood backs.

At 110,000BTU input 80% efficient. You need to move 1160CFM just to stay down to 70 temp rise of the heat exchanger.

Supposedly. A 2.5 ton duct system would be designed for only 1000 CFM. 1000 CFM on that same furnace would be a yemp rise of over 81F. his causes lots of troubles. Such as failed limits, damaged heat exchangers. Lots of down time with no heat.

A furnace that size should be moving around 1300 CFM.

When its installed on a duct system that is only sized for 1000CFM. It will be a gas hog.

I do not have dimensions on the ducting yet, but I would like to know how most contractors decide what size/geometry to go with. I am capable in the field of fluid mechanics, so I could calculate the pressure loss and flowrate for a given pump in a system if I took enough physical measurments on the ducting, but that will take a long time. Is there guidlines that contractors follow for duct layout based on minimum cross sectional area per CFM or anything like that?

I would prefer to install this furnace into a compatible duct system.

EDIT:

UPDATE: I applied for a mechanical permit, described my project to the comittee, and also ordered the furnace, a sheet metal duct adapter kit, and a wireless thermostat. Supposed to be delivered friday. Hope it isn't late : )

Last edited by KlamathMog; 10-12-2009 at 03:43 PM.
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Old 10-12-2009, 04:06 PM   #6
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Installing natural gas furnace in place of heat pump, please help


Most furnaces will have a External Static Pressure rating of .5"wc(Water Column).
After deducting supply and return grille pressure drops, along with air filter(standard cheap air filter) pressure drop. If you have no A/C coil. You have an Available Static Pressure of maybe .37"wc.



Your duct system should be designed to work within that amount of ASP.

If you ever intend to install A/C. Then you also have to allow for the coils pressure drop in the future.

The easiest way to determine what your current duct system will handle in air flow.
Is to run the blower. Measure its static pressure. And compare it to the units fan data chart.

Then use(new CFM/old CFM)squared) X old Static pressure.
This will give you what your new static will be at the higher CFM.

Cross section won't tell you the operating static pressure. because you won't know what resistance the fittings ells or grilles are.
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Old 10-12-2009, 04:21 PM   #7
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Installing natural gas furnace in place of heat pump, please help


Quote:
Originally Posted by beenthere View Post
Most furnaces will have a External Static Pressure rating of .5"wc(Water Column).
After deducting supply and return grille pressure drops, along with air filter(standard cheap air filter) pressure drop. If you have no A/C coil. You have an Available Static Pressure of maybe .37"wc.



Your duct system should be designed to work within that amount of ASP.

If you ever intend to install A/C. Then you also have to allow for the coils pressure drop in the future.

The easiest way to determine what your current duct system will handle in air flow.
Is to run the blower. Measure its static pressure. And compare it to the units fan data chart.

Then use(new CFM/old CFM)squared) X old Static pressure.
This will give you what your new static will be at the higher CFM.

Cross section won't tell you the operating static pressure. because you won't know what resistance the fittings ells or grilles are.

Awesome! Thanks for the info on that. The emperical method sounds like a better approach.

I alco checked oregon mechanical code. Supply and return ducts need to be a minimum of 2 in^2 for every 1000 BTU/h of heater output capacity for warm air blowers. For heat pumps they require 6 in^2. If the old system was to code (yeah, right....) it would have a 315 in^2 duct, and the new one requires a 176 in^2 duct. The old duct would have to be a 10 inch radius or about 1 foot by two feet to accomodate that cross section though. There is no way the ducts are that huge though.
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Old 10-12-2009, 05:10 PM   #8
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Installing natural gas furnace in place of heat pump, please help


While a 8"X22"(176 sq in cross) would move 1300 CFM at 1063FPM.
It would also have a friction rate of .15" per 100' of total equivalent length of supply duct.

While many guess. Or use arbitrary friction rates to size duct work(which is why so many are under sized).

The proper way is to determine total equivalent length.
And then determine ASP.
Then use ASP*100/total equivalent length.

EG: ASP.25*100=25/TEL365=.068 friction rate.
Then a ductulator would be used to determine size of all ducts from that point.

This is what is suppose to be done on new installs.

On an existing installation. Its easier to measure current static. And use the static pressure formula to determine if the duct work needs alteration for the new system.


PS" Total Equivalent Length is teh length of the longest supply run, and return run added together. Plus the equivalent length of all duct fittings in those runs.
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Old 10-13-2009, 06:22 AM   #9
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Installing natural gas furnace in place of heat pump, please help


Not trying to be short with you but why does everyone think a higher BTU rating will heat the house better??? And why does everyone think a larger A/C unit will cool the house better???

Let's think about common sense here, if the existing heater or A/C has been heating and cooling the house for several years why up size it other than if an addition was added.??????

If someone out there has a answer please inform me then we'll both know.
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Old 10-13-2009, 01:09 PM   #10
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Installing natural gas furnace in place of heat pump, please help


Quote:
Originally Posted by SULTINI View Post
Not trying to be short with you but why does everyone think a higher BTU rating will heat the house better??? And why does everyone think a larger A/C unit will cool the house better???

Let's think about common sense here, if the existing heater or A/C has been heating and cooling the house for several years why up size it other than if an addition was added.??????

If someone out there has a answer please inform me then we'll both know.

