How low can you (I) go? Water Flow that is

Hi All:

What an Awesome forum, Thanks!

Just finished a DIY open loop surface discharge geothermal here in New Brunswick, Canada. Running Single stage 3-ton McQuay Enfinity GSHP. The specs are as follows:

Flow: 6.7gpm
EWT: 50F (as measured at tap with thermometer)
LWT: 41.2
EAT: 70F
THA: 27,200 BTUH
Q: 34,668 BTUH

My question is with this open loop how much lower can I go with the flow? The hose kit from the Manufacturer came prebalanced at 6.7gpm (the minimum flow listed in their performance tables) with the THA (total heat absorbed) and all the other numbers listed above coming direct from their spec sheets.

Since Q = 500*gpm*DeltaT (approximately of course for water at ambient conditions)

If I lower GPM then DeltaT will increase which will result in a roughly the same THA by the water (it does go up slightly as flow increases). Soooo the question is, how low can I go with the flow on this HP before I start to experience issues. I was thinking of trying to balance down to 5gpm which would get me near freezing and minimize water usage. I am assuming though that as I lower flow since the Delta T must increase this must mean that the compressor must work harder. Any thoughts/technical experience or formulas? I have read numbers in the range of 1.5 gpm per ton for open loop? Is the only issue needing to stay away from freezing leaving water temperatures and let the compressor deal with the rest, or does this put increased load on the compressor, ultimately lowering COP and possibly compressor useful life?

I am an engineer that works on Commercial HVAC design and on a daily basis don't really get into stuff this hands on/technical, however I really want to learn about and tweak my system to get it running at peak.

I know there are well pump energy issues which I must consider (VFD etc...), however I will address these in later posts.

Thanks,
Glen

 

Surface discharge

As you noted, it is the surface discharge in your location that will limit how low you can go. You don't want to be anywhere near freezing.

Otherwise, you'll risk freeze up within the equipment itself if you start lowering flow too much.

1.5 gpm/tonne is the number, but you'll be likely increasing that due to the above.

 

Ok Thanks Chris! I am right in assuming then that the lower the flow the harder the compressor must work, thereby increasing energy consumption and ultimately minimizing compressor lifespan?

Thanks again

 

Ok I have done a bunch more thinking on this one and the manufacturer tables have listed flows of 6.7, 9.6 and 12.5 gpm. They don't go any less than 6.7 gpm (for their 3-ton units). So again how do I know that I can get away with 4.5 gpm or lower short of calling the manufacturer? I did a lot of math tonight trying to figure out the relationships in the manufacturers tables between THA, COP, and Total Heating.

The COP decreases slightly as flow decreases, however if I assume my 50F EWT and wanting to avoid freezing so say 32F, then my min flow is actually 3.3 gpm as below. There must be a point though where this reality/compressor/refrigerant physics overrules the theoretical 500gpmDeltaT formula?

Q=500gpmDeltaT
29,500 = 500(gpm)(50-32)
GPM = 3.3

Again any input is great, just trying to understand things. I obviously want to run as low a water flow as feasible since I am running open loop, however I would like to know more than a 'rule of thumb'. I guess I can always try it at a lower flow and see what happens? I assume there will be a safety switch that will trip at some point when the entering water flow and temp combine to result in a refrigerant pressure issue? Keeping in mind this is a 'commercial grade' unit with such built in safeties.

Thanks!

 

I don't think anybody will recommend you lower flow below manufacturer specifications. For pure number crunching, you can look at heat pump efficiencies, in particular the Carnot cycle. But a commercial unit is generally a lower quality heat pump, and you'd best be within manufacturer specifications.

Commercial units, while having high and low refrigeration lockouts may not have indicators specifying the cause. Also, they may not have freeze protection lockouts. Lockouts are at the extremes of operating parameters, and one does not want the wear and tear on the equipment from operating near them.

