Outdoor Reset applied to Bosch FS KWB-43-2 Condensing Boiler. Help me solve puzzle please.

Bromanowsky

I have a Bosch Floor Standing KWB 43-2 condensing Boiler. This unit has a DHW side and a boiler side. The DHW side is being used to heat a hot tub through a heat exchanger, so it doesn't figure into this problem.

As for the boiler side, I have two zones, baseboard hot water upstairs, and one for downstairs. Those zones are controlled by a system pump after the boiler, and two zone valves on the return to the boiler (one for each zone).

I have a "T" off of that loop, which has a pump feeding an INDIRECT water tank ("IWT"). That tank has it's own aquastat.

I have a Taco zone controller which controls my zone valves, which is connect to the "TT" terminal on my boiler.

I have a Bosch FW200 hooked up as well, which applies the Outdoor Reset Control ("ORC").

Because I have the ORC, the boiler runs typically at a temp. which is LOWER than what I need for the IWT. This results in a constant (or nearly constant) call for hot water to the IWT.

I called Bosch. They tell me that the machine will not take take the call directly from the IWT aquastat. They say that I could get rid of the ORC, and set up the IWT with a zone valve. They have no other solution for me.

I did some investigation. I see that TACO makes a "Fuelmizer" - SR501-OR-4. That unit (when matched with an Expandable Zone valve Controller) has it's own built in ORC, AND - here's the good part (i think), it will override that ORC when the IWT calls for heat, allowing the boiler to fire at full strength until that call is shut off. Then, it returns to the ORC control. That SEEMS to be what I need, BUT I still have questions:

1. Will that all work through the "TT" terminal on the back of my unit, or do I need to make sure the Unit accepts a 0-10 signal?

2. If this works, How do I wire the two pumps I have? Will they both be wired to the Taco controllers?

3. If this works, where do I connect the IWT aquastat?

4. If this doesn't work, is there a plumbing failure support group I can join somewhere, because I'm REALLY at my end....


I appreciate your guidance here. Thank you.

Ironman

Is that a combi or heating only boiler?

Bromanowsky

Hi Bob. It's a Combi, but please bear in mind I can't use the DHW side as it's servicing the hot tub.

Ironman

Adding ODR beyond the fw200 may gain some better comfort, but not efficiency since the boiler would have to be set hot enough to maintain the indirect.

If memory serves me correctly, there are are two heating curves in the fw200. You could set one high for the indirect and the other lower for your radiation. This obviously requires two thermostat inputs: one from the indirect and one from the space heating.

If you do this, you may wanna add a thermostatic mixing valve on the space heating side to limit the SWT when the boiler receives a call from the indirect since it's gonna go to the higher temp curve when that happens.

Bromanowsky

Thank you. I think the idea would be to disconnect the FW200 entirely, and use the TACO Outdoor reset. Unless I can use the FW200 to achieve the outdoor reset override when the tank calls for it?

Ironman

No, you don't wanna do it beyond the boiler because the boiler would not be resetting and would have to stay hot. Not much benefit there.

The two heating curves that I mentioned are two rest curves. The only time it would go to the hotter one is when the indirect would call. A space heating call would use the lower temp curve which would cause the boiler to operate more efficiently.

Ironman

The fw200 WILL do two heating curves. You'll need the IPM2 module.

Bromanowsky

thanks again. Is the IPM2 a separate part I need to buy?
on the 24v bus on the back of the unit, there is no place to connect the indirect water tank aquastat. So, it has to go to the zone controller. The zone controller then, would tell the boiler that the tank needs heat. The problem is that the boiler's hot water is not going to exceed whatever the outdoor reset tells it is necessary. In other words, the boiler thinks the call is for a zone, not for an indirect hot water tank.

the 24v Boiler Bus will accept a TT, and a tank sensor, but it won't accept the call from the aquastat.

Am I back to the Taco idea?

Joe Mattiello

hi
so your already have outdoor reset, and redundancy using fuelmizer is not going to correct your issue. I suggest using the SR504 with priority. use zone 4 for DHW, leave priority switch to on, so the controller recognizes DHW on that zone and fires boiler to high limit whenever zone 4 calls for heat. Appears rudimentary solution, unless I'm missing something. thanks

Bromanowsky

Thank you Joe! My thought is that I'd remove the FW200 entirely. In lieu of that, I'd use the SR504. The question I guess, is will the outdoor reset on the 504 work through the TT connections on the boiler? In other words, the SR504 will adjust the boiler temp through the TT terminal?

Or, are you suggesting I use the existing FW200, and somehow combine it with the SR504? How would that connection look? I think I'd need an EXP controller too, right?

Thanks again Joe.

ratio

I'm not sure how you have things wired together, but several people have said that the FW200 has two reset curves. Use the alternate curve for the indirect, with a setting of 0° for the reset. Use the aquastat to switch to the alternate curve as well as energizing the correct pump & hopefully turning off the heating pumps to avoid sending them water that's hotter than necessary.