Temp Rise Question

[MrDodge]

Good Evening,
I've spent a couple of days at my daughters trying to lower her elec. bill. Sealing attic ductwork, more insulation etc. I caught the outdoor temp. @ 47 this morning so I checked the temp rise at the air handler and 18 degree is all it would do.
She has a 2 ton 10 seer unit. I phoned the only HVAC guy I know and he seemed to think its performing as it should. I've done some online research and the output should be 22k btu @ 47 degrees, which on paper is a 24 degree temp rise.
I'm sure there's variables, but would 18 degrees be considered an acceptable temp rise? Any response welcomed, and thanks.

[kangaroogod]

Did you happen to measure the temperature of the large insulated line?

[tbone18]

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[MrDodge]

Kanga, Thanks for your reply. Didn't have a good therm. for the suction line but I'll get it tomorrow and repost. It was hot very hot all you could do to keep your hand on it.

[LKJoel]

22 BTUH at 47F ODT sounds a little optimistic to me, but it may be possible if all is tuned correctly.

I am curious though, how you calculated your expected temp rise without knowing your airflow

Either way, it sounds like it may be possible for a tech to tune up your HP and squeeze a few BTU out of it. It's probably well worth your money for a service call if you can shed some $$ off your hydro bill

Use the contractor locator map to find an Htalk guy, there are lots of guys here that are really good with HPs

[hvacrmedic]

As has been mentioned, you must know the cfm in order to calculate the expected temp rise. If this system has a 3 ton air handler set for 3 tons of airflow (1200 cfm) then the numbers would be pretty close. That being said, if that was the case, then lowering the airflow to 800 cfm would actually reduce the heating capacity and could potentially reduce the efficiency in heating mode as well, and this depends upon several factors. The good news is that it's a 2 ton system, how much electricity could it possibly be using?

ROI should be taken into account before energy saving projects are begun. It's difficult to recoup improvement costs when the heating and cooling expenses were already relatively small. What's the model number of the air handler? Are the ducts running through an unconditioned space? Did you measure the temp rise at the return and supply grilles, or at the air handler inlet and outlet?

[MrDodge]

LK & Medic,
True airflow is unknown, but I did go to emerg. heat w/10k strips and had a 39 degree TR. With that known info I looked up the online heating data which was 22k btu. @ 47 amb. A lil math later I had 24.8 target TR. Unfortunately 18 degrees only means about 16k if my math is right. AH is 2 ton on hi speed. Temp readings were @ filter & 12" off supply plenum.

[ChrisTechMech]

If you had only the electric heat strip on at 10kw = 34,120 btuh. 34,120=1.08xCFMx39. Therefore, CFM = 810 if you got a good accurate TR (approximately). It's probably running at the same speed as when the HP is running, right? Do you know if that's true for a certainty?

[MrDodge]

Chris, Yes confident about the strip TR, & yes fan speed is same strip or HP. Thanks

[jpsmith1cm]

tbone18

This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

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Additional infractions may result in loss of posting privileges.

[MrDodge]

Kanga, I took an accurate temp at a coil today. Suction line was 106 degrees @ 35 degrees ambient with a 13 degree temp rise. If the online data is correct I should have a 20 degree TR even with a 10 seer.
I cleaned the indoor & outdoor coil yesterday. Are these numbers in line for a 10 seer or should I have someone look at it? Thanks in advance.

[kangaroogod]

You have something going on there. Your line temp should be about 30 degrees warmer. I would have someone check the refrigeration circuit.

[MrDodge]

Thanks Kanga, For your timely response. Monday I started this endeavor with temps near the same as today (35). The HP wouldn't keep up (aux. would kick in). Today, after sealing the ducts, 36 bags of cellulose etc. the lil unit is off more than its on. But I'd like it to be a lil closer to spec so I'll have it serviced.
Thanks again