Tankless water heater water pressure drop....

[spf8298]

We’ve got a 3bay self serve with an IBA. We’ve got two Paloma ph24 mechanical tankless water heaters that supply us hot water to the self serve functions and hydrominders. We are having some issues with the water pressure dropping so much On the the hot side on busy days that the pumps cavitate. We don’t have any pump cavitation on the ‘cold’ water functions no matter how busy it is. I understand that tankless heaters operate off of flow, but my question is what is the proper size copper piping that should be feeding these heaters? I wonder if the piping supplying the heaters isn’t large enough. The piping is 3/4”. Any input would be appreciated.

 

[Dan-Ark]

Found a manual online, it appears each of your PH24 has about 3 gpm capacity. 3/4 is the correct size pipe for that heater. The colder the water coming in, and the higher the temp you have it set for the longer the water has to be in the burner. the heater restricts flow to maintain temperature. If your pulling more than 5 gallons per minute out of both heaters, you are going to get flow restriction. I am using similar heaters at my wash, however the only HP function that is hot is soap, a solenoid opens up that feeds hot water under city pressure to the pumps but if it isnt there, the pump will still pull from the tank as it does for cold functions. If you pumps are feeding from a tank, a back up float valve set to come on a if the water gets to a lower point than the hot water valve can be plumbed to cold water to keep the pumps happy if hot water runs short. this is the main disadvantage of tankless. I went with a boiler in my second wash.

 

[JGinther]

Actually, everything you read online about how a tankless water heater works doesn't apply on that particular Paloma water heater. Yours is the original 'old school' type which is purely mechanical. It will flow whatever it can and heat by whatever you turn the knob to. It has no idea what it's outlet temperature is. It modulates the heat by a Venturi activated diaphragm, which may restrict the water flow more than normal if it's starting to rot or has foreign material in the line. Typically you would need 1 heater per self serve bay if you are in an area with a consistently freezing winter.

 

[spf8298]

Actually, everything you read online about how a tankless water heater works doesn't apply on that particular Paloma water heater. Yours is the original 'old school' type which is purely mechanical. It will flow whatever it can and heat by whatever you turn the knob to. It has no idea what it's outlet temperature is. It modulates the heat by a Venturi activated diaphragm, which may restrict the water flow more than normal if it's starting to rot or has foreign material in the line. Typically you would need 1 heater per self serve bay if you are in an area with a conistently freezing winter.

So if I back the heat off by turning down the knob, will that theoretically allow it to flow more water?

 

[MEP001]

No, because JGinter is right. The PH24M doesn't throttle the water flow by design.

 

[soapy]

If you look at the inlet side of your Paloma you will see it probably has a restrictor installed. You can remove this and get more flow through the heater but with a little less heating of the water. That is how I have been running mine for 25 years.

 

[Randy]

Not being there or seeing pictures of the layout it's pretty hard to say what the problem is. I've got 1 on demand water heater for every bay. 2 of them have recently died. After watching a few customers wash I noticed that 90% of them only have the high soap on just long enough for the hot water to get out to the bay, so now I wondering do I really need hot water.

 

[wash4me]

A good fix is to put a float hooked to a solenoid valve in your rinse tank so it can fill with cold water if water level gets near near the bottom so the tank won’t run dry. If you don’t like the idea of warm/cold water in times of higher use you can put in a cheap tank water heater to feed this solenoid valve instead of feeding it cold.

 

[I.B. Washincars]

I tied my hot and cold tanks together at the bottom. I set the cold float much lower than the hot. Under normal circumstances the cold valve is closed tightly, and the hot valve will keep both tanks replenished. In the event that it gets behind and the level drops very low, the cold valve will open and help out.

 

[Earl Weiss]

Typically you would need 1 heater per self serve bay if you are in an area with a consistently freezing winter.

Seems like a lot because I have one for a Tunnel and it keeps up, but I don't use straight hot water. I have a mixing valve and adjust the cold / hot as needed. What is the max GPM of a typical large capacity on demand vs GPM on an .05 nozzle at 1200 PSI?

 

[JGinther]

Seems like a lot because I have one for a Tunnel and it keeps up, but I don't use straight hot water. I have a mixing valve and adjust the cold / hot as needed. What is the max GPM of a typical large capacity on demand vs GPM on an .05 nozzle at 1200 PSI?

Assuming 80% efficiency from a standard vent water heater (which is higher than you will really get in practice), you need around 200,000 btuh to heat 4.5 gallons per minute at an 80 degree f delta rise. (incoming water 40 degrees, output 120 degrees).

