Staging elements on Elctric boiler

jweber

Hi, I am hoping someone can help me.

I have an older but brand new electro industries 9 kw boiler ( non modulating) that I am looking for a way to stage the elements. The boiler is slightly over sized for my application. I have two zones independently controlled with thermostats with slab sensors. I would like to have the ability to drop one element at a set water temp and re-energize at a low temp. I did some testing by manually dropping off an element( pulling the jumper) after the boiler gets up to temp and my power consumption over time was considerably less to get the desired slab temp. Conversely I tested cycles with just using one element all of the time and it just ran way too long to efficiently get it to the appropriate temp. Should I do this with a simple aquastat or is there a more advanced programable component to use that would be a bit more accurate. Looked at a honeywell 775 temp controller as a possibility. Has some nice delay features that may add some efficiency. Yes, I could have bought a newer more appropriately sized model with modulation but I essentially got this one for free. It was unused and in perfect condition. I figured some folks have done this before and have some innovative solutions. Thanks for your help everyone.

Jamie Hall

Indeed you can do it. You will need a controller system -- pretty much your choice -- which will involve two sets of temperature set points, one to bring on and turn off the single stage, then one to bring on and turn off the second stage. The particular temperatures at which they are set will be pretty much your choice.

Then the controller will need to control a pair of relays -- one for the single stage element, one for the second stage.
You will also need to add in to the controls a positive, fail safe, high temperature shut down and low water shut down.

Except for two things. First, those relays will be handling a lot of current, and must be rated to do that.

It's an interesting, but not particularly difficult wiring problem.


Second, and perhaps more important, once you get done that boiler and its controls is going to be your puppy. No one else will touch it.

EBEBRATT-Ed

The Honeywell 775 would work but personally I Hate that control. Just didn't have good luck with it.

Personally I would use Johnson Controls 350 series. It's a bunch of modules that snap together. You can buy 1 basic control the A350 which will control 1 stage, add the temperature display module (if you want it's not required but makes operation easier) and then add as many stage modules as you need for the number of stages you need to control.

They are trouble free, easy to set up and don't seem to fail. It may seem more complicated than the 775 but will work a lot better

Others may differ JMHO

ratio

You can go as fancy as you want with the controls. A contactor (with an appropriate rating) can be used with a reasonably long duty cycle (they have an engineered electrical & mechanical lifetime that should be considered). An SCR is another option, which has the ability to be controlled by an analog signal. A mix of the two would be fun to operate, a contactor on one stage & an SCR on another, along with a T775 (sorry, @EBEBRATT-Ed!) would let you control to 100% turndown & exactly match the heating load.

NB: I have been told that I overcomplicate things. A simple aquastat/contactor combo like you mentioned will provide nearly identical operation for a lot less outlay.

jweber

Thanks for all of the replies. I too love to overcomplicate things but I also need to keep the bottom line in mind before ai spend so much money I might as well have bought the new version that modulates. (~$900)

Maybe if I supply a bit more info I can get a little more to the point. When boiler gets to 115 degrees I want to drop one element. If the boiler were to drop below 95 degrees I would like to have that element re energized. I did this manually and just want to replicate it without me standing there. It seems I could do this with the proper aqua-stat wired in line with the second element and right sensor that I can strap on the output pipe. Breaking contact at 115 and regaining contact at 95? This would cost around $100. The Johnson a350 seemed like a decent option. Not too expensive and with smarter controls. Is there a strap on sensor that can be added to this. I have not looked at how that would be wired but can this essentially break contact with the second element or do I need to add more components to make that work? And yes I know I have overused the term “strap on”.

Thanks again everyone and I am sorry if you basically already answered this. Just trying to wrap my head around it.

hot_rod

Could you just replace the elements with lower wattage versions and not bother staging. elements are around 15- 20 bucks.

Those staging relays are a maintenance issue, any contractor is. Most all the new electric boilers use triac relays to modulate elements. There triac relays are inexpensive also, but you still need a control to operate them bas on temperature.

Jamie Hall

If the aquastat you mention can handle the current and can be adjusted to those temperature ranges -- probably work just fine.

Keep in mind my comment on you modify it, you own it.

ratio

What's the wattage & voltage of the element? That'll determine if an aquastat can switch it direct or if you'll need a contactor. I'm leaning towards using a contactor anyway, because I wouldn't want to switch the contactor faster than minute-on minute-off, you may need a timer relay as well to slow this down a little.

The System 350 control will work fine, you can use any bullet-style probe, just zip tie it to the pipe & put a few wraps of foam insulation tape around it, but you'll definitely need a contactor as they're just pilot duty. A line voltage aquastat might be able to switch the load directly. Might even be able to adjust the differential to avoid short cycling the contactor.

jweber

The element is 240v 4500 watt. So an aquastat like this would work?
https://www.supplyhouse.com/Honeywell-L4008A1015-High-or-Low-Limit-Remote-Bulb-Aquastat-100-240F-5-30F-Adj-Differential-5483000-p

Has a 5-30 degree adjustable differential. Would I need a contactor? If so. Any recommendations there?

jweber

And also if I were to use the a350 powered by 24vac. I would use a contactor that would be triggered by the a350 with 24v which would then break contact of the 240v wired through it?

