Its about 30% depending on what size and type of valves you use.

Its really hard to say how much energy you will save with larger pipes, it really depends on how much water you want to move.

jparkfire,
2" plumbing is fine for most pools. It's plenty efficient for a 1/2 thru 1 HP pumps in most cases. Larger pumps will require larger plumbing.
Efficiency is not just pipe size. It's matching plumbing, pump, needed flow rate, type of application, and other factors to determine what works best.
To answer your question: there may be no upside to using 3-4" pipe. And, believe it or not some builders still use 11/2" pipe. It may, in fact, be apprpriate in some cases.

Efficiency gains can be quite extensive with larger pipes. Pipe head loss is inversely proportional to the 5th power of the diameter and fittings to the 4th power.

So as an example, a pool with a single suction line and a single return line and a typical 1 HP pump might have the following characteristics with various pipe sizes:

1.5" Plumbing: 74' of head at 58 GPM
2.0" Plumbing: 56' of head at 89 GPM
2.5" Plumbing: 46' of head at 102 GPM

In this case, pipe size can make a very big difference!!!

However, you can see that as diameter increases, the change in gain decreases so there is a limited return on upsizing. Also, in some cases it might make more sense to run multiple 2" lines rather than a single 2.5" line.

As a minimum, it is a good idea to have individual runs for each skimmer and main drain pair plus a return line for each pair of returns.

So for a pool with 3 suction lines and 2 return lines, the situation becomes much different:

1.5" Plumbing: 51' of head at 95 GPM
2.0" Plumbing: 36' of head at 113 GPM
2.5" Plumbing: 33' of head at 116 GPM

A big difference from the first case where only single lines are used. So larger pipe is not always better. It depends on how the whole system is designed.

Where does one find charts / formulas for head calculation?

I use a fairly detailed hydraulics model which in turn uses the Darcey Weisbach hydraulics equation for head loss. It can be found here:

http://www.engineeringtoolbox.com/darcy-weisbach-equation-d_646.html

However, you have to solve it simultaneously with the pump's head curve to get the operating point. The plumbing curve and the pump's head curve will cross at a single point which is the operating point of the system.

There are also tables located here for pipe loss:

http://www.engineeringtoolbox.com/pressure-loss-plastic-pipes-d_404.html

But you also have to include the friction loss for fittings which can be found here:

http://www.engineeringtoolbox.com/pvc-pipes-equivalent-length-fittings-d_801.html

In addition, there is head loss for the filter, heater, skimmers and main drains which are difficult to find so I use factors which I found by calibrating my own pool plumbing. In general, it is very difficult to get an accurate model for new plumbing and it is much easier to calibrate a model for an existing system.

You can also solve the problem graphically by drawing the plumbing curve over the pump head curve like this:

{{gwi:2050636}}

The cross overs are the operating points. Curves A-G are different pipe size as shown below:

{{gwi:2050637}}

Probably much more than you were looking for but I wanted to show how much is involved with doing that type of analysis.

Here is a link that might be useful: Hydraulics 101

Jparkfire...below is a link to literature that I used successfully to convince my PB that I needed larger pipes. These recommendations make it simple to select pipe size based on pump size. What I also like about this are the recommendations to bump pipe size for long runs of pipe. After countless hours of trying to convince my PB to upsize my pipes, this is what finally convinced them.

Hope this helps!

Here is a link that might be useful: Pipe Sizing Chart

I find it amazing that any pool builder needs hours of "convincing" that they should build with larger diameter pipe! Go to school...Get educated...Take a hydraulics course!!!

Robindigo...yeah they looked at me like I was from Mars when I was explaining pipe velocities and friction tables. Their response was "our plumber has been doing pools for over 20 years". I will give them credit for wanting to do the right thing in the end. A recent visit with them revealed they are more energy conscious these days. They were stocking a lot of 2 speed pumps and using large pipe. Prior to my pool they were using single speed and smaller pipe.

One thing that hasn't been brought up about using larger pipes is the greater exposure with heated water in piping to the ground. I am not sure how you quantify it, but if you have more water in a pipe below ground, you will get more heat loss. The question becomes is the heater loss more or less than the increased/decreased energy consumption by the pump.

Pipes in the ground act as heat sinks, and the larger the heat sink the more the heat loss.

Along those lines I plumbed a job where the client requested the return lines be insulated, using insulation similar to that found on process piping. Turns out we never did it; he used spray foam...

While it is true that a larger diameter pipe has a larger surface area, you also need to take into account the flow rate in the pipe. The water is traveling faster in a larger pipe so has less time to lose the heat in the pipe.

