Pentair Mastertemp HD 400K slow heating or too high of expectation?

[gocubs418]

I have noticed that the time it takes to heat this spa at my new house is significantly longer than the amount it took to heat my spa at my old house. I know this spa is a little bit larger but not overly larger.

Currently, I have the spa at 3275 RPM and a Mastertemp HD 400K BTU heater. I know it was cold last night around 38 degrees, and it took it about 2 hours to heat from 49 degrees up to 104 degrees. This is seemingly double the amount of time it took to heat my prior spa.

-I took out the thermal regulator and tested it, worked flawlessly.

-I have adjusted the pump speeds but haven't noticed anything substantially different.

-Filter seems clean

-Water under the heater is from pulling out the thermal regulator.

Kinda stumped... is this completely normal operation and I just need to get used to having a larger spa or could there be something at play here? I only got an error code when I lowered the GPM too low and it started cycling.

Any help would be greatly appreciated. Thanks!

 

[ajw22]

Is your signature up to date with your new pool?

Does your spa have a cover?

Lots of other thermal loss going on while heating the spa. All the ground and spa walls are cold and absorb heat.

I am not surprised at the time to heat from close to freezing water. I would not do a lot of surgery on the heater.

What size are your old and new spas?

 

[MSchutzer]

Are you sure that you are only heating the spa? Why does you pool temp show that it is going up 4 or 5 degrees while you are heating the spa?

I would double check that the Pool / Spa inlet and outlet valves are fully in the the spa position when heating the spa. You shouldn’t have any spillover from the spa to pool when the spa is running.

 

[JamesW]

2 hours to heat from 49 degrees up to 104 degrees.

That is 55 degrees in 2 hours or about 27.5 degrees per hour or about 0.458 degrees per minute.

How many gallons is the hot tub?

It looks really big.

If we estimate the volume at 1,465 gallons, then that is 12,190 pounds.

336,000/12,190 = 27.5 degrees per hour or about 55 degrees in 2 hours.

Check the stack flue temperature.

Check the gas meter to see how much gas is being used in 1 hour.

 

[JamesW]

2 hours to heat from 49 degrees up to 104 degrees.

That is 55 degrees in 120 minutes or about 27.5 degrees per hour or about 0.458 degrees per minute.

1,100 gallons is 9,153 lbs.

400,000 btu/hr at 84% efficient is 336,000 btu/hr.

So, if we use 1,100 gallons, the heating rate should be about 336,000/9,153 = 36.7 degrees per hour or about 0.61 degrees per minute.

If the tub is uncovered, then the heat lost can be substantial.

 

[gocubs418]

Is your signature up to date with your new pool?

Does your spa have a cover?

Lots of other thermal loss going on while heating the spa. All the ground and spa walls are cold and absorb heat.

I am not surprised at the time to heat from close to freezing water. I would not do a lot of surgery on the heater.

What size are your old and new spas?

Thanks for reply. My signature is not up to date, need to fix that, that is the old pool.

My spa doesn't have a cover but I would invest in one if that would significantly improve my heating time. I didn't really have an issue in my last house.

My old spa was 7x7 and my new spa is 9x6.

Are you sure that you are only heating the spa? Why does you pool temp show that it is going up 4 or 5 degrees while you are heating the spa?

I would double check that the Pool / Spa inlet and outlet valves are fully in the the spa position when heating the spa. You shouldn’t have any spillover from the spa to pool when the spa is running.

Thanks for reply. Well, that is my best guess. Unsure why my pool shows that it is increasing in temp.

Would you be able to expand on what to look for on the "pool/spa inlet and outlet valves"? I never had to really check this so not exactly sure what to look for. I did check the spill way today when I was doing a test run and didn't see any spillover why the spa was running.

That is 55 degrees in 2 hours or about 27.5 degrees per hour or about 0.458 degrees per minute.

How many gallons is the hot tub?

It looks really big.

If we estimate the volume at 1,465 gallons, then that is 12,190 pounds.

336,000/12,190 = 27.5 degrees per hour or about 55 degrees in 2 hours.

Check the stack flue temperature.

Check the gas meter to see how much gas is being used in 1 hour.

View attachment 551960

Thanks for reply. Spa is 9 x 6 x 3.5. subtract some for the giant benches. I would estimate 1,100 gals is probably pretty close.

Stack Flue meaning the exhaust on the heater? I will check the gas meter soon and report back.

That is 55 degrees in 120 minutes or about 27.5 degrees per hour or about 0.458 degrees per minute.

1,100 gallons is 9,153 lbs.

400,000 btu/hr at 84% efficient is 336,000 btu/hr.

So, if we use 1,000 gallons, the heating rate should be about 336,000/9,153 = 36.7 degrees per hour or about 0.61 degrees per minute.

If the tub is uncovered, then the heat lost can be substantial.

Thanks for reply. I don't have the exact gals off hand but it is 9x6x3.5 minus the bench space so maybe 1100-1250 gals?

It is uncovered, maybe I need to get a nice cover to throw while it heats.

 

[JamesW]

PoolMath

Discover the calculator behind the perfect pool, PoolMath. Dealing with pools has allowed TFP to become the original authors of the best calculator on the net.

www.troublefreepool.com

 

[JamesW]

Pentair MasterTemp Heaters - Further Reading

www.troublefreepool.com

While running, press and hold the On button to display the exhaust temperature (Older Models).

The new models use the Menu to get the SFS.

Here is what the normal exhaust temperature should be:

• Below 250 degrees...very low
• 250 to 290 ..................low
• 290 to 350.................acceptable
• 350 to 480 ..................high
• Above 480...................error/shutdown.

