Trouble with my Hayward 250FDN throwing LO error code

[gregg_hamilton]

I own a Hayward 250FDN Pool Heater and it starts intermittently. When the heater is cold I increase the flow rate on my pump as it is variable speed drive so that the LO flow error code clears. Once the low flow (LO) error code clears I switch the main panel from Standby over to Pool heating mode and then the heater ignites and starts heating the water. The heater runs for about 30 seconds and then it stops and the LO error code returns and Service LED is now lit up as well. If I wait for another 30 seconds, the LO error code disappears including the Service LED and heater goes into Standby mode. When I switch it back over to heating mode the second time it tries to ignite but then it throws the LO error code before the burner starts up.

At this point i suspect the water pressure switch is faulty but was hoping I could an opinion from this panel.

Thanks so much in advance.

 

[cptkirk]

When you increase the pump speed, what are you increasing it to? Also, is it possible that your filter is restricting flow so that the input is marginal?

 

[gregg_hamilton]

I increase the speed up to about 2380 rpm to 2420 rpm. I haven't changed the filter cartridges for a while and there currently is a 10psi pressure showing on the gauge on the top of the filter.

 

[gregg_hamilton]

There seems to be a good flow going into the pool so it doesn't visually appear that the filter is causing a restriction.

 

[gregg_hamilton]

I will try pulling the filters out to see if still throws the LO error code.

 

[cptkirk]

I have a H400FDN, which requires a minimum of 30 gpm (yours is 25 gpm). If my filter is clean, the minimum speed I can run the pump is 40% (1380). I normally run at 50% (1725), but when the filter gets a bit dirty the heater will throw an LO at that speed. Worth making sure you have sufficient flow before going further in troubleshooting.

 

[JamesW]

What is the elevation of the heater relative to the pool?

 

[gregg_hamilton]

Okay great, thanks @cptkirk, will pull the filters out after work today and give it a try.

The elevation of the heater relative to the pool @JamesW is no more 2'-6". It has been running for about 6 years though so don't suspect this represents a challenge unless you had another perspective. Thanks in advance.

 

[JamesW]

Sometimes installing eyeballs helps.

 

[gregg_hamilton]

Okay, that's fair, as it will result in more pressure being available to the heater as a result.

 

[swamprat69]

Having trouble understanding why you are trying to run your heater at the absolute minimum flow rate? At minimum flow rate, the exiting water temperature will be higher than at a higher flow rate but the total quantity of heat transfered will be lower. At a higher flow rate the exiting water temperature will be lower causing a higher temperature difference between the water in the heat exchanger and the flue gas resulting in a greater heat transfer. The flue gas temperature exiting the heater will also be lower since you are transferring more heat.

 

[JamesW]

Having trouble understanding why you are trying to run your heater at the absolute minimum flow rate? At minimum flow rate, the exiting water temperature will be higher than at a higher flow rate but the total quantity of heat transfered will be lower. At a higher flow rate the exiting water temperature will be lower causing a higher temperature difference between the water in the heat exchanger and the flue gas resulting in a greater heat transfer. The flue gas temperature exiting the heater will also be lower since you are transferring more heat.

The temperature of the combustion gas will be about 1,492 degrees Fahrenheit.

The average temperature of the water in a heat exchanger with an inlet temperature of 80 degrees will be about 88 degrees Fahrenheit at minimum flow.

The average temperature of the water in a heat exchanger with an inlet temperature of 80 degrees will be about 84 degrees Fahrenheit at double minimum flow.

The temperature difference will be 1,404 or 1,408 degrees.

1,404 ÷ 1,408 = 99.72%.

So, the difference in efficiency between minimum flow and higher flow is negligible.

Also, at higher flow, more water is flowing through the internal bypass.

So, even if you go to double the flow, a lot of the extra flow is not going through the heat exchanger anyway.

 

[JamesW]

Here is the temperature rise for a 400,000 btu per hour heater vs flow.

At a minimum flow of 40 gpm. the temperature rise is about 16.8 degrees from inlet to exit or about 8.4 degrees average.

Inlet 80 degrees. Outlet 96.8 degrees. Average temperature in the heat exchanger 88.4 degrees.

At double minimum flow of 80 gpm, the temperature rise from inlet to outlet is about 8.4 degrees or about 4.2 degrees average.

Inlet 80 degrees. Outlet 88.4 degrees. Average temperature in the heat exchanger 84.2 degrees.

 

[swamprat69]

In first example, you are subtracting water temperature TD from flue gas temperature??? In second example water temperature TD is ~ 4.75% of total water temperature but 50% of actual TD. In any case at a lower flow rate you are wasting input heat.

 

[JamesW]

The temperature of the combustion gas will be about 1,492 degrees Fahrenheit.

Heat transfer depends on temperature differential.

At 40 gpm, the average temperature differential between the combustion gas and the water inside the exchanger is about 1,404 degrees Fahrenheit.

At 80 gpm, the average temperature differential between the combustion gas and the water inside the exchanger is about 1,408 degrees Fahrenheit.

The temperature differential is almost exactly the same, which means that the heat transfer is almost exactly the same.