So just taking into account surface area and flow rate, the larger pipe can actually lose less heat. For example, a single 2" pipe has 33% more surface area than a single 1.5" pipe but the flow rate is 50% more that I sited in the example above. So the smaller pipe will lose about 50% more heat than the larger pipe.

For multiple pipes, 3 2" pipes still has 33% more surface area than 3 1.5" pipes but the flow rate difference is less, ~18%. So in this case, the larger pipe has more heat loss but the flow gain is much less and so using larger pipe is less important too.

Only about 10% of heat loss is in through the walls and plumbing. Because the walls of the pool/spa have a much larger surface area than the pipes, the pipe heat loss is much less than the walls. So overall, very little heat is lost through the plumbing.

Here is a link that might be useful: Heat loss in swimming pools.

jparkfire,
Looks like you got a lot of good info to sift thru. The one place you need to start is by asking how big is the vessel(gallonage) and how long do you want it to take to make one turn of the water. Good luck.

Good discussion and interesting point about heat transfer under ground. Mas985, from my HVAC experience and understanding of heat tranfer, turbulent flow improves heat tranfer while laminar flow impedes heat transfer. Wouldn't that suggest that one big pipe will not lose as much heat as multiple smaller pipes with higher pipe velocities. Howewver, I do agree that the heat loss via piping is small compared to other contributors with pools.

Laminar flow occurs for Reynolds numbers less than 2300 which is less than 2 GPM in a 2" pipe so it is very unlikely to occur in pool plumbing. Turbulent flow will be in both small and large pipe.

Increasing the surface area will = more heat transfer regardless of flow volume, or velocity. That is why larger heat exchangers are, well larger.

If you are increasing the volume, or velocity you are just decreasing the delta, but are you really altering the amount of energy used, or transferred to the ground?

There is a lot of heat lost in the ground, it would be interesting to see how much. It may be nothing, it may be a lot.

I wonder how we can measure this.

The real question one needs to ask is the amount of $ saved per $ invested. Whether it is larger, pipes, more efficient pump, larger filter one always wants to spend the money first where the savings is largest and then keep working down the list.

By this measure larger pipes (to a point) are one the best investments you can make. The cost to upgrade is small (labor is the same, only the cost of materials). In my book it is the very first thing one should do.

Next one should look at valves. Valves? Yes, if the goal is to reduce friction (head) so the pump doesn't have to work as hard (and therefore uses less energy) valves are a real source of loss. In fact the backflow valve can be one of the largest losses in the entire system. Again the cost to upgrade is almost nothing. The only downside is that manufacturers, being slow to recognize the advantages of low loss piping don't offer as many options in filters, valves etc in the larger low loss sizes. I had to call the local Pentair rep to get a line on their low loss backflow and even then they aren't yet in production of upsized valves for the rest of the system so I had to get them elsewhere.

Once you've handled these two the next issue is to get the flow rate down. Most pools run far in excess of what is really required. If you reduce the flow rate then the energy usage will plummet.

Finally, and only after you've done the above, buy a more efficient pump. I know it sound strange, but this is the right order in which to spend your money. Look at it this way, if the pipes are sized right and the system design to run and low gpm's then you can always buy a better pump later but the reverse is not true.

BTW my system runs at less than 20 gpm 24/7. Check the energy usage on a system like that and you will be amazed. Even if it isn't an Intelliflo its going to be pretty low.

I agree that the larger pipe will have better heat transfer for a given set of conditions (i.e. same flow rate). But flow rate does play a role in the rate of heat transfer. This is why the water is hotter coming out a heater when flow rate is decreased. The BTU delivered by the heater may be a constant but the water temperature coming out of it is dependent on the flow rate. Higher flow rates means a larger volume of water to heat in a given amount of time. The reverse is true as well.

Heat transfer is dependent on the conductivity of the material, the temperature difference of the material and the time that a unit volume of water is in contact with the material. Lower temp difference, lower rate of heat transfer. Lower contact time, lower rate of heat transfer.

So the less time the water is in the pipe underground, then less heat will be lost. Not only that but when the heater is running the temperature difference between the water and ground will be much higher with the smaller pipe so even more heat will be lost than the larger pipe.

