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I might have an oversized hvac system. When the furnace runs it runs for 9 mins and then stays of for 20 mins before kicking on again. The air conditioner on the other hand kicks on every ten min and has a run time of 5-10 and shots down for 5 before kicking back on . A heat loac calc said I needed at 39500 BTU system. I live in a 2001 1614 sq ft home no mods were made. I have a 90k BTU furnace matched up with a 3.5 ton system. I brought this up with the installer but he claims it runs fine. 3 questions
1. Is it possible to increase the heat load to make it run longer
2. Can the speed be ajusted
I really can't afford to replace the equipment further than already done. Thoughts ?
 

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Why would you want to increase the heat load? That will only increase your heating costs. 39000 BTU seems a little low, but I guess if it's a tightly sealed house then that may be true . For the heat, not much you can do . Is it oil fired or gas fired? If it's oil, you might talk to your maintenance guy about reducing the nozzle size. The important thing, is how long does it run on on the coldest day of the year. We aren't in the depths of winter yet and if your furnace is only running for 18 minutes every hour that doesn't seem to oversized to me. What has the a average outside temp been the last month where you live? Funny thing about France's, when they run, they cost me money when the they are off they don't . If you are comfortable then at this point I would worry about so,etching else. My oil boiler is oversize because at the time I was planning an addition and change over to insert domestic hot water. Well the addition never happened and the old oil fired Bock water heater is still kicking so it haven installed e indirect water heater. It may slightly short cycle, but I am still saving 35% in oil consumption over its predecessor, so I am not complaining.

Cooling however can be more of an issue if oversize in that if it does not run long enough, it can't bring the humidity down far enough to be truly comfortable,
 

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As long as the venting system is ok, CO shouldn't be an issue. Short cycling can cause other issues, comfort being one of them (rooms never get evenly warm) and reduced life of the unit it self (condensation and corrosion ).
 

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In a climate like yours, they have to put a bigger furnace to accommodate the a/c.

40000 btu furnace likely to have a blower only for 2 tons of cooling.

Your equipment is grossly oversized.

What you can do is put a thermostat with an adjustable cycle rate or swing, increase the swing/lower the cycle rate.

For a/c, same thing, u may be able to get the blower slowed down to improve dehumidification. You may have to run a dehumidifier.

Really if this is a new system the contractor should rip it out and put in the right size. His negligence will cost you more in energy, reduce comfort. You've been ripped off.
 

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Ok thanks I have talked to installer and he said there is nothing wrong that it is ok . The installer refuses to acknowledge that there is something wrong. The unit is only 7 months old
You pretty much have to be a very informed customer to not get shafted.
I think it's worse in the US than up here.
 
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If the quote didn't mention size in writing, you may be able to do something about it.

Also same goes for if the furnace isn't getting enough airflow and is tripping on limit. The oversized furnace won't run long enough normally to trip, but if the burners cycle on and off from overheating due to small ducts during an extended cycle, that constitutes a problem.

I don't know much about going after a contractor after the fact.

At the end of the day, especially if dealing with a bigger company, the salesperson may not know any more about HVAC systems than a used car salesman knows about the mechanical aspects of cars. But it's worse than that; the furnace and ac must be match to the house and ducts, properly setup/commissioned to work right.

The car a sleazy salesman sells is a complete system as is that was engineered to work together.
 

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so how do I try to convince my installer that my unit is too big to properly control my comfort level
Seven months later, they will do nothing.
They will keep saying it's OK and hope you stop calling.

Try threatening with legal action. But that's as far as you could go because it would cost more in legal fees than paying someone to replace it.

I'm sure this happens a lot and hopefully others read this and get educated before buying.
I am now educated. I was not before buying an over sized unit a few years ago.
 

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Over sized furnaces in Texas are commonly installed today because of the large A/C that is needed. Depending on brand, they may or may not make a smaller furnace with a 4 ton blower.

Unless your area has a code that says the furnace may not be over sized by more then X percent. Not much you can do now at this stage.
 

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I might have an oversized hvac system. When the furnace runs it runs for 9 mins and then stays of for 20 mins before kicking on again. The air conditioner on the other hand kicks on every ten min and has a run time of 5-10 and shots down for 5 before kicking back on . A heat loac calc said I needed at 39500 BTU system. I live in a 2001 1614 sq ft home no mods were made. I have a 90k BTU furnace matched up with a 3.5 ton system. I brought this up with the installer but he claims it runs fine. 3 questions
1. Is it possible to increase the heat load to make it run longer
2. Can the speed be ajusted
I really can't afford to replace the equipment further than already done. Thoughts ?
Am I correct in assuming your unit is in the attic?

Give us some outdoor temperature, relative humidity and want it indoor numbers to work with when you are timing the off and on minutes. Sometimes load calculations, especially those off the net may not be all that great and are a guideline without a brain or common sense.

And believe me when I say there are days when my 2.5 ton AC in a 1,800 sq. ft house would be switched to 3.5 ton in a heartbeat if it could be done with the flip of a switch. An example of those days are when I'd like it 76° indoor and the outdoor is 95° with RH 50% plus or 105° with a RH of 30 %.

Better keep what you have.
 

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And believe me when I say there are days when my 2.5 ton AC in a 1,800 sq. ft house would be switched to 3.5 ton in a heartbeat if it could be done with the flip of a switch. An example of those days are when I'd like it 76° indoor and the outdoor is 95° with RH 50% plus or 105° with a RH of 30 %.
In a climate that experiences a greater temp range, two-stage equipment may be a good idea.

But if the load calc is done to maintain your 76f at 105 outdoor, taking into account the ducts in the attic it should hold. The a/c unit's capacity needs to be derated for the 105f.

I think if anything the calcs over estimate the cooling load a bit; it's a sum of the maximum heat gains when in reality windows aren't getting hit with sun.
 

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A solution would be to get a multi-stage or modulating furnace that would probably operate on low fire all the time down in TX while keeping the larger AC (if indeed you really need that much AC) for the roasting hot summers. But the time to do that would have been when you got the system installed. Maybe they will meet you half-way in swapping it out.
 
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