Water flow thru heater to get best heating

[gary300]

Hello all. I have just started to heat my pool using a variable speed pump and new Raypak heater. With my plumbing setup I can disable flow to heater, bypass some water flow to heater or direct all flow thru the heater. For the most efficient heating, should I bypass some water or send all the water thru the heater? What speed should I run my VSP when heating the pool?

 

[mas985]

Given your electrical rates, you want to direct all flow through the heater at the lowest RPM possible. It doesn't matter if the heater needs to run a little longer because electrical costs will trump the gas cost.

Bypass should only be used if PH is too low and you want to protect the heater.

 

[JoelFitz]

I've gotten a heater to fire at 750rpm on a VS pump but mostly set them up for 1000rpm to be safe on long run situations and +1 on Mas985, close the bypass only when you're not using the heater or if you have chemistry issues.

The heater has a pressure switch and will not fire until it's closed. If the heater fires at a low rpm it should be okay.

GL!

 

[duraleigh]

Mas985,

I understand about directing all the flow to the heater but don't understand why to run the VS on low speed. Borrowing from my limited knowledge of Solar, I think we always want maximum flow through a heat source to get the greatest transfer.....this seems to go against that.

Help us (me) understand.

 

[bdavis466]

I believe because in this situation, electricity is far more expensive than natural gas so the inefficiency is a trade off for lower electrical consumption (and ultimately less money).

 

[mas985]

Yes, for heat transfer that is true. But my comments were about economics. Here in CA, the electrical costs are so high, that you are better off running a gas heater a little longer and using a little more gas, which is pretty cheap, so you can run the pump at a lower RPM and save money on electrical costs. There are limits of course because the heater requires a minimum flow rate.

 

[ps0303]

I would push all the water thru the unit and keep the pump at a medium speed. I know variable speed pumps say some $$ but I feel they are a huge rip off because of the cost. I also se and hear about them not lasting years and years like a traditional motor. The simpler the better and these new pumps have extra parts that will fail.

 

[duraleigh]

Thanks for the carification....that makes sense.

 

[gary300]

Thanks for the reply's... good info. What I would really like to know is what configuration and pump speed would heat my pool the fastest, regardless of cost. By the way, my electricity rate is 13 cents per KWH.

 

[mas985]

By the way, my electricity rate is 13 cents per KWH.

Isn't that the tier 1 rate?

But to heat the fastest, use the fastest flow rate. Although, I doubt it will be that much faster.

 

[smooth2222]

15000 gallon. 266,000 btu, i got about 3 degrees per hour, maybe a tad less, 2-3 degrees.

I have a very high flow rate

 

[bdavis466]

Isn't that the tier 1 rate?

Riverside has their own utility company with very reasonable rates. That's something I never appreciated until I moved out of the city.

 

[mas985]

Riverside has their own utility company with very reasonable rates. That's something I never appreciated until I moved out of the city.

Must be nice!

15000 gallon. 266,000 btu, i got about 3 degrees per hour, maybe a tad less, 2-3 degrees.

3 degrees in 15k gallons is 375k BTUh so that is not possible. At most your heater is 85% efficient which means it can deliver only 226k btuh which would be about 1.8 degrees per hour. And that doesn't account for heat loss.

 

[smooth2222]

Should i take pictures? I check temp, set timer and stove 1 hour, comeback 3 degrees higher, have done it multple times.

 

[smooth2222]

Calculator

2.8 hours 6 degrees, said. 4-6 degrees

 

[mas985]

Are you taking the temperature during the day when the sun is on the pool? If so, that would account for some of the gain and the 3 degrees is not just the heater. The bottom line is that size heater with that size pool cannot raise the temperature by 3 degrees in one hour by itself. It doesn't have enough BTU to do it.

Also, for that web site you have to include the heater efficiency in the Heat-Up Load (226k BTUh). When you do that and set temperature range for 3 degrees, I get 1.7 hours or 1.8 degrees/hr which is the same as I calculated above.

 

[gary300]

Isn't that the tier 1 rate?

But to heat the fastest, use the fastest flow rate. Although, I doubt it will be that much faster.

It would seem to me that a slow flow through the heater would allow more heat to be transferred to the water than a high flow rate would. Where have I gone wrong on this assumption?

 

[mas985]

Yes but is the net heat transfer to the pool that is important. Without heat loss, the heat transfer to the pool would be exactly the same no matter what flow rate you have as long as the heater is run for the same time period. If you halve the flow rate, the temp gain of the water is 2x because you are heating half the water/min with the same BTUs. But at the higher flow rate, 2x as much water is being heated 1/2 as much as low speed so net net, the heat transfer to the pool is exactly the same for the same period of time. It is all about the BTUs delivered to the pool and not the temperature of the water.

However, there IS heat loss in not only the heater but every part of the plumbing on the way to the pool and heat loss is proportional to the temperature difference of the water and everything else (air, ground, sky) around it. With a lower flow rate, the water is at a higher temperature so more of the heat will be lost on it's way to the pool. Even though the temperature gain of the water may start out a 2x with half the flow rate, by the time it gets to the pool, it will have lost some of the heat so it will be <2x the temperature gain. So there will be less heat delivered to the pool at lower RPM than higher RPM. How much different depends on the heat loss difference which depends on a lot of different factors.

 

[gary300]

Yes but is the net heat transfer to the pool that is important. Without heat loss, the heat transfer to the pool would be exactly the same no matter what flow rate you have as long as the heater is run for the same time period. If you halve the flow rate, the temp gain of the water is 2x because you are heating half the water/min with the same BTUs. But at the higher flow rate, 2x as much water is being heated 1/2 as much as low speed so net net, the heat transfer to the pool is exactly the same for the same period of time. It is all about the BTUs delivered to the pool and not the temperature of the water.

However, there IS heat loss in not only the heater but every part of the plumbing on the way to the pool and heat loss is proportional to the temperature difference of the water and everything else (air, ground, sky) around it. With a lower flow rate, the water is at a higher temperature so more of the heat will be lost on it's way to the pool. Even though the temperature gain of the water may start out a 2x with half the flow rate, by the time it gets to the pool, it will have lost some of the heat so it will be <2x the temperature gain. So there will be less heat delivered to the pool at lower RPM than higher RPM. How much different depends on the heat loss difference which depends on a lot of different factors.

Thanks for the explanation. Makes sense. I have 4 returns that wrap around to the opposite side of the pool from my equipment pad so I can see where heat would be removed from the water in those long runs more quickly if the water was moving slow versus fast moving water.

 

[danpik]

Another way to look at it is this...Think of it as if you are trying to cool the heater instead of heating the water. What would work better, low flow or high flow. Now as far as how fast to run the pump. At some point the heat absorption by the water thru the heat exchanger will level out. Put a thermometer in front of the return to the pool and measure the temp of the return water. Note that and turn the speed on the pump up and measure again. The return water temp should drop slightly. Keep increasing the flow untill the water temp no longer drops at the return. This is the ideal pump speed for max heat transfer into the pool. anything faster and you are just going to use more electricity and not gain any BTU load into the pool.