Solid State water heater control

[Electromatic]

I have a customer with a breaker panel in a guest bedroom. In the panel is a contactor that feeds the water heater (standard storage type) and is controlled by an ATS load shedding set of contacts. Of course, the contactor started to buzz. Instead of putting in another regular contactor which will inevitably also buzz, I'm thinking of using this solid state relay:

https://cdn.automationdirect.com/static/specs/ssclass6relays.pdf

I got the zero cross switching type and rated at 50A because it was almost the same price as 30A. Are there issues with having this kind of relay almost always "ON"? The only time it would open is if there's a loss of power AND the generator has to shed the water heater. Also, would there be any real need to use 2 of them and open both legs of the water heater? I don't see why.

Thanks as always.

 

[winnie]

The solid state relay will have a voltage drop which means heating when current is flowing. (There is also some heat from the control circuit but that is small.) So be careful about heat sinking.

They always leak a bit of current and thus cannot be considered a disconnect. Opening one leg for control purposes is fine.

-Jon

 

[PaulMmn]

Any possibility of inverting the control circuit? Relay with NC contacts. When the relay is energized, the circuit opens and the heater cools. Power the relay from the generator side of the ATS. Should be a relatively tiny load.

Only issue would be if the relay failed, and the water heater was powered by the generator.

Or, perhaps a latching relay that 'remembers' where the contacts are supposed to be.

 

[Jraef]

There are “quiet” contractors made for this very purpose. I helped on a hotel project in Las Vegas years ago (Wynn) where every room has a contactor in the small sub-panel that’s in the room, same problem. The contactor closes only when the room sensor detects that someone is in there, which means the contactor was buzzing only when the guest was trying to sleep. Not good.

Siemens had one specifically designed to not make noise. We tested them and they were acceptable. Can’t remember the series though, I’ll have to dig around in the half-dead brain cell section. May not have been big enough for a water heater though, as I recall I only needed 20A. I’ll post what I find.

By the way, we investigated SSRs too, wasn’t going to work. Watts loss is 1-1/2 W per running load amp, so even if you use only one pole, it’s like putting a 75W heater element inside of the panel, with no ventilation. It will cook itself.

 

[Electromatic]

This does come with a thermal pad, and I would be mounting it directly to the panel enclosure which should act as a heat sink, right? Or do you still think it would overheat? Is there a way from the cut sheet link in my original post that I could figure out how much heat it would produce? The specific model is: AD-SSR650-AC-280A

Of course, the water heater itself won't always be drawing current (i.e. when the water is up to temperature) so it would have times to cool down.

My other thought was to use a N.C. contactor, then put a relay in the ATS to change the load shed logic. I didn't want to get TOO fancy--under which category hunting down a specifically quiet contactor might fall.

 

[synchro]

To get heat dissipation in watts, I would multiply the water heater load current by the 1.6V RMS max voltage drop stated on the cut sheet. The voltage drop across SCRs/Triacs does not vary much with current and so the stated 1.6V is not excessively conservative.
Then use this calculated heat dissipation to estimate the thermal rise in the enclosure using one of the available on-line documents, etc. such as:


https://www.google.com/url?sa=t&sour...hT0PWCQVRAshPU


https://www.saginawcontrol.com/resou...al-calculator/

This is to estimate the temperature rise within the enclosure to make sure it doesn't significantly impact other components like breakers in the box.

The second to last page of your cut sheet provides the derating of the allowed current for various thermal resistances vs. ambient temperature. With a quick look I didn't find a good estimator for thermal resistance to ambient of steel enclosures. Because steel has a relatively higher thermal resistance than aluminum and because panels are not particularly thick, I would suggest putting a heat spreader of at least 1/8 thickness aluminum sheet in between the SSR and the panel. This is because most of the sheet steel thermal resistance will be local to the SSR attachment point.

 

[Jraef]

But a CB panel inside of a wall is unlikely to be free to dissipate heat fast enough into the ambient environment. No air flow even if it is not an insulated wall. If it's insulated, fugedaboudit...

