Delaying a 24v switch

Thread Starter

John Burdine Jr.

Joined Mar 8, 2019
6
I have almost no wiring/circuit experience. I am trying to modify a hot power washer (Sanitech Mark IV) to improve it's heat output. Our problem is that when we shut off flow (at the trigger), the propane burner turns off (as it should) but after we start spraying again it takes several seconds for it to kick back on. The result is we are not getting near as much heat as we'd like since our work flow requires frequent pauses. A flow switch kills heat instantly by opening a circuit tied to gas valve solenoid (24v). I wonder if there is any sort of "off delay" capacitor or the like that stores a bit of power that I can put in line after the flow switch so it stays "on" for a few seconds and skips the shut down process for those times when we are only closing the trigger very briefly. I think 10 seconds would be the max I would go for safety reasons. Again, I'm just trying to stop those short off-on cycles and want the heat to stay on even if water isn't flowing for 10 seconds. Linking to exact product if you know them would help alot since I'm not well versed in understanding the specs of this stuff.
 

Yaakov

Joined Jan 27, 2019
1,612
This is probably a very simple thing to do but it involves a safety feature.

Whatever you do with this, it must fail safe. I'm not to comfortable trying to work out how to keep a burner going myself. If I had to do it, I would be sure it had a watchdog of some kind.

I imagine you'll get some help but please consider what happens if the circuit fails on.

Good luck.
 

Thread Starter

John Burdine Jr.

Joined Mar 8, 2019
6
This is probably a very simple thing to do but it involves a safety feature.

Whatever you do with this, it must fail safe. I'm not to comfortable trying to work out how to keep a burner going myself. If I had to do it, I would be sure it had a watchdog of some kind.

I imagine you'll get some help but please consider what happens if the circuit fails on.

Good luck.
The circuit is normally off so if it fails I will just lose heat. The flow switch requires water to be moving to turn it on.
 

MaxHeadRoom

Joined Jul 18, 2013
20,719
Im with Yaakov on the safety issue, possibly a 10sec off delay timer to control the gas and some kind of heat sensor as a back up in case of error etc.
Max.
 

Tonyr1084

Joined Sep 24, 2015
4,543
I would suspect the reason why it takes several seconds to reignite the gas is to prevent an accumulation of flammable gasses that if ignited could result in an explosion, be it a "puff" or a "WOOF" or even a "BOOM". It could be minor and possibly lead to damaging the unit or it could be catastrophic and lead to shrapnel flying.

I'm sure we could come up with some sort of delay circuit to achieve what you ask for - but personally, I would not want to be the one responsible for someone getting injured by following my advice. So I will remain silent beyond this point. Unless specifically asked a question. But in the end, I'm not going to be the one to give you the solution you're seeking.
 

Tonyr1084

Joined Sep 24, 2015
4,543
Im with Yaakov on the safety issue, possibly a 10sec off delay timer to control the gas and some kind of heat sensor as a back up in case of error etc.
Max.
To this I would add a possible over pressure sensor since you are heating water, by pausing possibly too long you could create a boiler. And when a boiler fails it fails spectacularly. In addition to heat sensing I'd also consider pressure sensing as well. Heat because of the potential for scalding someone and pressure for the reason already stated.

So much for remaining silent.
 

Thread Starter

John Burdine Jr.

Joined Mar 8, 2019
6
To this I would add a possible over pressure sensor since you are heating water, by pausing possibly too long you could create a boiler. And when a boiler fails it fails spectacularly. In addition to heat sensing I'd also consider pressure sensing as well. Heat because of the potential for scalding someone and pressure for the reason already stated.

So much for remaining silent.
The unit has multiple burst disks (one for temp and one for pressure) and a pressure valve installed (though that is before the burner). I think the delay is just indicative of the time it takes the propane to travel down the tube after the solenoid to the igniter. I was hoping for something that charges itself from the current in the flow switch conduit itself (like a capacitor...but that might not be the right term)...that way when the circuit is opened upstream, the capacitor continues to send a signal downstream for a few seconds tricking the circuit into thinking the flow switch is still on. As soon as it runs out of juice (approx 10 seconds) it closed the solenoid like it normally would.
 

AnalogKid

Joined Aug 1, 2013
8,467
To clarify things,

What is the voltage available to run the timer?

What are the voltage and current of the power being switched by the timer?

ak
 

MisterBill2

Joined Jan 23, 2018
6,098
An alternative that would be safer would be to have a bypass valve on the discharge so that it would drain back to the inlet side of the pump. BUT then you must include an over temperature shutoff for the burner supply valve. The fire would still switch off for longer delays, but for short delays the water would stay hot, and you will still be safe because of the overtemp shutoff plus the original pump flow switch will need to be in the portion with the bypass valve, so that you will need flow to keep the fire on.
The down side is that you will need a high pressure valve for the bypass function. BUT you may be able to use a relief valve for that, set just a bit above your working pressure. Much cheaper and a lot safer and possibly easier to implement.
 

