Replacement Pool Pump

[Figgy]

I need help with determining what size pool pump to purchse. The current pump has burnt out after years of service.

While I realize the golden rule is to replace it with a pump of the same size/capacity, unfortunately the markings on it have worn off, and I do not know its stats.

This is a 40 year old pool with 1 1/2" copper plumbing running on the supply and return lines. The aboveground plumbing has generally been replaced with 2 inch PVC.

This is a 12,000 gallon IG. I have a 48 sq. Ft. DG filter. I have a Maxi-therm 330k BTU heater of recent vintage. The current pump is a brass Purex on the wet end and it appears as if the prior owner had the motor rebuilt or repalced a few years ago.

I don't know the DHP and I can't measure it with a broken pump. The filter PSI ran about 15 clean and twice that dirty. There are two supply lines from pool (one skimmer and one main drain) and one return line which splits in two somewhere below ground. The runs would be about 20' for the supply and 30" for the return. All the equipment is virtually level with poolsisd.

From everything I've read, here and elsewhere, the limiting factor in my case is the 1-1/2" pipe which can only take @ 44 GPM. The local pool people I've spoken with advise to go with at least a 1 HP pump based on the relatively large size filter, but I question that given the siz of the pipe and the potential strain on an aging system.

To demonstrate: Assuming that the DHP is in the nieghborhood of 50, a Whisperflo WFE-3 at 1HP with SF of 1.67 (BHP = 1.25) would have a flow rate of approx. 65 GPM. This is less than the optimal (73 gpm) for the 48 sq. ft. filter and appears to be too much for the 1-1/2" plumbing line. A smaller pump would reduce the efficiency of the filter even more, but would help on the piping issues.

What to do? Am I missing something?

Thanks for your help on this.

[Mark]

Flow rates for filters are normally speced as maximum flow rates. Optimum is usually less than 75% of maximum but you get better filtering with lower flow rates so lower is better. Also, given the size of your pool I would go with as low a HP as possible unless you need higher flow rates for something such as a water feature or spa. You might want to consider a two speed or variable speed pump to save some on energy costs.

[Figgy]

Thanks for the reply Mark.

The maximum on the filter is about 94 gpm, so the 73 GPM for the filter was the "optimal." We do have a attached spa, but since it has the same pipe limitations of 1-1/2", and the pool is switched off when the spa is on, I don't see how a larger pump would help. If the pipe can only handle 44GPM that prevents any benefit from a larger pump, correct?

I thought about a dual speed, but even the smallest I could find, a 3/4 HP with a SF of 1.67 yields a total brake HP of 1.25. While, I would use the lower setting for usual service, and could try to use the higher setting for spa, backflowing and chemical dispersion, again aren't i limited to the 44 GPM due to the piping? So the dual speed might really be ineffective as I would be stuck at using the lower power.

What problems might I have with the heater or the filter with an underpowered pump?

Thanks

[ps558]

Thanks for the reply Mark.

The maximum on the filter is about 94 gpm, so the 73 GPM for the filter was the "optimal." We do have a attached spa, but since it has the same pipe limitations of 1-1/2", and the pool is switched off when the spa is on, I don't see how a larger pump would help. If the pipe can only handle 44GPM that prevents any benefit from a larger pump, correct?

I thought about a dual speed, but even the smallest I could find, a 3/4 HP with a SF of 1.67 yields a total brake HP of 1.25. While, I would use the lower setting for usual service, and could try to use the higher setting for spa, backflowing and chemical dispersion, again aren't i limited to the 44 GPM due to the piping? So the dual speed might really be ineffective as I would be stuck at using the lower power.

What problems might I have with the heater or the filter with an underpowered pump?

Thanks

1 hp medium head pump will work great. hayward Super Pump

[Pool Clown]

1 hp medium head pump will work great. hayward Super Pump

+1

I would take the suggestion from your local pool guy, "At least a 1 Hp" and change it to 1Hp at the MOST.

[Mark]

Thanks for the reply Mark.

The maximum on the filter is about 94 gpm, so the 73 GPM for the filter was the "optimal." We do have a attached spa, but since it has the same pipe limitations of 1-1/2", and the pool is switched off when the spa is on, I don't see how a larger pump would help. If the pipe can only handle 44GPM that prevents any benefit from a larger pump, correct?

I thought about a dual speed, but even the smallest I could find, a 3/4 HP with a SF of 1.67 yields a total brake HP of 1.25. While, I would use the lower setting for usual service, and could try to use the higher setting for spa, backflowing and chemical dispersion, again aren't i limited to the 44 GPM due to the piping? So the dual speed might really be ineffective as I would be stuck at using the lower power.

What problems might I have with the heater or the filter with an underpowered pump?

Thanks

No the pipe size really doesn't limit flow rate only slows it down. The so called flow rate limits for pipes are more like suggestions and not hard limits. The recommendations take into account, head loss, noise levels and water hammer pressure so when the flow rates exceed the recommendations, the piping is more likely to be damaged in some way and the head loss gets to be excessive. But in reality these limits are pretty soft and usually have a margin of over 100% so exceeding them isn't all that critical.

However, if you don't need high flow rates, there is no reason to go with a high HP pump. If you can find a two speed in 3/4 HP, I would go with that. Again exceeding flow rate on high speed shouldn't be a problem.

[quantumchromodynamics]

I think that a flow rate of about 45 gpm would be good.

Note: In California, all new residential pool pumps 1.0 total horsepower (THP) or greater must have two or more speeds.

[Figgy]

Thanks for the suggestions.

Since the pipe size does not seem to be as much of a limitation as I though, I'm thinking I will go with a dual speed. I can avoid some significant re-plumbing by staying with the old wet end and replacing only the motor. I have determined that it is a Purex AH Series and based on the impeller size, I have been told it is probably for a 1 or 1 1/2 HP motor. I would go with a 1 HP with a sf of 1.3 (total effective HP of 1.3) and a low power of @ 1/2HP.

Is there any problem with using the old wet end with a dual speed motor?

[Mark]

There is no problem using the old wet end as long as it is in fairly good condition. Just make sure the new motor's BHP is at least as large as the old motor or it may be over loaded.