Evaporator sizing - is bigger better?

[AC_Hacker]

Originally Posted by jeff5may

This is in reference to the indoor coil in an air handler. With a heat pump, oversizing the indoor coil too much will kill your supply air temperature. Instead of 120 degF air coming out of registers, you will have 110 degF air, which feels "not so hot". Efficiency may rise a smidgen, but the overall "warmth" provided may be seen as diminished by the average user. Not a good thing to most.

This is very curious...

If your logic is correct, you have just defeated heat pump heating, and all other low temperature heating strategies.

Because heat from an oil fired heating duct can be uncomfortably warm, but does that mean that a heat pump heated house with the same inside air temperature would feel less warm, therefore less satisfying than oil heat?

If you wanted to actually enjoy the efficiency gain of a larger condenser, you would need to increase the volume of air through it, to get those BTUs into the room.

Or, if radiant floor was your heating method, you would need to increase floor efficiency through lower R-value materials above the heated part of the floor structure, and/or increased pipe density and/or increased water flow rates.

-AC

-AC

[Servicetech]

Originally Posted by AC_Hacker

This is very curious...

If your logic is correct, you have just defeated heat pump heating, and all other low temperature heating strategies.

Because heat from an oil fired heating duct can be uncomfortably warm, but does that mean that a heat pump heated house with the same inside air temperature would feel less warm, therefore less satisfying than oil heat?

If you wanted to actually enjoy the efficiency gain of a larger condenser, you would need to increase the volume of air through it, to get those BTUs into the room.

Or, if radiant floor was your heating method, you would need to increase floor efficiency through lower R-value materials above the heated part of the floor structure, and/or increased pipe density and/or increased water flow rates.

-AC

-AC

More indoor coil surface area is great for heat pumps in heating mode. Lower coil bypass factor will mean lower head pressure with the same discharge air tempatures. Airflow does not need to be increased to make it happen, although higher airflow rates would lead to further reductions in head pressure at the expense of lower temp at the register.

Register temp 100-110f is warm enough to reduce the "cold draft effect" but cool enough for efficient operation. This is where variable speed blowers help, no "cold blast" when the blower first kicks on.

[cobra2411]

Originally Posted by AC_Hacker

This is very curious...

If your logic is correct, you have just defeated heat pump heating, and all other low temperature heating strategies.

Because heat from an oil fired heating duct can be uncomfortably warm, but does that mean that a heat pump heated house with the same inside air temperature would feel less warm, therefore less satisfying than oil heat?

If you wanted to actually enjoy the efficiency gain of a larger condenser, you would need to increase the volume of air through it, to get those BTUs into the room.

Or, if radiant floor was your heating method, you would need to increase floor efficiency through lower R-value materials above the heated part of the floor structure, and/or increased pipe density and/or increased water flow rates.

-AC

-AC

With a heat pump the temperature rise over the coil is not as great as with a fossil fuel heater so you have to be careful about the airflow. Too fast and you don't absorb the heat and you throw cold air from the ducts and too slow and you saturate the air and needlessly wait around. The bigger surface area allows you to transfer more heat, faster.

Now, at the registers you have to find the proper balance. Too fast and you get the cold draft feel but too slow and you don't heat the room properly. Around 600 FPM is idea, give or take 50-100fpm. This gives you enough mass to get the heat into the room but it's slow enough that in the occupied zone you won't feel a draft. This is a function of the duct sizing and also the way it hooks to the register will affect how the air is distributed.

The ideal system is one that you can't tell if it's on or off.

Now with oil or gas heat you have plenty of temperature rise over the heat exchanger 40-60°f in most cases vs as low as 15°f with a heat pump. 60° return air + 60° temperature rise = 120° out of the heat exchanger that will be 110°+ out of the vents which will be warm enough that it will feel like warm air hitting you. Air velocities don't matter and most people will like a higher velocity that they will feel hitting them.

I haven't designed a ton of systems but I've done them both ways and in the end the house is comfortable. Coming from a forced air gas heater I'm used to feeling the blast of warm air coming out and it takes some getting used to when I'm in one of my rentals with a heat pump. The house is warm but you don't feel the airflow.

I believe there is some benefit to a slightly larger evap coil but I don't see any benefit to grossly over sizing them. I also believe the best benefit is for the air conditioning side in humid environments. The units I install that have fixed orifices typically have sizing for a 1/2 ton larger A-coil and also include the orifice for it so you get the proper superheat out of them. They typically do not have sizing listed for anything past 1/2 larger - i.e. they have the direct match and a 1/2 ton larger A-coil.

