I am about to lose my head...

taekwondodo

LifeTime Supporter
Apr 26, 2009
419
mas985 said:
It is pretty easy to slow down the flow rate (i.e. add head loss) and see if that helps the situation. If it works, then there are several options to consider.

But the thing I don't understand is why he said this:

, the smaller the plumbing , 1 ½”, the more pressure and the faster the water flows through the heater
Smaller plumbing reduces flow rate not increases it. The velocity of the water is higher in the PVC pipe but not the heater.
I'm with you - I don't agree. And told the tech that when I was on the phone w/him. I'm willing to do the work (and, if I can convince the customer, re-plumb most of the pad) and then call the tech back if it blows up and doesn't work.

Here's a shot I took while I was out there yesterday - don't laugh too hard (and I want to stress... I didn't plumb this):

 

madwil

Well-known member
May 2, 2011
370
lebanon tn
interesting that to much flow reduces heat exchange- so your car radiator at 70 MPH doesn't cool the engine as well as 55 MPH?
the electric booster fans on cars only come on at idle- they never come on at speed on the highway (well, they do with underpowered AC units, but not just the engine running)
Also, this would imply that putting more water on a fire would speed the fire up instead of putting it out... (well, sortof- just demonstrating the illogic of the comment)
 

RobbieH

TFP Expert
LifeTime Supporter
Aug 30, 2010
4,052
Dallas, TX
Madwil, that's right. So some people take their thermostat out on their cars thinking this will improve cooling. But, this allows the water to pass through the radiator too fast and actually makes your engine run warmer!
 

Andrew Sarchus

Active member
Apr 11, 2011
31
Seattle
mas985 said:
It is pretty easy to slow down the flow rate (i.e. add head loss) and see if that helps the situation. If it works, then there are several options to consider.

But the thing I don't understand is why he said this:

, the smaller the plumbing , 1 ½”, the more pressure and the faster the water flows through the heater
Smaller plumbing reduces flow rate not increases it. The velocity of the water is higher in the PVC pipe but not the heater.
It does and it doesn't. Reducing the pipe size will reduce the amount of water you can move (in GPH), but the water you do move will be moving faster (in feet per second) and may not be in the heat exchanger long enough to suck up much heat.
 

JasonLion

TFP Expert
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May 7, 2007
37,879
Silver Spring, MD
Higher water velocity improves heat exchange by increasing the temperature difference. It also increases the odds of there being turbulent flow, which also increases the heat transfer efficiency. However, higher water velocity also risks eroding the metal of the heat exchange pipes. It's a balancing act between the highest possible velocity to improve heat transfer and not going to quickly which would damage the heat exchanger.
 

mas985

TFP Expert
LifeTime Supporter
May 3, 2007
12,844
Pleasanton, CA
Andrew Sarchus said:
It does and it doesn't. Reducing the pipe size will reduce the amount of water you can move (in GPH), but the water you do move will be moving faster (in feet per second) and may not be in the heat exchanger long enough to suck up much heat.
Two parts of this statement I don't entirely agree with.

First, velocity is related to the pipe diameter and flow rate by

Water Velocity (ft/sec) = 0.4085 * (GPM / (Dia * Dia))

Even though the GPM is the same in the PVC pipe and heat exchanger, the velocity is not the same because the diameter of the PVC pipe and the heat exchanger are different. When going from 2" PVC pipe to 1.5" PVC only the diameter of the PVC is changed so even though the flow rate (GPM) is decreased due to extra head loss in the pipe, the velocity in the PVC pipe can go up due to the reduction in pipe diameter. However, the heat exchanger diameter remains the same and since the flow decreases, so does the exchanger water velocity. The heat exchanger will have lower water velocity with 1.5" PVC pipe vs 2" PVC pipe.

Second, while it is true that higher GPM through the heater has a lower exit temperature than lower GPM, that does not mean that the heat transfer rate is less. Heat transfer rates are dependent on the temperature difference between the water and the heat source. The larger this temperature difference the faster heat transfers. So higher GPM means less temperature rise in the exchanger but faster heat transfer as well. In addition, less heat is lost with higher GPM so overall a pool will heat faster with higher GPM than with lower GPM.

So in summary, 1.5" pipe will have lower GPM and a higher exit temperature from the heater but will heat the pool slower than 2" pipe albeit not by much.
 

mas985

TFP Expert
LifeTime Supporter
May 3, 2007
12,844
Pleasanton, CA
I would not go as far as putting in a bypass without making sure that it is a flow rate issue. But this is really easy to test without any modifications. Simply plug up all but one return eyeball. Head loss increases, flow rate will decrease and if the heater starts working, you have an answer. Then a bypass might be a good idea.
 

JamesW

TFP Expert
Mar 2, 2011
16,763
It is important to understand that the heat pump does not have a flow switch, it has a pressure switch that is supposed to act as a flow switch. With the unit being 8 feet below water level, it is possible for the pressure switch to close and the heat pump to run with the pump off and with no water flow, which will damage the heat pump. You have to install an actual flow switch or a pressure switch that is at pool surface level.