Jamie Hall

jweber said:

And also if I were to use the a350 powered by 24vac. I would use a contactor that would be triggered by the a350 with 24v which would then break contact of the 240v wired through it?

Yes. Don't even try to run 4500 watts (20 amps) through an aquastat. And keep in mind that you absolutely must break both legs of the 240 volt -- you need a DPST contactor. An aquastat is a single break, not double.

jweber

Something like this?

https://www.supplyhouse.com/Honeywell-Aube-RC840T-240-240v-Relay-w-Built-In-24V-Transformer

jweber

Also. Oke other thing to note. Instead of breaking the 240v directly, I was pulling a jump on the board that drops the second element. The tabs for the jumper were registering 25 volts DC. Would this be a smarter way to wire it and if so, one contactor would I use for that with the johnson a350?

jweber

*what contactor?

Jamie Hall

As far as I can see, that unit is SPST. Yes, it has the rating to handle the load. It does not break both legs of the 240 volt line.

What this means is that the element to which it is connected always has 240 volts live; it just isn't going anywhere. Now if a fault occurs -- or you should touch a terminal -- it will have somewhere to go. Will it work? Yes. Is it safe? Will it pass any sort of building or insurance inspection? No. If it causes a fire or accident, will your insurance cover you? No.

Can I recommend it? No way.

jweber

Thank you Jamie. What about using the DC jumper tabs in conjuction with the Johnson A350. Would this be a smarter bridge to gap? And if so what type of contactor would I use?

ratio

Strange that it's DC controls. Can you post a pic of the schematic, or the model number of the boiler?

If it's true 24 VDC, you should be good to switch it with the A350, but check & verify the contacts are rated for DC.

jweber

Its the electro industries emb-9. Pictures attached. Two jumper tabs shown. Then had thermostat call for heat. Reading shown.

jweber

Overview shot

jweber

Jamie Hall

You may have to use two relays -- one switched by your aquastat, and one controlled by that to control your power. Contactors with that power rating controlled by 24 volt coils -- either AC or DC -- are a bit thin on the ground.

jweber

Here is the basic diagram in the manual. It even has a suggestion of adding a normally closed contact device in place of the jumper to do what I am looking to do. See number 5

ratio

The IOM shows that unit being staged via a strap-on aquastat, see page 2 of the manual and note 5 on sheet BH302 P.1 (page 12 of the PDF)

jweber

Ratio. Thanks. So in this case I could just use the regular aquastat I listed earlier or the Johnson a350? Wired directly through both tabs? As long as I have the sensor strapped to the outflow pipe?

ratio

Yes. Mount the aquastat/sensor to the outlet piping at a convenient location (insulate the sensor, Just Because). Use the "break on rise" contacts to break the terminals that're jumped via the blue/white wire (cut & wire nut onto it, or replace it completely). If you go with the A350, you'll need to provide power to the module, it runs on 24VAC/DC. Sufficient power may be available on TB2 R (hot/+) & C (common/-).

I'd suggest the high-temp model (A350AA-2C), as the upper limit of the low temp module doesn't give you much headroom.

Study the paperworks of whichever control you're using. It always takes me a few minutes of reading & thinking to figure out which set of contacts I need to use!

jweber

Thank you for your help. I do already have a 24vac transformer on my radiant panel to power my thermostats and zone valves, zone controller. I can run it off of that.

ratio

It should be pretty straightforward then.

EBEBRATT-Ed

@ratio, I don't quit LOL Hate the Honeywell, love the 350s. And you can buy a A350P which has a 0-10 and a 4-20ma output and you can use both at the same time if need be. It will also drive an S350 stage module(s) if you just need contacts. They work good to drive VFDs too.

Take that Honeywell!!!LOL

Jamie Hall

ratio said:

The IOM shows that unit being staged via a strap-on aquastat, see page 2 of the manual and note 5 on sheet BH302 P.1 (page 12 of the PDF)

Oh ho! And Ah Ha! I see from the wiring diagram that that contraption has the elements controlled by built in, internal contactors. Thank you for the diagram!

In that case, yes, removing that jumper and using an NC contact on the aquastat will stage the second element -- and the aquastat is not handling line voltage, but control voltage. Nice design.

Wiring diagrams are handy...

Do note the caution regarding wire routing -- keep the 240 line wiring well away from any of the control wiring.

jweber

Which a350 is best for my application. The aa vs the p?

ratio

The A350P is an analog output device. You need a relay output.

You might consider the A350R reset controller, it'll allow you to use ODR to automatically change the temperature based on the outdoor temp—cooler outside = warmer water. I actually use one of these on my boiler, and will be adding a stage controller to it this summer as well.