I typically install 2-3.5 ton systems and with conventional split systems the range is 1.5 tons to 5 tons, I can't speak for mini-split sizing.

[jeff5may]

AC and SERVICETECH,

Yee haw! You're both right.

Because heat from an oil fired heating duct can be uncomfortably warm, but does that mean that a heat pump heated house with the same inside air temperature would feel less warm, therefore less satisfying than oil heat?

Most people who have had oil or gas blast furnaces in the past tend to expect that "shot" of hot air from the heater when it first starts up. This is one of the big reasons heat pumps took a long time to catch on, especially in Northern latitudes. As with the whole short cycling fallacy, they perceive a long cycle time of less "hot" air as inefficient and costing more in the end. We know this idea is backwards as a left hand thread, but you will never convince Joe Dirt.


More indoor coil surface area is great for heat pumps in heating mode. Lower coil bypass factor will mean lower head pressure with the same discharge air tempatures. Airflow does not need to be increased to make it happen, although higher airflow rates would lead to further reductions in head pressure at the expense of lower temp at the register.

Register temp 100-110f is warm enough to reduce the "cold draft effect" but cool enough for efficient operation.

The most important thing here is delaying the blower startup until the indoor coil has time to preheat. If the blower is not delayed, and starts up with the outdoor unit, the heat coming out of the registers will take time to develop. This goes against the previously mentioned fallacy. Joe Dirt will complain about it.

Oversizing the indoor coil will prolong this lag, more so if the airflow is increased. Even though the unit may gain 10-15 percent in COP, the perceived lack of "hotter" air at the registers may become a point of contention with the homeowner. The unit may cost much less to run every month, but other than that, the unit seems not to perform any better than before.

[Servicetech]

Many homeowners tend to flip the switch over to emergency heat when they feel temps below 90f coming from the registers or notice the unit running almost constantly. Variable speed blowers solve a lot of the low discharge temp issues since they can ramp up slowly and keep the outlet temp at optimum level. If they didn't charge such a price premium for them... You could also install a time delay/coil thermostat/head pressure switch on a conventional air handler to help with "cold blast".

[cobra2411]

First $1,000 electric bill will cure them of flipping the emergency heat on...

I haven't installed a HP system without a delay and variable speed blower. Combined with proper duct sizing I haven't had any problems. The only "problem" I ever have is from people with fossil fuel heaters that expect a blast of hot air when the heater comes on. I had one nit-wit I had to deal with on that who claimed the heater was broken yet the house was almost 80f when I got there with outside being in the 30's.

"But I don't feel hot air coming out... I keep upping the thermostat but I don't feel hot air coming out... Why am I sweating?"

Wasn't too bright that one...

[Servicetech]

I get a few of those also, think something is wrong when 120+ air isn't coming from the vents.

The biggest surprise to me is how few contractors install heat pumps in our area. 75% of all-electric homes use straight resistance heat, 25% heat pumps. Many claim "heat pumps don't work in Oklahoma". Then there are other contractors who install Dual Fuel setups to get rebate money, even though with current gas rates they make no sense.

[AC_Hacker]

Originally Posted by jeff5may

...Instead of 120 degF air coming out of registers, you will have 110 degF air, which feels "not so hot". Efficiency may rise a smidgen, but the overall "warmth" provided may be seen as diminished by the average user...

Getting back to your "smidgen" comment...

The formula for max theoretical heat pump efficiency looks like this:

...where all temps are in Kelvin.

So I did a little spreadsheet with a typical date set. I'm more interested in GSHP than ASHP and forced air heating (which in my humble opinion is obsolete), so the data set is typical for GSHP scenarios, but the basic principle still applies.

Then I graphed Max COP (values are %) vs T warm F (AKA: condenser temp) and here is what it looks like:

The first thing to understand is that the efficiency is theoretical, and will never be reached, but it is the theoretical efficiency that lies behind the real world efficiency.

Next to observe is that the efficiency gain from decreasing condenser temp from 120 to 110 is not as "smidgen-ish" as one might guess. Even more important is that further reductions of condenser temperatures enjoy an accelerating efficiency curve.

So how dd we take advantage of the accellerating efficiency curve?

Reduce heating requirements:

• Reduce infiltration
• Increase insulation
• Reduce heated area of the house
• Reduce thermal bridging
• Increase solar window gain
• Reduce window heat loss (high performance windows, reduced window area)
• High performance doors
• Locate all heat ducting inside the heated envelope
• Locate water heater inside the heated envelope

When that is done, lower temperature heating becomes possible.

• Increase area of heating coil or area and efficiency of radiant surface

Then you are moving out of the smidgen area and moving into a more exciting area of increasing effficiency.

Best,

-AC