Gas heater needs 2750 RPM to fire

Jump to latest Follow Reply
The header assembly is polymer, not metal.

Even if it was metal, the metal takes time to heat up.

Also, you have less than 3 seconds before the water boils.

In my opinion, the high limit is just not going to react that fast.

Try it on your heater if you feel like it won't hurt anything.

Most likely, the heater will start banging very loudly due to the water boiling and the heat exchanger getting expanded and warped.

Copper is very soft and delicate; it will get way too hot if there is no flow for even a few seconds.
 
Last edited:
165908_0_20178185642.jpg

J6755__24717.1594850057.jpg

PCSLAR-151-3232__35162.1587515618.jpg
 
JamesW said:
The header assembly is polymer, not metal.

Even if it was metal, the metal takes time to heat up.
Click to expand...
First the heat exchanger ends right at the header which is very close to the thermal limiters (inches away). This is why I said the limiters are thermally connected to the heat exchanger. So the water at the header is nearly at the same temperature as the heat exchanger metal and the heat exchanger metal is at the same temperature as the water inside the heat exchanger so I don't think that changes anything. But even so, since the sensor trips at 135F which is below PVC melting point so even if the water in the header reached 135F, no damage would occur and the water temperature in the middle of the heat exchanger should not be that much higher.

This is just an analogy but the surface temperature of solar panels is nearly the same as the water flowing through it even though the solar energy hitting it has a much higher temp. The exposed temperature sensor reads 110F in the sun but a IR temp sensor of the panel surface reads the same as the water temperature. Solar panels have much lower thermal conductivity as a metal heat exchanger but it still applies.

JamesW said:
Also, you have less than 3 seconds before the water boils.
Click to expand...
I think you are forgetting about latent heat of vaporization. It takes extra heat before it boils. Reaching 212F is not enough. But again, conductivity pulls the heat through the metal in the heat exchange to the limiters fairly quickly.

JamesW said:
In my opinion, the high limit is just not going to react that fast.
Click to expand...
I disagree. I don't think it will take much time at all and it won't be that much of a temperature difference as explained above.

JamesW said:
Try it on your heater if you feel like it won't hurt anything.
Click to expand...
I might just do that, stay tuned.

JamesW said:
Most likely, the heater will start banging very loudly due to the water boiling and the heat exchanger getting expanded and warped.

Copper is very soft and delicate; it will get way too hot if there is no flow for even a few seconds.
Click to expand...
Melting point of copper is close to 2000F. How does this matter? Even the solder is close to 500F. But again, I believe the thermal conductivity to the limiters is much higher than what you think it is.

I don't think the water temp will get anywhere close to boiling.
 
mas985 said:
Seems like the heater does a pretty good job of detecting low flow on it's own. Plus I am sure it has a thermal limiter that protects from overheating. An extra flow switch seems unnecessary to me.
Click to expand...
So, I should be able to leave the heater bypass closed completely at my base rate of 1750 RPM? Would I need to change the bypass to partially open in Pool Heat(2250) or Spa mode(3450) RPM's, speeds which are set in the original programming from the pool builder? The pool builder left the heater bypass open a bit but they had the speed permanently sent at 2750 RPM x 9 hours per day. They told me that in spa mode, the flap in the check valve after the heater should be half open. This issue arose as I was trying to reduce energy costs.

Thanks for all the expert advice on this forum!

Jim
 
Close the bypass completely and record the filter pressure at these speeds:

Pool Mode:

RPM.............Filter Pressure.
3,450 ...............
3,000 ...............
2,800 ...............
2,400 ...............
1,730 ...............

Spa Mode:

RPM.............Filter Pressure.
3,450 ...............
3,000 ...............
2,800 ...............
2,400 ...............
1,730 ...............

1692023796675.png
 
Last edited:
JamesW said:
Close the bypass completely and record the filter pressure at these speeds:

Pool Mode:

RPM.............Filter Pressure.
3,450 ...............
3,000 ...............
2,800 ...............
2,400 ...............
1,730 ...............

Spa Mode:

RPM.............Filter Pressure.
3,450 ...............
3,000 ...............
2,800 ...............
2,400 ...............
1,730 ...............
1691860662165-png.521718
Click to expand...
I will do it. The x side of the graph is filter pressure, I would think?
 
The Y-axis is the total head loss in feet.

The total head loss is the return and suction head loss.

