Thoughts on heater internal bypass

[vsman]

This goes against most of the analysis in the forums - specifically the assertion that we do not need an external bypass since an internal bypass exists in the heater- And placing one will let water sit in the coils for an extended period of time.
I would like to disagree based primarily on the diagram below- please correct me if my analysis is wrong.
The internal bypass and the thermal regulator valves are what I focused on.
1- when the heater is off- the thermal regulator valve is closed off. All the water flow is forced to go through the internal bypass. Any water that is in the coils remains there. If you don’t use your heater for months, that water will remain the coils for months.
So when the heater is off - the drop in psi is primarily across the internal bypass valve.
2- When the heater is functional the thermal regulator valve opens to varying degrees based on the temperature of the water coming from the coils. At very high flows if the temperature change is not significant the valve closes and more of the water is shunted through the bypass valve. Also at high flows- the bypass valve prevents too much flow through the coils. ( same thing as prior sentence but for a different reason- mechanical and flow balance versus appropriate heat transfer)

in essence, an external bypass it’s completely different than the internal bypass of the heater. When the heater is nonfunctional the external bypass gives the benefit of reduced TDH added by the internal bypass valve. with a VS pump I can further drop my RPM and get the same flows with significant energy savings.
No numbers- I would guess the internal bypass valve has a cross sectional area of a 1.5 PVC pipe? And the spring load- a 3-4 psi drop?
A first order analysis- please correct if there are other systems and bits I have overlooked.

 

[ajw22]

This goes against most of the analysis in the forums - specifically the assertion that we do not need an external bypass since an internal bypass exists in the heater-

TFP does not assert that.

Heater Bypass - Further Reading

www.troublefreepool.com

I also recommend you run your heater regularly for maximum life. Letting the heater sit idle for long periods makes it attractive for rats to nest in it and destroy the heater internals as well as water sit stagnant.

 

[Turbo1Ton]

I also recommend you run your heater regularly for maximum life. Letting the heater sit idle for long periods makes it attractive for rats to nest in it and destroy the heater internals as well as water sit stagnant.

@ajw22 - Not trying to hijack, but since you brought it up, what is considered to be 'regularly'? Weekly, monthly? I am curious because now that it is extremely colder out, we are not using the spa as much. I think it has been a little over a month. I ran the heater this week, just to exercise it, as noted. But if I should be doing this weekly, I will.

--Jeff

 

[ajw22]

@ajw22 - Not trying to hijack, but since you brought it up, what is considered to be 'regularly'? Weekly, monthly? I am curious because now that it is extremely colder out, we are not using the spa as much. I think it has been a little over a month. I ran the heater this week, just to exercise it, as noted. But if I should be doing this weekly, I will.

--Jeff

I think once a week is more likely to keep critters out then once a month.

And running once a week gets rid of moisture that promotes rusting and corrosion.

 

[Turbo1Ton]

I think once a week is more likely to keep critters out then once a month.

And running once a week gets rid of moisture that promotes rusting and corrosion.

Good info... Thanks Allen

--Jeff

 

[1poolman1]

Unless you actually have a bypass valve that allows you to completely stop water flow to the heater, pool water is circulated through the heater every time the circulation system is on. This keeps the heater clean internally. Bypass valves were quite common many years ago as heaters were not designed to accept the full flow from a modern pump. They had to be adjusted on-site according to each pools hydraulics. That has not been the case in decades. Sometimes pool owners would use them to shut flow through the heater off completely thinking that it, somehow, saved a bunch of money.

When you shut flow off to a heater, as you said the water just sits there. It may stagnate but a more serious problem is that it evaporates leaving deposits inside the heat exchanger. These can build up to the point that water can't flow through the heater properly, the heater becomes very inefficient, and there are other issues that can arise. If you can get ahold of an older heater manual you will find that most manufacturers would have instructions on how to "de-lime" a heat exchanger with a special, very expensive, tool. They may still do this, I haven't looked in a while.
This procedure can be done on heaters with straight-thru heat exchangers but not with Max-E-Therm/Mater Temp and the Jandy JXi heaters.

The Jandy VersFlo option is the only internal bypass that I am familiar with that shuts flow to the heat exchanger when the heater is not heating. But, according to their literature, it does not eliminate that flow. It severely restricts it but some water does still circulate through the heater when it is not heating.

 

[Dirk]

Pardon the hijack, but maybe this adds to the discussion... There seems to be some amount of contradiction here. I have a Pentair MasterTemp 250 heater. It can't be older than 10. It does not have an external bypass. I have never used it. Not sure if the previous owners did or not. But it hasn't been run for at least six years. Do I have to worry about the internals, stagnant water, or anything else? I don't expect I'll ever use the thing, but I did not consider I wouldn't be able to if I wanted to at some point due to negligence...

