recirculating pump size

[dh100m]

trying to get my rheem rtgh 95x tankless to work in closed loop with home made air handle
put in a taco 007 pump but it will not turn on the heater not enough pressure i think
people at rheem says the turn on pressure needs to be at least 25psi
does any one have experience that can help me

 

[DonL]

Not sure your problem is pressure, It could be Flow.

What is the flow on your homemade system ?

If it is a pressure problem, you may be able to add some restriction so pressure can build up.


Good luck on your project.

 

[Sponsor]

 

[Dana]

If you pump away from the tankless it lowers the pressure at the tankless. If you pump toward the tankless it raises the pressure.

Is this a closed system, or are you pumping potable water through the air handler? If the latter, what is the static pressure on your potable system?

There needs to be an expansion tank on the system on the input side of the pump to limit cavitation wear on the pump. If it's potable water, it needs to be a bronze impeller version, not iron, or you'll go through pumps pretty fast.

There needs to be a minimum flow for the tankless to fire up too, as noted by DonL. While it's unlikely that a Taco-007 wouldn't provide sufficient flow pumping against the modest head of a tankless HX + air-handler coil, any blockage or valves in the system that impede flow to under 1 gpm could be an issue.

 

[LLigetfa]

If you pump away from the tankless it lowers the pressure at the tankless. If you pump toward the tankless it raises the pressure...

I am trying to understand this. The OP described it as a loop so it would be both from and to.

The OP also described it as closed loop so there might not be the minimum 25 PSI that the tankless purportedly needs to operate.

 

[DonL]

Most of them heaters require a input pressure of some safe setting, just to insure there is water to heat.

If pressure it good, then it requires a AND logic with water Flow.

One of many safety devices.

If it is a true Closed system you need to add pressure, then if the pump can not keep up with flow, It is a No Go.


Me Thinks.

 

[Dana]

I am trying to understand this. The OP described it as a loop so it would be both from and to.

The OP also described it as closed loop so there might not be the minimum 25 PSI that the tankless purportedly needs to operate.

It's only from and to if you don't factor in the flow impedances imparted by both the heat exchanger in the tankless and the coil in the air handler (and the plumbing in the loop).

If you place the pump directly at the output of the tankless, it's trying to suck against all of those impedance elements. That makes the pressure at the air handler coil higher than the average system pressure, and the pressure at the output of the tankless the lowest pressure point on the system whenever the pump is running. These pressure differences are not just 1-2 psi. The actual pressure differences can be calculated if you know the "pumping head" of the coil, the tankless, and the loop plumbing. The higher the flow, the bigger the pressure differences (and conversely.)

Even if you pre-pressurize the system to 25psi (and keep it roughly there with an expansion tank) it doesn't take a huge flow to end up with the pressure at the tankless dropping below the requisite 25psi, when the output of the tankless heat exchanger is pulled down a handful of psi.

Since it doesn't matter what the pressure on the plumbing or air handler coil are, you gain advantage by placing the pump directly at the input of the tankless, which makes the input of the tankless the dynamic pressure HIGH point on the system.

The tankless controls don't typically care what the pressure is when there is no flow, only when it detects flow and is deciding whether it's OK to run the burner.

 

[LLigetfa]

I did not factor that the coil would be a significant flow restriction. I thought the greatest flow restriction would be on the output of the tankless. The Taco is rated to more GPM than the tankless.

 

[Dana]

The greatest flow restriction probably IS the tankless, which is exactly the reason you want to pressurize rather than depressurize it, to the extent possible.

Treating the intermediate plumbing and air handler coil as if it has low to no pumping head would be in error.

 

[DonL]

It all depends if you want to be sucked or blowed.

That pump should work but,

If it can't suck, it can't blow.

 

[LLigetfa]

If it is placed between the tankless outlet and the coil inlet, it will have less propensity to cavitate since the restriction on the intake of the pump would be less. The head on the pump though would be more since there is more friction loss between the pump outlet and the tankless inlet.

Anyway... I would expect the the flow restriction on the coil would not be significant enough to put it outside the 25 PSI prerequisite of the tankless.

 

[Dana]

Placing the expansion tank at the sucky end of the pump mitigates cavitation no matter where you place it in the loop.

The "...restriction on the intake of the pump..." is HIGHER if it's pulling from the highest-impedance section of plumbing, namely the tankless.

Air handler coils are not typically low-head at 3-6 gpm, but would be lower head than a tankless.

The biggest pressure drop on any part of the plumbing will be across the heat exchanger of the tankless, which is why you want to pump toward it in close proximity to the tankless (=low head plumbing between pump & tankless.) That makes the input to the tankless the highest-pressure point on the system. With the impedance of the loop plumbing and air handler coil in the path between the output of the tankless and the sucky side of the pump, the output of the tankless is higher than the average system pressure too. If you pump away on the output of the tankless it makes the output side of the tankless the lowest pressure point on the loop, which is something you're trying to avoid.

In short:

Pumping toward the tankless makes the input of the tankless the highest pressure point on the loop

Pumping away from the tankless makes the output the lowest pressure point of the loop.

Whether the pressure drop across the coil is 0.5 psi or 2 psi, and the pressure drop across the tankless is 4 psi, pumping toward the tankless will always yield the more favorable pressure. If the static pressure on the system is 30 psi, pumping away risks dropping below the 25 psi spec at least at the output side of the tankless. Pumping toward the tankless means that the input will always be above 30psi, and the pressure at the output side will be as high as it can be, but dependent upon the pumping head of the loop plumbing + coil.

 

[LLigetfa]

If the static pressure on the system is 30 psi, pumping away risks dropping below the 25 psi spec at least at the output side of the tankless.

Without seeing the technical details of the tankless, I am going on the assumption that the 25 PSI minimum is most likely a safety to assure adequate supply. I cannot imagine that they would be sensing it on the downstream side after their self-imposed flow restriction.

 

[LLigetfa]

Placing the expansion tank at the sucky end of the pump mitigates cavitation no matter where you place it in the loop...

Is it common practice to put an expansion tank on the hot side? I thought they would go on the cold side.

 

[Dana]

Without seeing the technical details of the tankless, I am going on the assumption that the 25 PSI minimum is most likely a safety to assure adequate supply. I cannot imagine that they would be sensing it on the downstream side after their self-imposed flow restriction.

Which is exactly why having the pump located directly at the supply of the tankless is preferable, since that takes all of the head from other elements in the loop completely out of the equation. It simply doesn't MATTER what the head of the plumbing and air handler coil are if you pump toward the tankless in close proximity to the tankless. If you are pumping away from the tankless those other pressure drops matter.

The common (or at least the correct) practice it to place the expansion tank on the input side of the pump, which nulls the pressure variations there, limiting cavitation. That is true independently of whether the pump is on the hot or cold side of the boiler/tankless.

 

[DonL]

That tank-less system is not made for that application, Period.

Can it be made to work, Yes. Adding a tank is not the answer.

Should You do it, Hell NO. Unless you understand how it works.

Not knowing what you are doing is not a Plus. +

 

[Dana]

Air handler/tankless solutions are sold pre-packaged by from some tankless vendors (eg Rinnai) , with all of the pumping & coil math already worked out. It's a bit presumptious to say that they're not made for that purpose- some clearly are, though with others ANY space heating application voids the warranty. I have no idea whether Rheem would take issue with it.

Takagi actively encourages use of their tankless units in hydronic heating applications.

 

[DonL]

I get your point.

The "trying to get my rheem rtgh 95x tankless to work in closed loop with home made air handle"

May not fly, and can have many variables, Including HUA.