In your example, you are running around 2.8 gpm assuming you meant a nozzle size of 5. So you could almost supply 2 bays with one heater, but not quite.

If using an 'electronic controlled' water heater, which the Paloma example above is not, you need to plan for the water heater guaranteeing the output temperature. This means the heater will restrict the flow of water until the setpoint is met. And that means undersizing will leave you with a dry tank on a busy winter day. The low fill cold water valve mentioned above solves that.

 

[Earl Weiss]

Assuming 80% efficiency from a standard vent water heater (which is higher than you will really get in practice), you need around 200,000 btuh to heat 4.5 gallons per minute at an 80 degree f delta rise. (incoming water 40 degrees, output 120 degrees).

In your example, you are running around 2.8 gpm assuming you meant a nozzle size of 5. So you could almost supply 2 bays with one heater, but not quite.

Here is what I am missing. Water to the bay doesn't need to be 120 degrees. As long as it is not "Cold" at point of use you can mix n Hot water. I don't know what the calculations are for sure but assuming city temp is 40 and Hot water from heater is 120 the difference is 80 degrees, If you add 30% city water to the cold at 8 degrees for every 10% that would reduce the temp to 96 degrees, still plenty warm, and now your 4.5 gallons of Hot is 5.8 Gallons of 96 degree water. So, that is 3 bays assuming they are all using HP at the same time.

 

[wash4me]

Here is what I am missing. Water to the bay doesn't need to be 120 degrees. As long as it is not "Cold" at point of use you can mix n Hot water. I don't know what the calculations are for sure but assuming city temp is 40 and Hot water from heater is 120 the difference is 80 degrees, If you add 30% city water to the cold at 8 degrees for every 10% that would reduce the temp to 96 degrees, still plenty warm, and now your 4.5 gallons of Hot is 5.8 Gallons of 96 degree water. So, that is 3 bays assuming they are all using HP at the same time.

Also this would only happen in your busiest times and probably nobody’s even going to notice it.

 

[JGinther]

I supplied generalized information for hot water, and industry typically sets the bar for that at 120 degrees since its the threshold for possible scalding situations. A mixing valve being used to reduce temperature at point of use will obviously reduce the amount of hot water needed. However, in the example of 96 degree water, that is a lower temperature than the body temperature of the customer. If they were to feel the hose or the water, they would think its definitely not 'hot water' which is about all the more detailed a standard self service customer would get in their analysis. Its true that most wouldn't notice, but many would and just not say anything... Probably mumble 'cheap skate car wash owner' and move on.

 

[New Washdog]

We recently experienced an issue with our high efficiency on demand hot water heaters. Turned out to be corrosion/deposits in the secondary heat exchangers. Have you flushed your boilers lately? We weren't flushing the boilers like we should have been, as we didn't know we should be flushing them annually at least and the end result was we had to remove the heat exchangers and clean them via pressure washing. All good now!

 

[Earl Weiss]

However, in the example of 96 degree water, that is a lower temperature than the body temperature of the customer. If they were to feel the hose or the water, they would think its definitely not 'hot water' w

"Hot" obviously is subjective. I think if I go outside when it's 96 degrees it feels "Hot" . I will have to do a water test. If my water heater at home is set at 120 degrees I don't use straight hot for washing.

 

[mjwalsh]

"Hot" obviously is subjective. I think if I go outside when it's 96 degrees it feels "Hot" . I will have to do a water test. If my water heater at home is set at 120 degrees I don't use straight hot for washing.

Just to add: We do have customers during extremely cold days who start with our rinse which is not heated. They have said the water initially freezes on the surface & grumble about it. I say they have to be careful what they wish for .... just thinking there could be some windshields might crack or at minimum have a crack get worse from the too quick of temperature change.

Our soap tends to hold at about 100° F. Our rinse & wax are cold & if & when we get our undercarriage set up it is planned to be cold only.

I would think even during the summer there would be a danger of hot water on cars. Not sure at what temperature threshold there could be car paint damage &/or other risks???

On the laundromat side we tend to maintain a range of 120° to 130° F for clothes. The dogwash tends to be always at very close to 100° F with a PID - Belimo PLC setup. I can bring up a graph on either touchscreen ... in case we are wondering on how consistent the water temperature is maintained. Veterinarians recommend washing dogs to be on the "cool" side when it comes to the error side!