1,404 ÷ 1,408 = 99.72%.

The heat transfer at 40 gpm is at least 99.72% of the heat transfer at 80 gpm.

There's only a 0.28% difference.

The amount of power required to move the water at double the flow is 6 to 8 times as much.

If it takes 250 watts to move 40 gpm, it will take at least 1,500 watts to move 80 gpm.

The internal bypass is designed to bypass any excess flow around the heat exchanger.

So, even if you go from 40 to 80 gpm, the amount of flow going through the heat exchanger is only going to increase by a marginal amount.

The flow through the exchanger might go to 50 or 60 gpm at best.

So, going above the minimum really does not help and it costs a lot of energy in pump power input to do it for no gain.

 

[JamesW]

See this chart where the pump uses 239 watts at 1,600 rpm and 1,800 watts at 3,200 rpm.

That's 7.53 times as much power to move the water at twice the speed.

That's a lot of power to use to try to save less than 0.3% of the heat produced by the heater.

You will save about 1 cent per hour in natural gas but it will cost you about 16 cents per hour in electricity.

 

[JamesW]

If the air entering the heater is 80 degrees Fahrenheit and it exits the heater at 320 degrees, then it has gained 240 degrees.

If the heater is 84% efficient, the amount of heat that caused the temperature rise for a 400,000 btu/hr heater is about 16% of 400,000 btu/hr = 64,000 btu/hr.

If 64,000 btu/hr causes a 240 degree temperature rise, then 400,000 btu/hr will cause a 1,500 degree rise.

If the air entering the heater is 80 degrees and the temperature rises by 1,500 degrees, the combustion temperature is 1,580 degrees.

As the air passes the heat exchanger, 336,000 btu/hr transfers from the combustion gas to the water through the heat exchanger, which lowers the temperature from 1580 to 320 degrees.

If you wanted to check the efficiency difference between the minimum flow and a higher flow, you could run the pump at the minimum flow and then check the exhaust temperature.

Then, increase the flow by 10 gpm or so and recheck the exhaust gas stack flue temperature.

If the stack flue temperature is lower at higher gpm, then that suggests that the efficiency is improved.

To check the stack flue temperature, turn the heater on. The fan will come on for about 30 seconds, then you will hear a click, and the heater should fire.

At that moment, which ever thermostat you are using (POOL or SPA) hold that button down for ~10 seconds.

Display should change from water temp to exhaust temp.

Normal temps should be in the range of 290-350.

Each degree lower represents about 267 btu/hr of extra heat saved.

267 ÷ 400,000 = 0.067%

For example, if the exhaust temperature went from 320 degrees to 310 degrees, that would be about 2,670 btu per hour or about 0.67%.

The cost to run a 400,000 btu/hr heater is about $3.81 per hour.

0.67% of $3.81 = 2.55 cents.

The cost to run a pump is about 10 cents per hour for 1,000 watts.

The breakeven point is if the extra pump power is 255 watts more.

In other words, if you increase the pump flow and it costs an extra 255 watts to get a 10 degree reduction in exhaust temperature, the savings in gas cost equal the extra costs in electricity.

If you can get a 1 degree reduction in exhaust temperature for less than 25 watts, it’s worthwhile increasing the pump speed.

 

[JamesW]

At a minimum flow of 40 gpm. the temperature rise is about 16.8 degrees from inlet to exit or about 8.4 degrees average.

Inlet 80 degrees. Outlet 96.8 degrees. Average temperature in the heat exchanger 88.4 degrees.

At double minimum flow of 80 gpm, the temperature rise from inlet to outlet is about 8.4 degrees or about 4.2 degrees average.

Inlet 80 degrees. Outlet 88.4 degrees. Average temperature in the heat exchanger 84.2 degrees.

which lowers the temperature from 1580 to 320 degrees.

We also have to look at the average temperature of the exhaust.

If it hits the heat exchanger at 1,580 degrees and exits the heat exchanger at 320 degrees, the average temperature is 950 degrees.

So, the difference is 950 - 88.4 = 861.6 degrees.
and 950 - 84.2 = 865.8 degrees.

861.6 ÷ 865.8 = 0.995149 or 99.5149%.

So, a 0.4851% difference.

However, that's if all 80 gpm goes through the exchanger, which it won't.

The best way to tell if extra flow will improve efficiency is to check the stack flue temperature at minimum flow and then at 10 gpm higher to see if the stack flue temperature is lower.

If it remains the same, then efficiency is the same.

If it drops, then efficiency is improved.

The flue temperature will naturally fluctuate and you have to eliminate the noise to see any real efficiency improvements.

Normal variations will probably be several degrees.

 

[JamesW]

The thermal regulator opens at or above 120 degrees Fahrenheit.

So, even if the flow going to the heater is increased, the temperature in the heat exchanger won't be below 120 degrees.

Once you meet the minimum flow requirements, it is unlikely that any additional flow sent to the heater will go through the heat exchanger.

The best that you can do is about 121 degrees in the heat exchanger.

[Edit} This heater does not use a thermal regulator.[End Edit]