The return head loss is the filter pressure X 2.31.

For example, 10 psi is 23.1 feet of head.

You can measure the suction pressure with a gauge or try to estimate it based on the return head loss.

If we assume that the suction head loss is about 1/3 to 1/5th of the return head loss, we might choose the following for an estimate.

25 psi is about 58 feet of head loss for the returns and you need to add the suction, which we can estimate to be about 12 feet of head loss for a total of about 70 feet.

If you know the pipe size and length for the suction, you can use head loss charts, but this assumes that you know the flow rate.

If we use 70 feet of head at 3,450 RPM, we can see that that corresponds to about 98 gpm.


1692024167523.png

1692024705017.png
 
Last edited:
If H1 = 70 feet and Q1 is 98 gpm, then we get the following system curve.

Using Head Loss as the Y-axis and Flow in GPM for the X-axis

Y = (Head Loss1 / GPM12 ) (X)2

Y = (0.00728863) ( X2 )

That would make your system curve slightly better than Curve "C".

You can superimpose the system curve onto the pump performance curve to find the operating point at any pump speed.

If you measure the pressure (Head Loss) at different speeds and graph the data onto the graph, it should produce the estimated system curve.

You should be able to produce the system curve from any pump speed and pressure reading.

The system curve will change depending on the system configuration.

1692026543889.png



1692026890334.png



1692026938954.png



1692027206763.png



1692027449778.png



Y = 0.00728863(X)^2, y from 0 to 80 - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.
www.wolframalpha.com www.wolframalpha.com

aqua.Pumps_Slide2_0115.png



aqua.Pumps_multispeed_0115.png
 
Last edited:

Enjoying this content?

Support TFP with a donation.

Give Support
Also, check the temperature rise for each pump speed and this will help us to estimate the flow in GPM using the formula GPM = 672/(Temp Rise).

This will give us 2 different ways to estimate the flow at different speeds.

Hopefully, they will be close.

Be careful about operating the heater while changing speeds as you do not want to operate with insufficient flow.

You can also get a vacuum gauge to measure the suction head loss and a flow meter, but these things are rarely worthwhile as you can usually adjust things well enough without a flow meter or a suction gauge.

For the temp rise at 98 gallons per minute, we would be looking for about 6.8 degrees.

So, if the heater inlet temp is 82 degrees, the cell temp should be about 89 degrees.



1692024705017-png.522207
 
jmcmd1980 said:
New pool 6/8/23
Pressure at 3450 is about 22-25
3 returns
Not sure of pipe size, see picture, looks pretty big, I took a picture of an eyelet.
Click to expand...
Assuming the filter gauge is 4' above water level and I guessed right at the suction side plumbing...

If the filter pressure is 22 PSI @ 3450 RPM, the operating point I get is:

114 GPM @ 62' of head.

If the filter pressure is 25 PSI @ 3450 RPM, then the operating point becomes:

101 GPM @ 68' of head.

You can fiddle with the input numbers here:

docs.google.com

Pool Pump Tools v027 - jmcmd1980

ReadMe Version:,v027 Disclaimer,These spreadsheets are provided as is without any guarantees or warranty. Every effort has been made to produce reasonable estimates with these modeling tools. However, unknown measurement errors in the CEC/APSP/EnergyStar data may significantly affect the results...
docs.google.com docs.google.com
 
Last edited:
  • Like
Reactions: JamesW
22 PSI could be in pool mode where there are two returns, one set to the pool and another to the spa. The suction side also has two lines which reduces the suction side head loss.

img_1250-jpeg.521681
 
Do you have any advice on how much the bypass valve should be opened, if any, when I’m running the filter at baseline 1750 RPM, versus pool heat 2250 RPM or spa/spa heat mode at 3450 RPM? The pool builder advised the 42-50% bypass open you note from my photo when using the heater to make the check valve flap open 50%. Sorry for my confusion about what to make of all of this. Jim
 
For the filter pressure measurements, you want it completely closed (i.e. no bypass). We are interested in the performance without any external bypass using only the internal bypass.
 
  • Like
Reactions: JamesW

Enjoying this content?

Support TFP with a donation.

Give Support
You must log in or register to reply here.
Thread Status
Hello , This thread has been inactive for over 60 days. New postings here are unlikely to be seen or responded to by other members. For better visibility, consider Starting A New Thread.
Top Bottom
Back
Jump to latest Follow Reply