However, as I pointed out before, the heat loss in pipes is so small compared to loss in the pool walls, conductive heat loss and especially evaporative heat loss. Perhaps you missed the link I gave earlier but heat loss in swimming pools has been studied many times and have multiple articles regarding how much is lost. General consensus is that conductive heat loss through the walls and plumbing of a pool is less than 10% of the total heat loss. Evaporative and radiative heat loss is 90%. Thermal conductivity of concrete is about 2x the conductivity of PVC. In addition, the surface area of a pool and even a spa is much more than the surface area of piping. So the heat loss through the pool walls will dominate the conductive heat loss part (10%) and if you compare the pipe heat loss to all the other heat loss, it is ceratinly less than 5% of the total heat loss and probably less than 1%. The point is, it doesn't matter what is lost in the pipes because it is insignificant.

The real question one needs to ask is the amount of $ saved per $ invested. Whether it is larger, pipes, more efficient pump, larger filter one always wants to spend the money first where the savings is largest and then keep working down the list.

By this measure larger pipes (to a point) are one the best investments you can make. The cost to upgrade is small (labor is the same, only the cost of materials). In my book it is the very first thing one should do.

Next one should look at valves. Valves? Yes, if the goal is to reduce friction (head) so the pump doesn't have to work as hard (and therefore uses less energy) valves are a real source of loss. In fact the backflow valve can be one of the largest losses in the entire system. Again the cost to upgrade is almost nothing. The only downside is that manufacturers, being slow to recognize the advantages of low loss piping don't offer as many options in filters, valves etc in the larger low loss sizes. I had to call the local Pentair rep to get a line on their low loss backflow and even then they aren't yet in production of upsized valves for the rest of the system so I had to get them elsewhere.

Once you've handled these two the next issue is to get the flow rate down. Most pools run far in excess of what is really required. If you reduce the flow rate then the energy usage will plummet.

Finally, and only after you've done the above, buy a more efficient pump. I know it sound strange, but this is the right order in which to spend your money. Look at it this way, if the pipes are sized right and the system design to run and low gpm's then you can always buy a better pump later but the reverse is not true.

BTW my system runs at less than 20 gpm 24/7. Check the energy usage on a system like that and you will be amazed. Even if it isn't an Intelliflo its going to be pretty low.

" But flow rate does play a role in the rate of heat transfer. This is why the water is hotter coming out a heater when flow rate is decreased. The BTU delivered by the heater may be a constant but the water temperature coming out of it is dependent on the flow rate. Higher flow rates means a larger volume of water to heat in a given amount of time. The reverse is true as well. "

??????

We do a lot of combustion analysis on heaters for utility grants. Flue temperature is something that we measure during our analysis. Flue temperatures actually are much cooler with faster water flow, which typically means that more heat is entering the water. The temperature that the flame burns at is constant, when the inlet air temperatures are the same.

The problem with running water to fast through a exchanger is condensation, and in some cases erosion.

T

WOW, thanks guys for a lot og good information from true professionals. You bring up an interesting point about heat loss. I don't expoect is is very imnportant when heating the pool but whwn heating the SPA and using a heater that is a goodly distance away from the SPA heat loss might make a difference. I asked a couple of PBS if they ever heard of anyone insulating the pipe in the ground. Of course I never got an answer. I was thinking about insulating the SPA piping with 1/2" or 3/4" high density polyethelene ot closed cell urethane pipe insulation. If the insulation is sealed well it should add efficiency to the heating of the spa. I am a strong believer in doing little things that save money in the long run. If there is a 5 year payback it makes sense, especially as energy rates escalate. I'd be interested to hear if you have ever seen the pipes insulated. As mas985 says the loss is probably insignificant.

sbedelman makes good sense talking about valves. It doesn't make much sense using larger pipes and restricting the flow with tiny valves. Can you point me in the direction of the valves you used? Thanks.

Thanks for the Re number classification mas985...so now we know this is a turbulent flow comparison. I would tend to agree with racket and lean towards one pipe offering better heat loss properties when compared to a multiple pipe system under ground. This will likely depend on other factors that could make one configuration better than the other depending on the situation...this would be a good thesis project for a college student!

Sbedelman...well said..couldn't agree more with your levels of importance to maximize return on investment. I also ran into the same problems with valve availability. At the time I was building our pool, Jandy was a few months away from releasing their versa-plumb system with big low-loss valves and fittings. Did you check on these new valves from Jandy when you were building your pool? How are the venturi skimmers working out so far?

"Can you point me in the direction of the valves you used?"

Waterway Plastics and Jandy make large valves that will accept 3" pipe.

racket and trhought,

I think you may still be missing my point.