 

[synchro]

But a CB panel inside of a wall is unlikely to be free to dissipate heat fast enough into the ambient environment. No air flow even if it is not an insulated wall. If it's insulated, fugedaboudit...

Agree 100%. I guess I glossed over the comment that the panel is in a guest bedroom. So it's probably recessed in a wall... unless they put their guests in the basement

 

[growler]

Ever thought of using mercury contactors . I just looked and they still make them and a 60 AMP 2 pole is only like $130.00.

I used some back in the 1980s and they worked out well.

 

[Jraef]

I found the Siemens contactor (3TG series), it's limited to 20A, not going to help you here. But try doing a search for "silent contactors", people make them for sure, specifically because of this issue.

Mercury contactors can make noise too, there is still an electromagnet and a plunger. I tried that and the problem is that mercury is so scary for anyone to use now, the choices of suppliers are severely limited. The two that I tried were both too loud.

I found these, referred to as "Silent Contactors":
http://www.attr.com/contactor.htm

No UL listing though.

 

[texie]

I have a customer with a breaker panel in a guest bedroom. In the panel is a contactor that feeds the water heater (standard storage type) and is controlled by an ATS load shedding set of contacts. Of course, the contactor started to buzz. Instead of putting in another regular contactor which will inevitably also buzz, I'm thinking of using this solid state relay:

https://cdn.automationdirect.com/static/specs/ssclass6relays.pdf

I got the zero cross switching type and rated at 50A because it was almost the same price as 30A. Are there issues with having this kind of relay almost always "ON"? The only time it would open is if there's a loss of power AND the generator has to shed the water heater. Also, would there be any real need to use 2 of them and open both legs of the water heater? I don't see why.

Thanks as always.

Consider an Aube RC840 relay. Super quite. Made for resistive heat load. They last forever. 24 VAC coil.

 

[Jraef]

Consider an Aube RC840 relay. Super quite. Made for resistive heat load. They last forever. 24 VAC coil.

Nice, but only 22A, he needs 50A. I'm going to keep those in mind for something I have in the works though, thanks.

 

[texie]

Nice, but only 22A, he needs 50A. I'm going to keep those in mind for something I have in the works though, thanks.

I'm thinking the OP has a garden variety 4500 watt 240 water heater.

 

[texie]

Consider an Aube RC840 relay. Super quite. Made for resistive heat load. They last forever. 24 VAC coil.

On second thought, consider the RC840T that has a built in control transformer. Just run a 2 conductor class 2 cable to your dry contact in the ATS.
But as Jraef questioned, I'm not sure of your actual heater watts.

 

[kwired]

Electric heat relays that are bimetal actuated and driven by a 24 volt heater coil - those are quiet compared to magnetic contactors and most common ones are used on ~5kW elements so they are rated for the current.

 

[neonbkw]

Electric heat relays that are bimetal actuated and driven by a 24 volt heater coil - those are quiet compared to magnetic contactors and most common ones are used on ~5kW elements so they are rated for the current.

But they are too slow to be used in a load shed application, as requested by OP. Ask me how I found that out...

Sent from my SM-T820 using Tapatalk

 

[GoldDigger]

How you found out?

 

[kwired]

But they are too slow to be used in a load shed application, as requested by OP. Ask me how I found that out...

Sent from my SM-T820 using Tapatalk

good point.

 

[Electromatic]

Well, shoot! Thanks for all the replies even though it sounds like everyone thinks this thing will just burn up in the panel which is not what I wanted to hear.

That RC840T does look like an option though and it seems to be available and not too expensive.

BTW, the water heater is a standard residential storage type, so 4500W when it's heating. The panel is actually mounted in a utility room unfinished wall with its back to the finished bedroom wall.

 

[LarryFine]

BTW, the water heater is a standard residential storage type, so 4500W when it's heating.

So, 18.75 amps.