I think i didn't effectively communicate why I am installing a 110,000 BTU furnace in place of a 70,000 BTU heat pump. I had a heating company rate the house for capacity, and they said at 80% efficiency I would need a little more than a 70,000 BTU unit. Currently on sale online are a 49K and a 110K. I went with the 110K. It is overcapacity, but I got the one with the smaller variable dspeed blower, so it should be the closest match to what I would order if price were no option.

Thanks again for explaining in detail why contractors do what they do and what the best practace is. Where can a person buy a static pressure gauge or measurement device? I sure don't own one yet
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Old 10-13-2009, 03:12 PM   #11
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Installing natural gas furnace in place of heat pump, please help


What brand is was the 70,000 BTU heat pump.
Since no residential heat pump is made in a 70,000 BTU size.

Its possible the strip heat package was 70,000 BTUs. But, that doesn't mean it needed that much heat.

Many companies just put in the largest heat package they can.

Doing your own heat load calc. Could save you lots of money, and headache.
HVAC Calc
The 49 bucks it cost, could save you hundreds in the first year alone.
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Old 10-13-2009, 06:53 PM   #12
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Installing natural gas furnace in place of heat pump, please help


Quote:
Originally Posted by beenthere View Post
What brand is was the 70,000 BTU heat pump.
Since no residential heat pump is made in a 70,000 BTU size.

Its possible the strip heat package was 70,000 BTUs. But, that doesn't mean it needed that much heat.

Many companies just put in the largest heat package they can.

Doing your own heat load calc. Could save you lots of money, and headache.
HVAC Calc
The 49 bucks it cost, could save you hundreds in the first year alone.

The old pump was from the 70s... and the company that quoted a replacement unit said it was a commercial style rooftop unit. They ran calcs and got 56,000 required for output based on the house data they gathered.
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Old 10-13-2009, 07:13 PM   #13
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Installing natural gas furnace in place of heat pump, please help


If you need 56,000 BTUs of output. Then a 70,000 BTU input 80% will meet that.

And a 70,000BTU input 90% will exceed that.

Is the old unit by any chance a Carrier?
If so. They may have read the model number incorrectly. Carrier used misleading model numbers on their roof top units.

Either way, 110,000 is TOO big.
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Old 10-14-2009, 01:07 AM   #14
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Installing natural gas furnace in place of heat pump, please help


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If you need 56,000 BTUs of output. Then a 70,000 BTU input 80% will meet that.

And a 70,000BTU input 90% will exceed that.

Is the old unit by any chance a Carrier?
If so. They may have read the model number incorrectly. Carrier used misleading model numbers on their roof top units.

Either way, 110,000 is TOO big.
If I could have found a 70K unit I would have bought one. The place I ordered mine from had a few units on sale, and this was the next size up from a 49K. I kinda wish this issue would have been brought up a little sooner. The unit is supposed to arrive Thursday.

When I spoke with the service rep on the phone (who seemed to know what he was talking about when I asked him some installation questions) he said that the unit would not run at maximum efficiency due to shorter cycle times, but that would be the only drawback. I added that the increased cycling may decrease the life of the product and he said that is also possible, but that he wasn't very concerned about it since it is only about 50% larger than the minimum recommended BTU/h capacity for the application.

At this point I think I have three options:
Return this unit, let the tenants move out, and install the correct one
Modify this one to reduce the heat capacity (not going to do this)
Install the oversize unit and see what happens

Unless there is a reasonable way to tone-down the burners or blower or both, I am going to install it in the factory configuration and hope for the best.

This unit has a variable speed blower as well. I would think that it would adjust to the heating load to run longer rather than run full blast and overheat something or blow at a higher rate than necessary.

It is made by carrier if anyone cares to know.
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Old 10-14-2009, 04:34 AM   #15
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Installing natural gas furnace in place of heat pump, please help


Quote:
Originally Posted by KlamathMog View Post
I kinda wish this issue would have been brought up a little sooner. The unit is supposed to arrive Thursday.

Sorry. You didn't post it a couple days sooner then you did. Then I might have said something about your jumping in feet first without looking, a little sooner.

When I spoke with the service rep on the phone (who seemed to know what he was talking about when I asked him some installation questions) he said that the unit would not run at maximum efficiency due to shorter cycle times, but that would be the only drawback. I added that the increased cycling may decrease the life of the product and he said that is also possible, but that he wasn't very concerned about it since it is only about 50% larger than the minimum recommended BTU/h capacity for the application.

Its his job to sell. Not to say look some place else for the proper size.

At this point I think I have three options:
Return this unit, let the tenants move out, and install the correct one
Modify this one to reduce the heat capacity (not going to do this)
Install the oversize unit and see what happens

Unless there is a reasonable way to tone-down the burners or blower or both, I am going to install it in the factory configuration and hope for the best.

This unit has a variable speed blower as well. I would think that it would adjust to the heating load to run longer rather than run full blast and overheat something or blow at a higher rate than necessary.

It is made by carrier if anyone cares to know.
VS blowers, tend to get real loud on under sized duct systems.
How loud, varies with how under sized.

Along with noise. An under sized duct system also tends to decrease the life span of VS blowers. if its under sized too much.

Doubt the guy you talked to mentioned this either.

Since it has a VS blower, its a 2 stage unit then.(as I recall, Carrier doesn't have a 110,000 input furnace, could be mistaken though). Hmmm.
So you can wire it to just use first stage. This will be 65% of second stage firing rate. It won't be as efficient for the year. Nor provide the comfort 2 stage is suppose to. But, it will heat.

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