Note: HD models can be up to 75 degrees higher. HD models use a cupro nickel exchanger and the efficiency is slightly lower resulting in less heat transfer and more waste heat.

 

[JamesW]

I will check the gas meter soon and report back.

A 400,000 btu/hr heater should use about 400 cubic feet of natural gas per hour.

 

[gocubs418]

A 400,000 btu/hr heater should use about 400 cubic feet of natural gas per hour.

James, really appreciate the replies. I will follow up tomorrow on the exhaust and the gas reading.

 

[TopJimmy]

The greatest heat loss is by evaporation which is magnified depending on how low ambient temp is. My spa is covered, 900 gal and I maintain it at 70 deg with a MasterTemp 125. It takes about 2.5 hrs to go from 70 deg to 99 deg. So just a little planning ahead when I want to get in.
Pentair says you shouldn't regularly heat the water from below 68 deg due to acidic condensates which will destroy the heater. So I'd advise you maintain at least 68 deg and get a cover. Benefits: it will take a lot less time for your 400k btu to heat up your spa; and you'll save energy, water, and your heater.

 

[JamesW]

HEAT LOSS FROM WATER SURFACE AT VARIOUS AIR AND WATER TEMPERATURES AND AIR VELOCITIES AT 70% HUMIDITY.

https://apps.raypak.com/wp-content/uploads/2019/02/RP-P-902-6-REV-C-Residential-Pool-Catalog.pdf

 

[JamesW]

So, if we use 1,100 gallons, the heating rate should be about 9,153 x 27.5 degrees per hour = 252,000 btu/hr net gain rate.

336,000 - 252,000 = 84,000 btu/hr loss.

That is about 1,555 btu/hr per sq ft.

 

[gocubs418]

The greatest heat loss is by evaporation which is magnified depending on how low ambient temp is. My spa is covered, 900 gal and I maintain it at 70 deg with a MasterTemp 125. It takes about 2.5 hrs to go from 70 deg to 99 deg. So just a little planning ahead when I want to get in.
Pentair says you shouldn't regularly heat the water from below 68 deg due to acidic condensates which will destroy the heater. So I'd advise you maintain at least 68 deg and get a cover. Benefits: it will take a lot less time for your 400k btu to heat up your spa; and you'll save energy, water, and your heater.

Thanks for reply. Do you have a cover you recommend that I should purchase? Seems like a very reasonable investment to make.

View attachment 552031

https://apps.raypak.com/wp-content/uploads/2019/02/RP-P-902-6-REV-C-Residential-Pool-Catalog.pdf

So, if we use 1,100 gallons, the heating rate should be about 9,153 x 27.5 degrees per hour = 252,000 btu/hr net gain rate.

336,000 - 252,000 = 84,000 btu/hr loss.

That is about 1,555 btu/hr per sq ft.

James, thanks for reply, not entirely sure how to read this, but I assume that I am losing a ton of heat via evap so further evidence that I should invest in a cover. Would you happen to know a good product?

Also, I did check the SFS and it was at 325. I have not had a chance to check the gas usage.

 

[JamesW]

I did check the SFS and it was at 325.

That is in the acceptable range, so the heater is probably working at the correct capacity.

Check it periodically.

The heat losses are probably pretty high.

The volume is not known exactly, so getting a better, more exact volume would be helpful.

I do not have a specific cover that I can recommend, but any cover will reduce evaporation, which is the main issue.

You can also see how much gas is used once the spa reaches the set temperature.

For example, if the amount of gas used in 1 hour at set temperature is 84 cubic feet, then that would match with the estimated 84,000 btu/hr loss rate.

Once you reach the set temp, all further gas usage is due to maintaining the temperature.

So, the heat required is only due to heat losses and not trying to heat the water from a lower number.

 

[JamesW]

For a 50 degree air temp and a 104 degree water temp and a wind velocity of about 3.4 mph, the estimated heat loss is about 600 btu/hr per sqft.

6 x 9 = 54 sqft, so that is 54 x 600 = 32,400 btu/hr loss expected/predicted/estimated.

The spa look bigger than 6x9, are you sure about the dimensions?

HEAT LOSS FROM WATER SURFACE AT VARIOUS AIR AND WATER TEMPERATURES AND AIR VELOCITIES AT 70% HUMIDITY.

 

[gocubs418]

For a 50 degree air temp and a 104 degree water temp and a wind velocity of about 3.4 mph, the estimated heat loss is about 600 btu/hr per sqft.

6 x 9 = 54 sqft, so that is 54 x 600 = 32,400 btu/hr loss expected/predicted/estimated.

The spa look bigger than 6x9, are you sure about the dimensions?

HEAT LOSS FROM WATER SURFACE AT VARIOUS AIR AND WATER TEMPERATURES AND AIR VELOCITIES AT 70% HUMIDITY.

Yeah, I'm pretty sure I measured it right. I thought I bought a 10x10 solar cover but I bought a 8x8 on accident. I tossed it on anyways and heated up the spa.

Based on my time starting at 57 degrees, it took roughly 1.5hrs to get to 102 degrees.

Seems like similar performance with or without cover but 2ft of the hot tub was exposed since the cover was too small so I will repeat the test again.

I'm also going to lower the spa RPMs from 3250 to 2850 and see if that impacts the performance.

If after that, it doesn't do anything. Then I am just going to accept that the spa is that much larger than my old spa and it does require that much heating time + many other variables to get it up to temp.