First, as far as heat transfer was concerned in my previous posts, I was comparing a single pipe at one diameter to another single pipe at another diameter. I showed flow rates for multiple pipes but I wanted to focus on the case with a single pipe for the suction and return lines. However, a group of multiple pipes will behave similarly to a single larger pipe so this analysis is applicable to a group of N x 1.5" pipe and a group N x 2.0" pipes. Either way, the analysis is meant to compare the different pipe diameters.

Second, I never argued the fact that the heat transfer rate of the larger pipe is more than the smaller pipe given the same water temperature in the pipes and the same time in the pipes. The issue is around the total heat lost due to the pipes in the total volume of water.

Next, separating the pool and spa into two different scenarios and including some examples will hopefully clarify things. The examples are simplified somewhat to make the calculations easier but the trends remain the same.

For the pool scenario (excluding a heater), assume the water temperature initially is the same for either pipe diameter. Since turnover for the pool is dependent on flow rate, the slower the flow rate, the longer the pump will need to be run for the same amount of water to be filtered. So for example:

10k gallon pool & 1 HP pump would have the turnover times for each size pipe:
2" pipe @ 89 GPM = 1.9 hour turnover
1.5" pipe @ 58 GPM = 2.9 hour turnover

The heat transfer rate in a cylindrical pipe is not directly related to the surface area of the pipe but is related to the difference in the log radius of the pipe and the soil surrounding it (see http://en.wikipedia.org/wiki/Heat_conduction). It is a somewhat complex calculation that is dependent on soil temperature distribution, soil heat transfer coefficients, depth of the pipe and even the air temperature above the soil. Using these assumptions:

Pipe Depth: 3 ft
Air Temp: 85F
Water Temp: 85F
Soil Temp: 70F @ 6 ft depth & 78F at 3 ft depth & 85F at surface
Heat Transfer of Dirt: 0.6 Btu/(ft hr F)
Pipe runs: 20 each direction

The heat transfer rate can then be estimated as:

Heat transfer rate in 2" pipe @ 89 GPM = 391 BTU/hr
Heat transfer rate in 1.5" pipe @ 58 GPM = 356 BTU/hr

So the heat transfer rate of the 2" pipe is bigger than the heat transfer rate of a 1.5" pipe. However, when the turnover rate is included, the total heat loss for each pipe is:

Heat loss in 2" pipe = 743 BTU/turnover or Delta T = 0.009F
Heat loss in 1.5" pipe = 1032 BTU/turnover or Delta T = 0.01F

The smaller pipe actually loses more heat over the course of a turnover. When the pump is off, only the water in the pipe loses heat which is small compared to the pool volume. But either way, you can see that the change in temperature is very very small due to just the pipe and not really worth considering.

For the spa, the situation is a bit different. Here we can assume the run times are the same but the temperature of the water coming out of the heater will be different for the different size pipe. The inlet to outlet water temp gain of heater can be calculated by:

Water Temp Gain = (Heater BTU/hr) / (GPM * 60 * 8.34)

So as the flow rate increases, the delta T decreases and the water temperature in the smaller pipe will have hotter water than the bigger pipe. For example with a 400k BTU heater:

Temperature Gain 2" pipe @ 89 GPM = 9 deg F
Temperature Gain 1.5" pipe @ 58 GPM = 13.8 deg F

Since the heat transfer rate is also dependent on the temperature of the water this will affect the transfer rate of each pipe size. So for example at a starting water temp of 85F the return path is:

Heat transfer rate 2" pipe @ 89 GPM & 94F = 430 BTU/hr
Heat transfer rate 1.5" pipe @ 58 GPM & 98.8F = 505 BTU/hr

The suction path is:

Heat transfer rate per ft of 2" pipe @ 89 GPM & 85F= 196 BTU/hr
Heat transfer rate per ft of 1.5" pipe @ 58 GPM & 85F = 178 BTU/hr

Both Paths are:

Heat transfer rate 2" pipe @ 89 GPM = 626 BTU/hr
Heat transfer rate 1.5" pipe @ 58 GPM = 683 BTU/hr

So even though the 1.5" pipe loses less heat in the suction path, it still loses more heat overall because the return path is worse. Also, as the water heats up, all the transfer rates will increase.

Of course after heat up the heater will cycle on and off and the average heat loss will depend on other factors. But if the cycle time is about 40%, then the heat loss in the pipes should be about the same for each pipe diameter.

My main point here is that flow rates need to be considered when determining the total heat loss in the pipe. But at least for the scenarios outlined above, the 2" pipe will actually lose less heat than a 1.5" pipe and it is all because of the higher flow rate.