Posted By Don Lloyd on 21 Apr 2010 01:20 PM
... my W to A system delivers heat at 165 F ...

Time to fix your thermometer?

Leaving air temp (at the unit) should be somewhere in the neighborhood of
20°F above the thermostat setting, and 25°F-ish above Entering air temp.

For example, my 3-ton WF NDV038 is spec'd to deliver 93°F to 97°F LAT in
stage2, with 70°F EAT, 40°F EWT. That implies a tstat setting of about 72°F.

...it's a space heater, not a popcorn popper,

Looby

I wonder if Don L is referring to the temperature right at the compressor discharge where the high pressure gas has a relatively large amount of superheat.

It is a bit of a subtlety, but it is important to understand the nature of heat transferred by refrigerant. The amount of heat available at such high temperature is very low - on the order of 5-10 % of the total system capacity. An especially useful place to put that small amount of relatively high grade heat is into domestic hot water via a DSH coil.

Specific heat of superheated refrigerant gas is quite low, which is why it takes only a few tenths GPM water flow per system ton through a small DSH coil to drop that hot gas by 30-50 degrees. The bulk of the remaining heat, ~90%, is transferred to house air (or radiant water) by the condensation of the refrigerant gas at a much lower temp, around 100-120. A bit more (~5%) comes from subcooling the liquid refrigerant a few degrees below saturation.

The fact that refrigerant emerges from the compressor at such a high temperature relative to saturation temperature is a function of several effects in more or less chronological order:

1) Superheat originating in the evaporator (low side)

2) relatively low specific heat of gas (small heat input = large temperature increase)

3) cooling the compressor's internals (compressors are cooled by refrigerant flowing through them)

4) heat of compression

I hope this is correct, clear, and helpful.

I could be wrong, but I would think that if you had a water to water heating water to 100F or water to air heating an air coil to 100F the COP should be very similar.

I have always assumed, as pointed out earlier, that in most water to water systems the output side is much warmer than what is typical in a water to air system.

I have a water to water and in my case I am only heating pool water to 90F so my water output tops out at 100F.

I concur - if a radiant system is configured to use 90-100 degree water it should offer COP similar to air

Yep, the whole thing seems explainable with the different load temps. I watched my source delta today after I had the unit off for a while, the buffer tank cooled down, when I turned it on my source delta was 5.7 degrees at a load EWT of about 80F. It went down to 3.9 when the when the load EWT was 120F. The amp drawn and the flow where the same, however the heat extracted where significant less at higher load temps. Thus a much lower COP at higher load temps.

I'm quite suprised to read that the amp draw remains constant as load EWT rises. I would expect such a rise to lead to higher condensing temps which means higher condensing pressures which means higher compressor amps.

Heat pump COPs drop as delta T between source and load rises. This effect is due to both increased power use and decreased capacity

Well you experts have a lot to say, but if you have answered the question I am too dumb to know it. Why does Energy Star have higher W to A COPs than W to W as their minimum efficiencies to meet the partnership agreement?

Posted By Don Lloyd on 26 Apr 2010 09:27 AM
Well you experts have a lot to say, but if you have answered the question I am too dumb to know it. Why does Energy Star have higher W to A COPs than W to W as their minimum efficiencies to meet the partnership agreement?

EnergyStar works by being better than x% of your peer group. It's something of a moving target so that what passes one year may not the next. The idea is that it will keep ratcheting up the standard. However, it leads to strange things like absolute standards losing meaning.

Engineer - in my (limited) experience you are correct - compressor power is strongly dependent on hydronic side temp. In addition the output drops so COP decreases rapidly with increasing temp. I'm attaching a plot of power vs hydronic temp and ground loop temp (color) from a couple of years ago.

I think the original question has been answered - the primary reason is that w-w heat pumps are rated at 104F EWT which is probably 112-116F LWT in most cases. W-A pumps are rated at 68F return air temp (I think) which translates to significantly lower condensing temps.

A secondary reason is that as tinoue stated the ratings are relative and w-w have traditionally been a pretty small part of the market - I would assume most R&D and optimization goes into the w-a lines.