 

Usually the manufacturers specify 1.5 gpm/ton flow at EWT above 50F, and 2.0 gpm/ton at 50F or below. Cut off usually is the low pressure switch corresponding with a LWT of 32F (some manufacturers at 30F). The reason for that is that higher flow increases the LWT, so they ususally like to keep it above 40F if not freeze protected. Now the discharge pipe exposed to outside below freezing temperature is not the main concern, the coldest part is usually the end of the source heat exchanger. Refrigerant can be still very cold, and if water flow stops there is the chance that the heat exchanger will freeze. They like a few degrees for safety margin. The tables are not really the operational limits of the heatpumps, obviously they have no problem with LWTs down to and below 25F in closed loop systems (they are rated at 32F EWT, which usually reflects 27F LWT). It is really more about protecting the coil from freezing.

 

Thanks to you both. Right now I have been running 5gpm (+/- 5%) with my pressure independant balancing valve for about 5 days with no percievable issues with the heat pump. My temps measured are 50 EWT and 39 LWT at the outdoor discharge (haven't figured out a cheap/easy way to install thermometers in my 3/4 Pex without multiple fittings) with my pool thermometer.

Since I have measured the 39F LWT at the discharge if I do the calcs for Total Heat Absorbed by the water Q=500gpmDeltaT as above it would seem to me like I could put in the 4.5 gpm orifice without dropping the LWT too low.

I assumed that the manufacturers tables were there at rated test conditions and that the high and low flows in the tables aren't necessarily the upper and lower limits.

As for the general lower quality of the commercial units, is there a way I can verify the quality quantitatively (althought it is kind of too little too late at this point). Can I get the model number off the compressor, coil manufacturer, HX make/model, would this tell me anything about quality, or is the quality statement more of an experience based subjective thing?

Thanks again guys, loving the forum and expertise, hoping my posts are helpful/useful to others.

 

Again it's not just GPM but demand and EWT all mixed in. There might be cases where less than 1 GPM would do but......
Climatemasters communicating systems now have modulating valves so we can dial in Delta T.

 

Why do you want to lower the water flow?

Mark

 

Don't change a winning team. You start to touch margins you should not touch. A delta T of 11F and a 39F LWT is something I would not expand on....

 

Any number of reasons:
Reduced well demand
Reduced pump size/flow
Reduced water usage

To me it just makes sense if I can heat/cool with less water then why not, obviously within reason. 1 gpm may 'work' but also doesn't really make a lot of sense as mentioned AMI.

I am a tinkerer/tuner (vehicles, small engines and anything mechanical for that matter) and by times to my demise, but for this one I think I am satisfied with the 5 gpm and the resulting Delta T and COP based on extrapolation from manufacturer tables.

 

Excellent advise, Thanks Doc.

With the 39F LWT it gives me some room for the ground water temp to drop a couple of degrees over the winter and still be away from that freezing point.

 

Keep in mind that the water leaves the heat exchanger with 39F, but the walls of the heat exchanger can be "charged" with 0 degrees refrigerant, so there is the possibility of ice build up in the heat exchanger at LWT above freezing.
I assume you have a slow closing water control valve, and the compressor shuts off way before the valve closes. I have seen coils freeze shut within seconds....

 

Again thanks for the info Doc. I do have a slow close solenoid and the MicroTech controller that comes with built in controls to make sure the water valve closes long after everything else is shut down.

 

Certainly 1.5 GPM isn't crazy, but don't forget, less is less. IOW less GPM brings less BTUs. You actually get higher COP with higher GPM (of course you also get higher pumping cost). So there is a trade off.

 

That is what I have been confirming with my calcs, and I think that where I am right now is the 'sweet spot'. I am done tuning for a month or two now and see what the utility bills look like when they show up.

I am a fidget and always tweaking though so I may revive this one again but I don't plan on it.....

Thanks again to all

 

I am not sure that you will see the difference in the few percentage points of the COP on your electricity bill accumulated over the period of a month, especially when the well pump is on the same electric bill. In the range we are talking about the difference is too small.

 

Agreed. At this point I am just trying to find a baseline for a couple of months on the overall system as I haven't been up and running for a month yet while actually leaving things fixed for a bit instead of forever adjusting things.

 

I want to go the other way. I have a flowing well and would want to push as much water as I could to increase cop. I was told to keep it around 5 gpm. I can do more easily, but haven't. I need a new unit now after 25 years due to Freon leak.

 

Soon or later you pay more for pumping power to push the water through the heatpump, than the higher COP gains you. The is specially true for open systems, where you have to overcome the static pressure of pumping the water up the well.