How would you wire this system?

GroundUp

I try not to dabble in wood boiler systems anymore, but sometimes it just happens. I was called out to look at adding an electric boiler (newly installed PV solar array he wanted to put to work) to an existing 2 zone radiant slab system which was being heated via an unpressurized Central CL5036 outdoor wood boiler. He had it set up decently with mixing valves and circs for each zone, but no W2W heat exchanger. Heat loss necessitated a 23kw boiler so I added one as well as a 40 plate HX and pressurized the system along with some repiping but reusing the original AM-1 mixing valves and Taco 007-ZF5 priority zoning circs. Homeowner is an electrician and obviously wanted to wire everything himself, but I left him with some of my limited knowledge (I'm a hydronics installer, not a service guy or sparky) on the subject. He fought with it for weeks and I ended up going back yesterday to troubleshoot and try to wire it properly so that the electric boiler would back up the system automatically if the external aquastat sensed the OWB water was below 120F, it would run the electric.

My original thought was to use a ZVC403 and wire the existing stats as normal, then run the ZV outputs to the 007-ZF5 zone circs, and run one of the end switch wires through the aquastat to interrupt signal until the OWB water was below setpoint. That's what he did, and it did not work. After about an hour of messing around with it, I did finally figure out a way to make it work, but I'm not very confident that it's proper.

What would you do? Obviously switching out the circs and using as SR502 is a viable option, but ideally we keep the priority zoning circs as I'd intended. Just looking to see if there is a better option than what I ended up doing- which I will share after a few responses from those smarter than me.

Photos of before and after, for reference. Thank you

hot_rod

A two stage t stat can work, and I’ve seen it done with two separate stats set 5 degrees apart. The electric should only run when the wood isn’t keeping up? Do you want them to run together also.

Zman

Does the OWB have glycol or does it need some heat to protect it from freezing when it is not stoked? Does the fan coil need to run on the electric boiler? Would a DPDT aquastat on the OWB be a solution? When the boiler OWB temp drops the OWB circ would stop and the electric boiler would be enabled.

GroundUp

The stats are existing heat-only Honeywells and are working as-is. There is no glycol in the OWB, so a bit of backfeed for freeze protection was figured into the equation using the return glycol from the slab to reheat the OWB water as it passes through the HX in a constant circ loop. The unit heater does not need to run off the electric boiler. There is an aquastat on the OWB supply line, so that it enables the electric when the OWB supply temp falls below setpoint.

My question, is how would you wire this? Maybe I was unclear about the Taco circs- they are the type with the internal transformer and switching relay, so they plug in to a constant 120v and the signal from the thermostat tells them when to turn off and on. Again, my thought was to wire the existing stats into a ZVC403 and wire the ZV outputs to the R & W in the circ panel as if they were zone valves. This did not work, as there was no end switch signal from the ZVC. Jumpering 3 & 4 in the ZVC activated the end switch, but it would stay on forever until a wire was disconnected- meaning the circ ran forever regardless of the thermostat status.

GroundUp

Nobody? Okay then.

So what I ended up doing was landing the R & W from the circs to the same R & W terminals in the ZVC as the stats and jumping the 3 & 4 terminals for zone 1 and 2 in the ZVC. The ZV outputs on the bottom of the ZVC are empty, and the XX end switch makes a series loop through the aquastat on the OWB supply line and up to the electric boiler's TT. Seems like a waste of a ZVC if its only use is essentially acting as an end switch, but it works. Now that I ousted myself, can anybody tell me what I did wrong?

hot_rod

Switching between the two is a simple double throw relay like a RIB. What threw me is the unit heater not running when the electric boiler is. And backfeeding the OWB from electric boiler produced heat, to prevent freezing? That seems like an. energy intensive proposition.

If they were piped P/S or with a sep, then you have a few more control options, like both could run if the wood still had some heat to give.

My go to, as you know, is a differential (solar) control instead of a setpoint, as it watches both turn on and turn off temperatures, by using two sensors not one like a setpoint control. Instead of a fixed setpoint. It maximizes the output from both heaters.

For the way it is piped your solution works, I think a single Caleffi relay would also work as it has 3 pump outputs and you can configure how the relays work together or independently.

At the end of the day, so many ways to control, I think we all end up with what we are most familiar and comfortable with, using off the shelf components.

One suggestion I would have is write down the control logic, and a drawing, even a hand sketch. So the next person to work on it doesn't start from scratch like you did.

Jamie Hall

nice to have a written commentary, too, on how it is supposed to work, to go along with the sketch...

mattmia2

You probably would get more response if you drew it out. i am having a lot of trouble following it as just text. I wouldn't be able to figure it out without drawing it if i were doing it myself.

GroundUp

mattmia2 said:

You probably would get more response if you drew it out. i am having a lot of trouble following it as just text. I wouldn't be able to figure it out without drawing it if i were doing it myself.

Drew what out? I was asking the rest of you how you would wire it. If I drew a wiring diagram of what I did, it would have defeated the purpose of the question. I realize it's a lot of banter, but I didn't know how better to explain it without giving away the solution I came up with. Whether it's correct or not, it's working and nobody else seems to have an opinion so I guess I'll just have to wait and see.

hot_rod

A sketch, even a hand drawn one will help keep everything clear in you head. A piping drawing, then a ladder drawing of the wiring, and an essay or simple explanation of how it operates, what happens when.

This is a bit more elaborate, but it gives you an idea of how the flow, goes.