What's the dealio?

 

[vsman]

Unless you actually have a bypass valve that allows you to completely stop water flow to the heater, pool water is circulated through the heater every time the circulation system is on. This keeps the heater clean internally. Bypass valves were quite common many years ago as heaters were not designed to accept the full flow from a modern pump. They had to be adjusted on-site according to each pools hydraulics. That has not been the case in decades. Sometimes pool owners would use them to shut flow through the heater off completely thinking that it, somehow, saved a bunch of money.

When you shut flow off to a heater, as you said the water just sits there. It may stagnate but a more serious problem is that it evaporates leaving deposits inside the heat exchanger. These can build up to the point that water can't flow through the heater properly, the heater becomes very inefficient, and there are other issues that can arise. If you can get ahold of an older heater manual you will find that most manufacturers would have instructions on how to "de-lime" a heat exchanger with a special, very expensive, tool. They may still do this, I haven't looked in a while.
This procedure can be done on heaters with straight-thru heat exchangers but not with Max-E-Therm/Mater Temp and the Jandy JXi heaters.

The Jandy VersFlo option is the only internal bypass that I am familiar with that shuts flow to the heat exchanger when the heater is not heating. But, according to their literature, it does not eliminate that flow. It severely restricts it but some water does still circulate through the heater when it is not heating.

However- to my original argument- external bypass or not- if you do not fire up the heater- the water remains in those coils and what you described above still has potential to occur.

 

[vsman]

TFP does not assert that.

Heater Bypass - Further Reading

www.troublefreepool.com

I also recommend you run your heater regularly for maximum life. Letting the heater sit idle for long periods makes it attractive for rats to nest in it and destroy the heater internals as well as water sit stagnant.

Noted- stand corrected not all posts- some posts.
In the further reading section- why do you need the check valve? The thermal regulator and internal bypass are one way valves.

 

[Dirk]

So you can bypass the heater altogether if you needed to (for repair, or if it was leaking)...

 

[ajw22]

Noted- stand corrected not all posts- some posts.
In the further reading section- why do you need the check valve? The thermal regulator and internal bypass are one way valves.

Heater bypass valve without the CV on the heater output will not work if the heater coil is leaking or if you want to remove the heater for maintenance or replacement.

 

[mas985]

in essence, an external bypass it’s completely different than the internal bypass of the heater. When the heater is nonfunctional the external bypass gives the benefit of reduced TDH added by the internal bypass valve. with a VS pump I can further drop my RPM and get the same flows with significant energy savings.
No numbers- I would guess the internal bypass valve has a cross sectional area of a 1.5 PVC pipe? And the spring load- a 3-4 psi drop?
A first order analysis- please correct if there are other systems and bits I have overlooked.

The head loss through the heater and actually any piece of equipment is proportional to the GPM squared so it is not a fixed amount. The lower the GPM/RPM, the lower the head loss.

With the regards to the RPM setting, while it may be true that you can run at a lower RPM for a given function (heating, SWG, skimming, etc) with an external heater bypass, I think you will find that the difference in energy consumption is not likely to save you a significant amount money when you are running at lower RPM.

Using my pool as an example:

@30 GPM, the pump uses 112 watts (7.2' of head) with the heater/internal bypass and 103 watts (6.3' of head) with the external bypass. Even at $0.30/kwh that I pay and 9 hours per day run time in the summer, it would save me only $0.73 per month. Hardly worth the effort and expense to add it.

 

[Dirk]

Pardon the hijack, but maybe this adds to the discussion... There seems to be some amount of contradiction here. I have a Pentair MasterTemp 250 heater. It can't be older than 10. It does not have an external bypass. I have never used it. Not sure if the previous owners did or not. But it hasn't been run for at least six years. Do I have to worry about the internals, stagnant water, or anything else? I don't expect I'll ever use the thing, but I did not consider I wouldn't be able to if I wanted to at some point due to negligence...

What's the dealio?

Can anyone answer this question for me?

 

[mas985]

Do I have to worry about the internals, stagnant water, or anything else?

IMHO, no.

First, it is my belief that the thermal shut off valve is likely less than 100% effective (non-flexible material for thermal stability) and it is very likely that at least a tiny amount of water gets by that valve.

But even if it were 100% effective, so what? Unless the water is significantly aggressive, there would be no harm having it sit in the coil. Also, I don't believe that the water would become aggressive over time since there is really nothing for it to react with that would alter PH and/or cause corrosion.