Topic split from locked post located here for purposes of educational discussion and substantiation of potential Heat Pump damage claims, discussed further below. The continuation of discussion will only be allowed as long as the replys remain free of public personal dispute, which if necessary should be handled via PM - dmanb2b

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This is totally incorrect. Any NG/LP heater has to go last in line after the filter. Never put a NG/LP heater before a Heat Pump, period.

Additional, are the multiple heat pumps in series or parallel? All heat pumps need to be in parallel, in other words, they all need to be drawing fresh cold filtered pool water and passing it back to the pool (not to any other heat pump) or on to a NG/LP heater.

Do you mind expanding on that "period" and explaining why, so that we may learn from your knowledge

Also, your statement about the heater having to go "last in line" has me confused vs. the statement about plumbing in parallel .

I'd like to see the reason for that as well. We're talking pool heaters here, so you're only heating the water a few degrees per pass.

Yes what you said is true, but that is only true after the water mixes back into the thousands of gallons of pool water, not the current gallon in the 2 inch line direct from the NG/LP heater! The very hot water directly from an NG/LP Heater (remember its a 300 to 400K BTU heater) will ruin a heat pump if it is running and being feed water from the outlet of a running NG/LP Heater IE there is no where to "Exchange" the heat made by the Heat Pump.

Why would the heat pump even be running if you were using a gas heater? And if you hook it up as you say in parallel, its not an issue anyway.
If you run hot water thru the heat pump and it's not running, there is still no issue. I'm not even sure its an issue if it were running. I've heard this argument before, but not sure it has been substantiated.

I personally prefer to see gas fired units after heat pumps. The main reason has to do with the amount of heat the return side of each unit will inject. A gas fired unit's return flow is more likely to approach a heat pumps set point, shutting it off. There goes an extra 75 to 90 KBTUs that could be injected into the flow.

Bypasses can and should, but not must, be added in systems that push less than 110 GPM. Systems that push higher flows should be configured in an off-line configuration. That, though is usually how you would find a commercial pool like in an aquatic center (YMCA).

Scott

That one i'll buy.

The damage argument, eh, not so much

20 years ago, the damage issue did hold some merit. With heat pumps, the use of titanium has eliminated issues there and Cu-Pro Nickel in gas fired units has reduced erosion and chem wear significantly when compared to straight copper exchangers.

While non-titanium and straight copper units are available, fewer dealers sell them.

Scott

Psst...bk gave that recommendation in his first post to this thread...

Can you define very hot? Assuming that 2 inch line you are referring to is made from PVC wouldn't that piping show damage before any inline heat pump's metal element was able to be damaged? Again, for full disclosure, I do not own either a HP or NG/LP heater, but wouldn't the output plumbing of an NG/LP Heater required to be metal if it is was really that hot?

I am not at all questioning the efficiency debate and I understand PoolguyNJ's point about tripping the HP set temp, I just question how even let's say 105 deg water can damage the HP element. I also agree with the suggestion about plumbing heaters in parallel, but quite frankly I do not think Bama was that far off in his statement as you are making it out to be. Heck, I would even consider adding a gas heater if the rise in temp is that significant.

When water passes through a heater, most is bypassed internally. The water coming out of the exchanger then mixes with the rest of the flow, cooling it.

Sometimes, the heat is too much for schedule 40 pipes and a few feet of schedule 80 is needed.

Metal is never used anymore.

Scott

I still stand by my original statement. In the context of the OP's situation it won't make much difference.

Dman said it best. Before you can do any damage to an HP that PVC pipe between the two is gonna have serious problems.

Nobody said the heat pump would be running, be it could be couldn't it? The question was were should a NG/LP heater be placed, common sense says always put the NG/LP heater last after all other heaters.

The Parallel only applies to multiple heat pumps on the same pool not necessarily multiple heater types. Multiple different heater types only need be placed in series in the proper heater type order as in Solar, Heat Pump, NG/LP Heater back to pool.

And yes if you run hot water through a heat pump that is not running there is no issue, but you can ruin a running heat pump if you run hot water directly from a NG/LP heater into a running heat pump so why chance it, just put the NG/LP Heater last. And its not a wife's tale, its true! What would happen to your car engine if after the water went through the radiator you reheated it hotter then it was before it entered the radiator and then sent that to the running engine? I will tell you, in time a ruined engine! A heat pump is designed to exchange heat it creates to water through the heat exchanger and that heat has to be removed. If the water the heat exchanger is in is hotter then the total exchange amount, heat will not be removed and the heat pump can be damaged.

Of course there is no law that says it has to be this way, more over, its a free country and people can still do what they want in their own back yards. But even having said all that, again common sense says that one wants to avoid any unnecessary problems/accidents and that means that a NG/LP Heater should always go last after the Filter and before the Chlorinator. Its just plain old good common sense and proper pool practices.

Have any references for any of this"damage" u say that can happen if the HP is running and warm water is running thru it? We usually don't deal with undocumented claims on this site that are dubious at best.

This thread is locked until we make a determination on splitting some of it to The Deep End, deleting parts of it or both. The discussion, while not uninteresting, is not really helping the OP.

Everyone please try to stay more civil when/if it's reopened.

Topic has been re-opened and edited to omit personal or interpreted to be personal comments. Thanks

When properly installed a gas heater should only raise the water temperature by a couple of degrees (ideally less than 10, but sometimes a bit more than that), which would not make much difference. There are going to be some exceptions, a large heater used in a fairly low flow rate system when the filter is very dirty could raise the water temperature by 20 to 40 degrees, but by and large the water temperature will never be hot enough to damage anything.

Of course, heaters are not always properly installed. It isn't too difficult to imagine a situation where an improperly designed plumbing system was allowing the heater to raise the water temperature by more than 40 degrees. In that range, a heat pump that skimped on protection circuitry could have problems.

Efficiency of the heat pump is a much clearer issue. Heat pump efficiency is fairly sensitive to the incoming water temperature. Higher water temperatures mean less efficient heating. This is the best reason to place a gas heater last. Gas heaters are much less sensitive to the incoming water temperature.

There will be a much higher peak temperatures right near the heater if the pump and heater turn off at the same time. Heat inside the heat exchanger continues to heat the water after the flow stops. This hot water won't migrate too far, because the pump is off, but it can cause problems very near the heater. Ideally, the heater should turn off before the pump to allow the heat exchanger to cool down. If it doesn't, the water in the pipe within a foot or two of the heater can get extremely hot. Typically, special high temperature pipe (generally either gray CPVC or metal pipe) is used for a foot or two after the heater because of this.

If a second heater, or a SWG, or any of several other things, is plumbed in directly after a gas heater, without a couple of feet of pipe between them, and the pump doesn't provide a cool down period, there can sometimes be real problems. Some heaters are designed to minimize this risk, using heat exchangers with lower thermal mass and/or putting in spring valves to stop the water flow when the pump goes off and keep the heat inside the heater.