104F EWT is high for some systems, but even so, this would argue that users of W-W heat pumps would get higher efficiencies if they added a water-air heat exchanger on the return path from the radiators to extract more heat (ie, a hybrid system). Might be practical if ducts are already in place. Or use heat pumps designed for air and water output.

Such an arrangement has three shortcomings which spring immediately to mind:

1) Extra power - running both pump(s) to circulate water and blower to circulate air

2) Lukewarm air - return water would likely be able to heat air by 10-15 degrees. Tepid air is already a problem with some heat pumps. Air so slightly heated and on the move would feel positively chilly

3) Added complexity and expense for installation and control.

I don't believe that the air and water heat pumps out there can heat both at the same time, likely for the reasons above.

Engineer, Well, went back and measured again today. Amps were constant between 90 degree and 120 degree load, however heat extracted went down by about 25%. Yes, higher condensing temps and pressure necessary at higher load temps, but I always thought that the compressor needs to run longer to build up higher pressure, and since the power supplied (or drawn) is constant, meaning decreased capacity per hour as meassured in BTU/hour. I admit I might be missing something here....

Not sure where you are measuring current / power, but am quite certain that compressor amps vary widely relative to discharge pressure, which in turn depends on high side condensing conditions.

Power supplied / drawn is NOT constant. Compressor builds required pressure fairly quickly during the first minute or so of a run cycle. Compressor pushes hot gas into condenser, and pressure builds fast until high enough that refrigerant condensation begins (when saturation pressure / temp exceed whatever is going on on the other side of condenser - air or water flow drawing heat off condenser) Refrigerant condensation makes room for continued refrigerant addition to the condenser. Steady state occurs when refrigerant condensation rate matches rate of gas input to the condenser from the compressor, typically 10-20 degrees above other side of condenser. Steady state conditions continue until control mechanism (thermostat or aquastat) decides enough is enough and ends the call for heat or cool. Compressor stops and system returns to off state equilibrium, depending on whether metering device bleeds.

I hope this makes sense and helps out.

Or more precisely, power draw changes with the difference between high and low side pressures and the flow rate.

Posted By docjenser on 25 Apr 2010 11:25 PM
Yep, the whole thing seems explainable with the different load temps. I watched my source delta today after I had the unit off for a while, the buffer tank cooled down, when I turned it on my source delta was 5.7 degrees at a load EWT of about 80F. It went down to 3.9 when the when the load EWT was 120F. The amp drawn and the flow where the same, however the heat extracted where significant less at higher load temps. Thus a much lower COP at higher load temps.

I concur that higher press/temps and lower Delta Ts diminish COP and that is as close to a short answer as we can give OP.

I maintain, we are trying to heat air which is going to make the water to AIR system most suitable for the job (by design). I think it couldn't be more obvious with or without the math! If we were trying to heat water, I'm going to guess that the water to WATER system will do that more efficiently:)
j

It's seems obvious to me that we are trying to heat people and that only involves heating air to some extent - it also involves heating surfaces.

Curiously in the heating world the appliances only try to heat thermostats. They are indifferent to people and other surfaces.

Posted By Don Lloyd on 18 Apr 2010 11:17 AM
Here is a question for you experts. We know that an open loop system is more efficient than a closed loop because water is a more efficient conductor than dirt. So why is a water to Air system more efficient than a water to water system?

This is a misleading statement.  Technically an open loop is no more efficient than a closed loop, its the generally higher entering water temperatures (EWT) that gives an Open Loop a higher COP than a close loop system.  An open loop system is pulling water from a aquifer that has millions of gallons of water in it, instead of a few hundred in a closed loop situation.  The aquifer is a lot less likely to be affected by geothermal system dumping heat into it.  Every house and business in town would have to be geothermal to have any impact to the overall aquifer temperature.  Closed loops also have an advantage at the beginning of the heating season, since the ground is somewhat warmer than the aquifer water after a cooling season, but they quickly lose that edge once the heating season begins. The higher power requirements for the open loop system's pump cost more to operate the open loop than a close loop, but the generally higher COP more than makes up for it. Also Open loop installations are somewhat cheaper to install.