Which heats quicker High or Low speed pumping

[JohnT]

So High flow will heat faster but increase electrical cost since the pump costs more. I have to decide the if the trade-off is worth it.

Will it heat 10% faster 50% .. 80% Any guesses ?

The only advantage of the low flow would be less per hour pump cost. Assuming the pump's electrical use is in direct proportion to its flow, the high flow would win hands down because it is more efficient and the total amount of heat needing to be added is the same.

[chem geek]

The specifics of whether a high speed or low speed are more or less efficient is dependent on how your system curve (feet of head vs. flow rate) intersects your two pump curves. Let's just assume that the system curve intersects your pump curves at their most efficient points. At very slow flow rates the "fixed" inefficiencies of electrical and other losses make the overall efficiency lower. For example, the Intelliflow variable-speed pump has an output power (product of head and GPM scaled to appropriate units) to electrical power efficiency ratio and other parameters as follows:

 RPM   GPM   Head   Watts(Input)   Efficiency
3450   100    77       2780          52.1%
2070    60    28        316          50.2%
 690    20     3         95          11.9%

However, note that from a heating perspective, the output power is not relevant since it doesn't matter what the pressure is, only the flow rate. Essentially, half the flow rate is achieved at one-fourth the electrical power (the formula is more complex than that, but this is close enough). So lower pump speed is more electrically efficient from a flow rate (GPM) perspective, but such efficiency improvement is far less at very slow speeds (flows). I don't know the specs on your specific pump, but it would certainly be true that the slower pump speed would be more electrically efficient in terms of GPM (even if efficiency relative to output power were the same).

As for how much of an efficiency difference there is vs. flow through a gas heater, I do not know, but I can refer you to what happens in solar panels at this link where the efficiency through a panel at 1 GPM was 60%, at 2 GPM was 70%, and at 4 GPM was 80%. Obviously at some point, doubling the flow rate won't continue to add 10% since you can't get to 100% efficiency. Typical gas heater efficiencies are around 80% unless you have a super-efficient heater at 90%. These are probably rated at typical flow rates so my best guess is that at your lower flow rate which is 37.5% of your higher flow rate, that the efficiency difference might be around 15% less. So if your high flow rate was 80% efficient, then perhaps your low flow rate might be 65% efficient.

Furthermore, be aware that gas heaters not only specify a maximum flow rate (typically around 120 GPM), but also a minimum flow rate that varies depending on heater size. The minimum flow rate appears to be approximately 1/10,000th the output BTU so your 400,000 BTU heater (assuming that's output BTU and not input BTU) probably has a minimum flow rate requirement of 40 GPM. This is to make sure that the heat transfer pipe and other internals of the heater do not get too hot. If the hot is not transferred as efficiently, that means that something else (i.e. the heater itself) gets hotter and that can cause damage.

So this is a hard one to determine because the slower pump speed, if it isn't TOO slow, will cost you a lot less overall though you will heat your pool perhaps 15% more slowly due to the lower thermal efficiency. However, if the slow pump speed is very electrically inefficient, then you may not realize a net cost savings (though my hunch is you probably still will). Also, if the pump speed with the 30 GPM is lower than your gas heater can handle, then obviously that is not an option for you.

Richard