Why are evaps located upstream from heat strips?

[tipsrfine]

Or does it not even have anything to do with the heat strips, but rather has to do with the blower? I notice that on downflow all electric air handlers with built in evap coils, the evap coil is located on top on downflow units. Is this simply to keep the blower from blowing condensate off the coil?

[gregp]

heat pump application it is so the heat generated from the electric heat strips does add heat to the coil and increase the refrigerant pressure.

[Kevin O'Neill]

Good answer. In fact, the correct answer. Besides increasing the discharge pressure in heat mode, it lowers efficiency too.

[coilcleaner]

I would think on an upflow straight ac with heatstrips it would prevent water from leaking on the strips in summertime which in turn would damage the strips.

[supertek65]

possible but heat strips are made of nickel and chrome wit a lil silicon.
not sure how mush the water would hurt them but it may???????

I would think on an upflow straight ac with heatstrips it would prevent water from leaking on the strips in summertime which in turn would damage the strips.

[coilcleaner]

Think of this someones coil freazes h.o. turns on electic heat, coil starts defrosting raining on heatstrips.

[mgenius33]

Think of this someones coil freazes h.o. turns on electic heat, coil starts defrosting raining on heatstrips.

Happens all the time with downflow applications.
We do have a slight improvement with the Trane's Hyperion. The blower is located above the coil which also benefits by having positive pressure instead of negative pressure coil.
The downflow situation plagues many high humidity areas. It's critical to have a proper charge, clean coil, and unit pitch to provide the best drainage.
Whenever possible, we try to convert those units into upflow by ducting alteration.

[gregp]

Happens all the time with downflow applications.
We do have a slight improvement with the Trane's Hyperion. The blower is located above the coil which also benefits by having positive pressure instead of negative pressure coil.The downflow situation plagues many high humidity areas. It's critical to have a proper charge, clean coil, and unit pitch to provide the best drainage.
Whenever possible, we try to convert those units into upflow by ducting alteration.

and the blower is BELOW the coil on the hyperion (upflow application) pushing the air not pulling it through. This also eliminates the need for a trap on the condensate drain.

[John Markl]

As stated earlier, the strips are after the coil for heat pump applications.

In the old days, you could buy an electric furnace, with heat strips, and put a regular cased coil on top.

Then, most distributors went to carrying the heat pump air handler for both HP and electric heat applications to reduce inventory.

The first heat pump I ever installed was a "Heatwave". And the components came seperately. The coil was simply installed ahead of the "furnace".

[coilcleaner]

Yea I'm sure the sole reason was to reduce inventory.

[JPJ13]

We all know the coil is before the heat strips on a heat pump air handler to keep the refrigerant pressure from spiking. However, what do you think would be wrong with the blower first, then the coil, then the heat strips? If your answer is moisture from the coil in the summer, let me remind you that pressure is still pressure, whether negative or positive, the same force can wick water away or force it away.

[John Markl]

We all know the coil is before the heat strips on a heat pump air handler to keep the refrigerant pressure from spiking. However, what do you think would be wrong with the blower first, then the coil, then the heat strips? If your answer is moisture from the coil in the summer, let me remind you that pressure is still pressure, whether negative or positive, the same force can wick water away or force it away.

That's what I've wondered for years.....now Trane and American Standard have done just that.

I'm wondering how many service calls have been generated over the years by dry traps causing sink, bathtub, or washing machine drains to make "thump, thump, thump" sounds....

[BigBacardi]

We all know the coil is before the heat strips on a heat pump air handler to keep the refrigerant pressure from spiking. However, what do you think would be wrong with the blower first, then the coil, then the heat strips? If your answer is moisture from the coil in the summer, let me remind you that pressure is still pressure, whether negative or positive, the same force can wick water away or force it away.

Personal pet peeve of mine.........

negative pressure??????

I freakin hate that term......negative pressure does not exist...oxymoron...

[John Markl]

Personal pet peeve of mine.........

negative pressure??????

I freakin hate that term......negative pressure does not exist...oxymoron...

I've always wondered why those little round things with the hose to the inducer on a gas furnace were called "pressure switches"....

[supertek65]

negative pressure does exist!
but not in an evaporator or a trap!

Negative pressures

While pressures are, in general, positive, there are several situations in which negative pressures may be encountered; When dealing in relative (gauge) pressures. For instance, an absolute pressure of 80 kPa may be described as a gauge pressure of −21 kPa (i.e., 21 kPa below an atmospheric pressure of 101 kPa). When attractive forces (e.g., van der Waals forces) between the particles of a fluid exceed repulsive forces. Such scenarios are generally unstable since the particles will move closer together until repulsive forces balance attractive forces. Negative pressure exists in the transpiration pull of plants, and is used to suction water even higher than the ten meters that it rises in a pure vacuum/ The Casimir effect can create a small attractive force due to interactions with vacuum energy; this force is sometimes termed "vacuum pressure" (not to be confused with the negative gauge pressure of a vacuum). Depending on how the orientation of a surface is chosen, the same distribution of forces may be described either as a positive pressure along one surface normal, or as a negative pressure acting along the opposite surface normal. In the cosmological constant.

[Samshawn]

Isn't the heatstrips used to control the humidity in the ducts too?

[Kevin O'Neill]

Isn't the heatstrips used to control the humidity in the ducts too?

No.

The humidity coming off the cooling coil will be near 100%. Running the strips with the cooling will ruin the efficiency and capacity in cooling. Strips are used only in heat and defrost modes.

[Samshawn]

Thank you Kevin O'Neill for the information.

[gregp]

Isn't the heatstrips used to control the humidity in the ducts too?

some applications where the customer runs the a/c to de-humidify a space and runs the electric heat strip(s) so the space does not get too cold. These are usually in labs, etc. where they are not concerned about energy consumption, only controlling the climate in the space. But,as was stated, the heating elelments themselves do not control humidity.

[mark beiser]

some applications where the customer runs the a/c to de-humidify a space and runs the electric heat strip(s) so the space does not get too cold. These are usually in labs, etc. where they are not concerned about energy consumption, only controlling the climate in the space.

I've set up a number of systems like that for applications where temperature and humidity control were the only concern, and energy efficiency was not a consideration.