Electrical experts needed - Help with circut design

[Weedwaka]

I need some help with the design of my heating and hot water system.

What I want to do is run two similarly sized Seisco units in our carriage house. 1 unit ( RA-22 or RA-28 ) will look after all our hot water needs and a 2nd unit ( SH-11 or SH-14 ) will look after our radiant heat in just the garage slab.

Now I would like to make it so that when unit #1 kicks in to provide hot water ( aprox 22000 watts or 28000 watts / 90 - 120 amps depending on model) unit #2 can not kick in until unit #1 is done its duty's.

Basically I want it to be wired so that only one can operate at a time with priority going to the hot water heater ( RA-22 or RA-28 ) .

Is there an easy way to do this ?

 

[LoneGunman]

Very easily done for someone who does motor controls, not so easy for the average homeowner. I know nothing about these units so youll need to provide some information. Basically, youll need to figure out what controls the #1 unit, is this an instant hot type or something like a regular water heater?

Either way, When number one kicks on, youll have voltage going to something, youll have to come off of that line and use it to energize a coil on a relay. I'm assuming number two is controlled by a thermostat? If so, the thermostat line will go to the relay on a normally closed contact. When the coil in in an unenergized state your #2 unti can operate whenever it wishes. When #1 calls for hot water it energizes the coil which in turn opens the normally closed contact preventing #2 from coming on.

 

[timgr]

Looks to me like these are both on-demand water heaters.

How complicated do you want this to be? I would guess that the control could be done with low voltage. All you need to do is shut off #2 when #1 is running. However, you may need some delay between shutdown and startup unless the startup lag is long enough to overcome any possible overlap. The manufacturer should be able to help you with this. They may have some suggestions about other approaches too.

However, to me, it seems like you may as well add a properly sized mains circuit for each heater rather than try to make both heaters run on one circuit.

 

[Weedwaka]

Thanks for the reply's guys !!


Im trying to keep the amp load for our carriage house down as much as possible. We have the option of running a 200 amp service or a 400 amp service to the carriage house with the 200 amp costing 805.00 and the 400 amp costing 1705.00 plus installation etc. Once the main house is built the power will be pulled from the service at the carriage house. Now if we can keep the load down enough on both then the 200 amp service should be plenty.

The two biggest loads on the carriage house would be the seisco heaters and with no need for the floor heater to be run at the same time as the water heater I thought it would make sence to run them like this.

We dont want to hook up to gas on the carriage house but when the main house is built it will be mostly gas stove, gas heat etc so the draw should not be that large.

I did not think about the short overlap which would probably occure if the space heating unit (#2 ) was energized while the water heating ( #1) first starts kicking in. That would throw a wrench into my plans me thinks.

Maybe I should just bite it and get the 400 amp service installed .

 

[mmg440]

just a thought

It might just be cost effective to go with a tank type of heater that draws a lot less amps (just for a longer time). You would only have the high amp load for the in floor heat and 200 amp service probably could handle both. My guess is a 200 amp service could work. Of course you still would have to do the math to make sure.

 

[ddawg16]

I had a quick look at the model online and couldn't find anything that suggest that you could turn it off using low voltage means. To shut off the 230 Vac feed you would need a couple of multipole contactors....the cost of that and related circuits alone is going to be more than the cost of one of the units or the cost of a larger service.

Why not go with gas? With that much demand, you would be better off gas....no issues with overlap or demand...if sized correctly....and I can't imangine the gas line install costing more than the electrical.

 

[Weedwaka]

Yeah I think i will go with the 400 amp service just to be safe and scratch the idea of switching between the two seisco's.

Gas here is much more expensive then electric at over 11.00 per GJ vs 6.25 kwh and with only 1 service we will save the minimum charge while we are living in the carriage house for about 4 years min. Plus with both of us working full time we are only using hot water after work ( showers ) and for the dishwasher every third or fourth night and for washing hands etc so the electric on demand unit should be really cheap.

When it comes time to heat the main house I will probably go with a high efficiency gas boiler but for now the cost is too high for such a small space. The stainless venting required is a bit of a pita also and $$$.

Thanks for the help guys !!