MasterTemp 400 too hot

[JamesW]

We had literally almost this exact same set up in our previous home and we did not have this problem.

What was the heater size for the previous system?

A 400,000 btu/hr heater is probably too big for a hot tub.

A 400,000 btu/hr heater is nice for heating a pool quickly, but it's oversized for a hot tub as you are finding out.

A 400,000 btu/hr heater will provide 3.36 degrees per hour for a 12,000 gallon pool.

A 200,000 btu/hr heater would provide 1.68 degrees per hour, which isn't too bad.

If the pool is going to be kept at temperature, a smaller size heater would have been fine.

If the pool is only going to be heated occasionally, then the bigger heater is nice.

I think that a 200k btu/hr or a 250k btu/hr might have been a better choice.

 

[JamesW]

Assuming an input temperature of 104 degrees Fahrenheit, the output temperature is going to be as follows.

The input temperature would not be 104 degrees, it would be closer to 102 to 103.

So, the temperature rise will be 12 or 13 degrees to get to 115.

A 13 degree rise indicates a flow of about 52 gpm, which better matches the data.

A 12 degree rise indicates a flow of 56 gpm.

82 gpm gives a rise of about 8.2 degrees.

Starting at 103, the exit temperature will be about 111.2 at 82 gpm at 3,450 rpm.

 

[wildpony]

James, I'm attaching some pictures of our equipment setup. I want to make sure you saw my note above where I indicated that the readings are for the pump on the pool side. I tried again this morning and I can't figure out how to run this test where the spa is operational. I added the missing value, for 3450. When I re-ran the pump at 3450,I noticed that the values are not static. They vacillate quite a bit.

RPM........Filter PSI......Pump PSI........Pump GPM
1,500.............7 psi...........7 Psi................19 GPM
2,350........14psi .............17 psi .............51 GPM
3,110...........22 psi ...........29 psi ...........73 gpm
3,450........27 psi..............35 psi............84 gpm

On this latest test, the filter PSI vacillated, but seemed to settle at 22 PSI.

If it's relevant, the suction lines for the pool are more 40-45'.

Thanks again for your help.
--Lee

 

[wildpony]

What was the heater size for the previous system?

A 400,000 btu/hr heater is probably too big for a hot tub.

A 400,000 btu/hr heater is nice for heating a pool quickly, but it's oversized for a hot tub as you are finding out.

A 400,000 btu/hr heater will provide 3.36 degrees per hour for a 12,000 gallon pool.

A 200,000 btu/hr heater would provide 1.68 degrees per hour, which isn't too bad.

If the pool is going to be kept at temperature, a smaller size heater would have been fine.

If the pool is only going to be heated occasionally, then the bigger heater is nice.

I think that a 200k btu/hr or a 250k btu/hr might have been a better choice.

The heater size for the previous system was also 400,000 BTU. The previous setup was very similar. The pool is only heated occasionally which is why we wanted the bigger heater.

 

[JamesW]

Check the cell to see if there is any debris clogged in the screen or the plates.

What is the white thing after the cell?

Is that a tab feeder?

I would get rid of that. That's probably creating excessive restriction.

I would bypass any flow over 40 gpm around the heater.

You can use a 5 psi check valve or you can set the bypass valve and install a 40 gpm flow switch going to the heater to make sure that the heater can only run when the flow is 40 gpm or higher.

When the system is in Service mode, you can change from pool mode to spa mode at the outdoor panel to check the spa readings.

Or, you can manually switch the automated valves by using the switch on the valves.

I would have used a 420 or 520 square foot cartridge filter instead of only 200 square feet.

The spa suction is too small.

It should have been a 3" pipe instead of a 2" pipe.

The return is also too small.

It should have been 2.5" or 3".

 

[wildpony]

James,

That is a tab feeder. We can get rid of it if that helps.

I will try again tomorrow to get those readings on the spa side of the system.

Thanks again.

--Lee

 

[JamesW]

I would do the following:

1) Remove the tab feeder.

2) Do a heater bypass.

3) Install a flow switch on the line to or from the heater

4) Install a flow meter.

5) Check the salt cell for debris.

6) Change the filter to a 520 square foot model.

7) There is nothing that can easily be done about the suction and return lines.

 

[JamesW]

Once you get the spa numbers, you can plot them on the graph to see what the spa system curve looks like.

Once you make the changes, you can plot the new system curve to see how much improvement you were able to make.

You can also draw in estimated pump performance curves.

For example, here is the chart with a curve at 2,730 rpm added by going about half way between the existing curves since the rpm is half way between 3,110 and 2350.

Based on the estimated curve, the flow at 2,730 rpm should be about 62 gpm.

 

[wildpony]

I would do the following:

1) Remove the tab feeder.

2) Do a heater bypass.

3) Install a flow switch on the line to or from the heater

4) Install a flow meter.

5) Check the salt cell for debris.

6) Change the filter to a 520 square foot model.

7) There is nothing that can easily be done about the suction and return lines.

James,

Here's how the readings came out for the spa:

RPM........Filter PSI......Pump PSI........Pump GPM
1,500.............4 psi.............7 psi................27 gpm
2,350..........14 psi ............17 psi ...............44 gpm
3,110...........25 psi ...........30 psi ...............58 gpm
3,450..........31 psi.............37 psi...............64 gpm

I'm going to check the salt cell later this week. I played with the pump speeds while sitting in the spa yesterday. At a setting of 3,110, the pressure is uncomfortable. I'm thinking a comfortable maximum RPM setting is in the 2,500-2,800 range. We already have a heater bypass valve, but I'm not sure if you're thinking of a different type of one in your suggestion above. If I can find a reasonably comfortable temperature setting for the spa output with the valve we have, I'm thinking of putting a valve actuator there. That way I could make easy adjustments through the automation. Thanks again for all your help.

--Lee

 

[JamesW]

Here is a graph with the pool and spa curve with a pump curve at 2,730 rpm added in.

The spa curve is worse than the pool curve.

The flow at 2,730 is going to be about 51 gpm, which means a temperature rise of 13.4 degrees and an output of 116.4 degrees.

The heater bypass isn't going to do much good if you don't want more that 51 gpm coming out of the jets.

If the flow is 51 gpm, the output is going to be 116.4 degrees.

If you're not going to increase the flow, the only other thing you can do to reduce the output temperature is to go to a smaller heater.

A 200,000 btu/hr heater will have a temperature rise of 6.7 degrees and an output temperature of 109.7 degrees.

How many jets do you have?

 

[JamesW]

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