Wiring smart thermostat to two boilers

BMurrins

Hey guys,

I'm trying to connect a Honeywell T9 thermostat to control two parallel hydronic boilers both fitted with a L8148.

Can someone please tell me how to properly wire a dpdt switch i have here (r8222d) or a RIB2421B spdt switch that i also have here, without having to worry about burning out the transformers? For some reason I cant figure this one out probably out of fear of burning them out and making an expensive mistake.

Ill attach a simple diagram i drew to show how theyre wired so far. Mind you that this diagram doesnt include the vent damper molex connection that my aquastats have. sort of irrelevant to the question im asking so i just went for the first diagram that i saw.

Boiler 2 on the left, Boiler 1 on the right

EBEBRATT-Ed

Are you planning on running both boilers at the same time off the same stat? Or are you going to run 1 boiler or the other? Switch boilers manually or automatically?

BMurrins

EBEBRATT-Ed said:

Are you planning on running both boilers at the same time off the same stat? Or are you going to run 1 boiler or the other? Switch boilers manually or automatically?

same time off the same stat

EBEBRATT-Ed

You can do it with the R8222D but you will need an additional 120-24 volt transformer.

wire term 1 on the R8222d to TV on one aquastat
wire term3 to T on the same aquastat

wire term 4 to tv on the other aquastat
wire term 6 to t on that aquastat

mount new transformer and power line side 120 volt

on the 24 volt side the common goes to the coil of the R8222D and to common on the thermostat.

the 24 volt hot goes to R on the thermostat

w from the thermostat goes to the coil of the R8222D

There may be cleaner easier ways to do this but with what you have this will work

HVACNUT

Sorry its crude. You don't need an additional transformer. The one in the aquastat is 40 va.

northlakes

Out of curiosity and because I’m currently trying to figure out a solution for a two boiler system, Is your application as simple as you need both boilers to fire every time in order to satisfy demand at all times? 

BMurrins

hey guys thanks for the responses so far i really appreciate it.

northlakes said:

Out of curiosity and because I’m currently trying to figure out a solution for a two boiler system, Is your application as simple as you need both boilers to fire every time in order to satisfy demand at all times? 

yeah this is for both boilers to fire simultaneously

Zman

I think HVACNUT has given you the solution to firing both boilers at the same time. The question of "why" is pretty valid. When you control boilers that way you often end up with one boiler actually firing more than the other (one aquastat being a little more sensitive) and both boilers short cycling. A simple two-boiler controller will auto-rotate the boilers run times and only fire both when the load requires it.https://www.supplyhouse.com/Tekmar-261-Boiler-Control-Two-Stage-Boiler-Setpoint-4153000-p

BMurrins

it it’s not the most ideal setup that we have here but it’s a large apartment building being heated by 2 400k btu crowns. Horrible design but to modernize it, we’d be way over budget right now. Looking to get through the winter and redesign it come summer 2023

BMurrins

HVACNUT said:

Sorry its crude. You don't need an additional transformer. The one in the aquastat is 40 va.

This is brilliant. Thank you so much for taking the time to draw this

HVACNUT

BMurrins said:

HVACNUT said:

Sorry its crude. You don't need an additional transformer. The one in the aquastat is 40 va.

This is brilliant. Thank you so much for taking the time to draw this

Yes, it's so brilliant my wife put it on the refrigerator with a gold star. Move over Honor Roll kids, Daddy's coming through.

EBEBRATT-Ed

The transformer in the L8148 is rates at .8 amps @ 24 volt. More like a 20va. I don't think it's a 40 va.

The r8222d coil inrush is 20va sealed is about 10va