Thread Starter

John Burdine Jr.

Joined Mar 8, 2019
6
An alternative that would be safer would be to have a bypass valve on the discharge so that it would drain back to the inlet side of the pump. BUT then you must include an over temperature shutoff for the burner supply valve. The fire would still switch off for longer delays, but for short delays the water would stay hot, and you will still be safe because of the overtemp shutoff plus the original pump flow switch will need to be in the portion with the bypass valve, so that you will need flow to keep the fire on.
The down side is that you will need a high pressure valve for the bypass function. BUT you may be able to use a relief valve for that, set just a bit above your working pressure. Much cheaper and a lot safer and possibly easier to implement.
That's not an option for a number of reasons, the pump isn't rated to handle hot water, it's generally a bad idea to not have the unloader valve very first after the pump, AND the flow switch wouldn't work in that application.
 

Thread Starter

John Burdine Jr.

Joined Mar 8, 2019
6
To clarify things,

What is the voltage available to run the timer?

What are the voltage and current of the power being switched by the timer?

ak
All I know is that this part of the machine is 24 volts. I doubt there is much current going through the flow switch even though its rated for 250v and 3 amp (Suttner ST6), since it is really just sending a signal to the circuit board on whether or not power should go to a gas solenoid, but I don't know maybe the power to that solenoid is flowing through this switch.
 

Tonyr1084

Joined Sep 24, 2015
4,543
One thing you should take the time to find out - that's whether the valve turns back on as soon as you hit the trigger or not. If there's a (oh, lets say) five second delay then the issue may be NOT that the valve shuts off but when the valve turns back on. I'm thinking back to high school days where in auto shop we had a steam cleaner. It consisted of a propane tank and a water tank that was heated. When the temperature reached a specific temp the burner shut down, leaving just a pilot light.

What I'm imagining for your washer is a flash heater where the water going through the heater core is flash heated. Hence, that would explain the cold water for the few seconds before the burner comes back on. So that's two things you should be able to answer for us - 1) If there's a time delay for turning the burner back on; and 2) If it's a flash heater (or for that matter exactly what kind of heater it is).

Yeah, I know, I know. I said I was going to be silent. But just look at me! How can a cute face like that resist the urge to speak out?
 

Thread Starter

John Burdine Jr.

Joined Mar 8, 2019
6
One thing you should take the time to find out - that's whether the valve turns back on as soon as you hit the trigger or not. If there's a (oh, lets say) five second delay then the issue may be NOT that the valve shuts off but when the valve turns back on. I'm thinking back to high school days where in auto shop we had a steam cleaner. It consisted of a propane tank and a water tank that was heated. When the temperature reached a specific temp the burner shut down, leaving just a pilot light.

What I'm imagining for your washer is a flash heater where the water going through the heater core is flash heated. Hence, that would explain the cold water for the few seconds before the burner comes back on. So that's two things you should be able to answer for us - 1) If there's a time delay for turning the burner back on; and 2) If it's a flash heater (or for that matter exactly what kind of heater it is).

Yeah, I know, I know. I said I was going to be silent. But just look at me! How can a cute face like that resist the urge to speak out?
Those are good questions...the heater has a coil not a tank...so the flame runs beneath and within the coil. Also, there is a delay for the burner to re-fire...it includes a delay for the solenoid to open (seems to vary from 1 to 5 seconds...not sure why) then within another 1 or 2 seconds you can clearly hear the roar of combustion. That 3 to 6 seconds of water moving through without being heated seems to cost up 20 to 30 degrees of temp at the wand (in typical usage) compared to being constant on for 5 minutes straight. I suppose cutting that ignition delay out would help somewhat but leaving it on for 10 seconds would help tremendously and is my preference if possible.
 

MisterBill2

Joined Jan 23, 2018
6,098
Those are good questions...the heater has a coil not a tank...so the flame runs beneath and within the coil. Also, there is a delay for the burner to re-fire...it includes a delay for the solenoid to open (seems to vary from 1 to 5 seconds...not sure why) then within another 1 or 2 seconds you can clearly hear the roar of combustion. That 3 to 6 seconds of water moving through without being heated seems to cost up 20 to 30 degrees of temp at the wand (in typical usage) compared to being constant on for 5 minutes straight. I suppose cutting that ignition delay out would help somewhat but leaving it on for 10 seconds would help tremendously and is my preference if possible.
OK, I got the response to my suggestion and now let me clarify: Yes, of course the relief valve has to be quite close to the discharge port. BUT you can add a second one set for some lower pressure, with a shutoff throttle valve, downstream of that. It would be set to allow just enough flow for the flow switch to stay on and to keep the burner lit, so you would need a flow control also to reduce the waste to a minimum amount. That valve would need to close when the pressure dropped when the spray switched on. You would not need to recirculate the relief valve discharge but you would need to do something with it. And you would probably want to turn that off if the pauses in spraying were more than a few seconds.
 
Top