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Old 12-09-2011, 01:58 AM   #1
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Boiler's 24VAC overloaded?


Hi,

I have a Buderus 124X boiler that uses a Honeywell L8148E aquastat. I think the installation is overloading the transformer of the aquastat.

I have 3 thermostats on 3 valves. The thermostats used to be simple, very low power honeywell. I replaced them with powered (need Blue/C) ones. When one is asking for heat all is well, but if all 3 at the same time are asking for heat, the pilot doesn't lit.

I initially thought it was a problem with the gas valves but it doesn't appear to be. The demand does go all the way to PV. However, if all 3 thermostats are asking at the same time (implicitly, all 3 valves are also opened since they ultimately control the mixing demand) the voltage is about 20.5V at PV - not enough to open the valve. I measured 31VA just for the thermostats and valves being kept open. When only one thermostat requests the voltage is about 23.9V and it lights-up every time.

Looking at the spec for the L8148E aquastat they mention it is designed for 1 valve controlled only - 2 in some circumstances. It is now powering 3 valves, 3 home-automatable fancy thermostats in addition to the gas valves.

Here's the current connections. The boiler is connected to a Tekmar 360 mixing controller. The mixing controller has an isolated demand (will consider any voltage on the demand as a demand), and the boiler request is also isolate d (just a relay). The valves are White-Rodgers 13SH16-102.

The wiring is this… (omitting the valves for simplicity, which are the one ultimately calling the mixing demand when they are opened)

Boiler T (White) To Tekmar 360 Boiler 8
Boiler TV (Red) To Tekmar 360 Boiler 7 to Red on all Thermostats
Boiler W (blue) to Tekmar 360 Demand 2 to Blue on thermostats
White from zone valves to Demand 1 on Tekmar 360

There are 3 thermostats, each linked to their own zone valve. When the valve open it energizes the white wire back to the tekmar to initiate the mixing demand.

Now my question is… could I isolate the boiler and use an auxiliary 24V supply? That would mean, I think… disconnect the blue (W) wire from the aquastat - connect to one side of auxiliary 24VAC. Disconnnect Red from boiler 7 on Tekmar and connect to other side of aux 24VAC. Giving something like this…

Boiler T (White) To Tekmar 360 Boiler 8
Boiler TV (Red) To Tekmar 360 Boiler 7
Boiler W (blue) to Tekmar 360 Demand 2
White from zone valves to Demand 1 on Tekmar 360
24VAC+ to Red on thermostats
24VAC (common) to Blue on thermostats

I was thinking using a Honeywell AT175 (a little overkill) 24V transformer.

Does this seem correct? I'm an electrical engineer but I've spent all my life in software - so I'd rather check before I fry a bunch of boards…

Thanks,
Benoit.

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Old 12-09-2011, 07:14 PM   #2
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Boiler's 24VAC overloaded?


Ok I got a White Rodgers 40VA transformer and intend to do this tomorrow. If anyone has an opinion on the correctness of this, I would highly appreciate hearing it... Thanks!

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Old 12-09-2011, 07:44 PM   #3
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Boiler's 24VAC overloaded?


the valves and the thermostats are draining on the unit transformer thats why it's not lighting especially with grabbing that C wire for a constant 24Vs at the stats.your valves and stats should be sepearate from that unit TR let the valve action end switch run the boiler call and circ relay.if you have a long run up into the house on the stats i might suggest 60VA #3TZ69 if short runs up a #3TZ67 40VA 24V/120/208/240 circuit breakers on the 24V side....protected. all availble at your local Grainger supply...if you don't have a source
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Old 12-10-2011, 07:47 PM   #4
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Boiler's 24VAC overloaded?


A 40 va transformer is good for three valves and maybe 3 stats because the stats draw so little.
My opinion is that you use the 75 va instead.It will for sure be big enough and the extra capacity will never hurt you.
Only a fool walks the edge when he doesn't have to.
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Old 12-10-2011, 08:16 PM   #5
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Boiler's 24VAC overloaded?


Thanks that is useful!

I still have the 75 on order - and I like the built-in breaker - they just didn't have it in stock. Since the 40 is so cheap and I have it in hands I think i'll set it up tonight and check the actual load (my 31 VA max I read might have included more than the stats & valves). If it's anywhere too close I'll replace with the 75 as soon as I get it.

These things are big. Where do you mount them? I have a 4x4 mounting plate (with 1/2" hole) but it means the whole transformer is sticking out right? Or is there a better way... Just want to do it right.
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Old 12-10-2011, 10:43 PM   #6
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Boiler's 24VAC overloaded?


still might make it with a 40VA ...wire run out to the stats pulls on the VA also along with the actual stat. mounting it might think of a double 1900 stacked with a blank cover.mount it above the zone valves bringing over the 115V then dropping the 24V might even want to get a black terminal strip with jumpers for all the Rs and Cs coming back

Last edited by biggles; 12-10-2011 at 10:52 PM.
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Old 12-11-2011, 12:36 AM   #7
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Boiler's 24VAC overloaded?


It works!

My actual load is about 20 VA - so I think I'm good. That' little 40 VA transformer is almost double the size of the one in the aquastat that had to multitask! No wonder it was getting so hot! It is now barely warm to the touch.

I was able to mount it under a Taco SR502 zone relay box box I have too for the pool/upstairs radiators. However that 40VA has the 24V sticking out the other side which makes for a lame job. I'll still replace it with the 75 once I get it because all the wires goes on the same knock-out and it has a built-it breaker.

Thanks again for your help guys - it was a 30 min job - the local HVAC contractor wanted $450 + taxes - and that's after paying $120 to have the guy come over and me just explaining the problem (with him nodding) I don't mind paying - people need to earn a living - but $600 for that just ain't right....
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Old 12-11-2011, 06:30 AM   #8
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Boiler's 24VAC overloaded?


installers will cheat 24Vs off there mother not to install a TR on a new job might get away with the zone actuators but not the stats,and as you saw the dragging of the boiler controls surprised you didn't see the stat working slower...especailly with that R/C pull direct

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