Location of the heat pump above or below the elevation of the pool water surface can also affect operation of the switch. In general, the pressure switch can be adjusted to accommodate this effect if the heat pump water connections are no more than six (6) feet [1,8 m] below the pool water surface or no more than fifteen (15) feet [4,6 m] above it. See instructions for pressure switch adjustment in the heat pump start up section of this manual. If the heat pump is installed outside of this range, an external pressure switch may need to be installed in the plumbing upstream of the heat pump.

Installation of Pool Heat Pump Below Pool Surface
A water pressure switch is integrated into the system to ensure that the unit only operates when water is flowing through the heat exchanger. If the heat pump is installed below the pool surface, the backpressure of water from the pool can cause problems with the internal water pressure switch setting. The water pressure switch is located in the control panel of the heat pump. Adjustment knob – counterclockwise decreases pressure set point, adjustment knob – clockwise increases the set point. Backpressure from the pool can close this switch, whose function is to tell the heat pump that the filter pump is running. This pressure switch setting should be adjusted so that the heat pump is running only while the pool filter pump is operating.

If the heat pump runs for more than a couple of minutes without the filter pump running, the unit can become overheated and damaged.

http://www.hayward-pool.com/pdf/manuals ... lation.pdf
 

taekwondodo

LifeTime Supporter
Apr 26, 2009
419
mas985 said:
It is pretty easy to slow down the flow rate (i.e. add head loss) and see if that helps the situation. If it works, then there are several options to consider.

But the thing I don't understand is why he said this:

, the smaller the plumbing , 1 ½”, the more pressure and the faster the water flows through the heater
Smaller plumbing reduces flow rate not increases it. The velocity of the water is higher in the PVC pipe but not the heater.
Keep in mind, that the heat pump itself has 2" piping/unions that go to/from it. The unit itself is "unaware" of the head pressure, all it cares about is "flow": "Am I getting too little/too much flow?"

According to the flowmeter I installed yesterday (and got paid to do it :wink: ), we're getting 65GPM through the "system". Below 70, and above 20. Before I make any arguments back to the vendor, I want to make a statement here and have it "knocked around":

"The unit itself is carrying the required 65GPM (between 20 and below 70) - pressure and flow through the unit is directly proportional, i.e., head (and all of the issues with the external plumbing) become irrelevant inside the 2" plumbing of the unit... More specifically, we can now rule out flow/pressure/head issues as a reason the unit will go into 'HP' mode."

I'm not sure the above statement is correct...

[edited to note: There's a fair chance we may be doing a complete pad re-plumb in the near future, to clean-up the builder's plumbing mess]
 

mas985

TFP Expert
LifeTime Supporter
May 3, 2007
12,844
Pleasanton, CA
taekwondodo said:
Keep in mind, that the heat pump itself has 2" piping/unions that go to/from it. The unit itself is "unaware" of the head pressure, all it cares about is "flow": "Am I getting too little/too much flow?"
Actually, I think the opposite is true. Most heaters have a pressure switch that will shut the unit off when pressure gets too low. However, I am not aware of any heater that has a flow switch like an SWG although some may. However, the pressure in the unit itself is dependent on the the head loss in the rest of the plumbing which in turn is dependent on the flow rate. So there is a dependency on flow rate but the heater measures pressure.


taekwondodo said:
"The unit itself is carrying the required 65GPM (between 20 and below 70) - pressure and flow through the unit is directly proportional, i.e., head (and all of the issues with the external plumbing) become irrelevant inside the 2" plumbing of the unit... More specifically, we can now rule out flow/pressure/head issues as a reason the unit will go into 'HP' mode."
Presssure loss through a plumbing system is approximately proportional to the square of GPM and the fifth power of pipe diameter. But the heat exchanger in most heaters is much smaller than the couplings used to connect to the heater to increase efficiency. My heater uses 1" pipe for the heat exchanger which has multiple parallel paths. Also there is a built in bypass so when flow rates get higher, more water is bypassed around the heat exchanger. Most heat pumps also have a bypass so if the bypass valve is broken and all of the water is being bypass, then the pressure switch in the heat exchanger will shut off the heater.
 

JasonLion

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May 7, 2007
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Silver Spring, MD
The pool water pressure inside the heater is probably higher than what it is expecting. The flow rate is near the upper end of the acceptable range, and the unit is below water level, so the pool water pressure reading will be higher than a typical install. That doesn't match up with my understanding of an HP error, which usually means over-pressure on the refrigerant side of the system. But it is a remarkably similar situation, just on the pool water side.
 

madwil

Well-known member
May 2, 2011
370
lebanon tn
so, if it is backpressure due to "head" from pool- wouldn't it fix it just to "lift" the heater 6 inches when the replumb? this would reduce the backpressure due to height below pool level...
If I understood Jason's comment, anyway, seems like that would be a relatively easy fix (of course, I would have expected like Jason that HP was on refrigerant cirguit, not the water circuit)
 

mas985

TFP Expert
LifeTime Supporter
May 3, 2007
12,844
Pleasanton, CA
If the HP is on the refrigerant side and not on the plumbing side, raising or lowering the heater will probably not do anything.

But if the unit was charged incorrectly with a little too much freon, then under certain water flow conditions (i.e. heat transfer), the refrigerant pressure switch could trip while in others conditions it might not trip at all.