control options for zone valve system


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Old 08-14-06, 06:08 PM
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control options for zone valve system

Hi all. I have two space heating zones and one indirect water heater. All three are controlled with Taco 570 zone valves and their respective thermostats (and the indirect's aquastat). One circulator drives the whole system.

During heating season the DHW production competes heavily with space heating, leading to very long DHW recovery times.

I need to priority zone the indirect. Can I do it with this setup using something like a Taco ZVC404? Or am I stuck unless I get the indirect its own circulator?

Thanks.
 
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Old 08-14-06, 06:54 PM
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Do you have thermometers on the supply and return lines of both the heating loop(s) and the DHW loop? If so, what are the temperature differences?

When all three zone valves are open is the boiler temperature dropping while the burner is firing? Does the burner cycle while all zones are calling for heat?

I'm not really sure just what this Taco unit is supposed to do, my expertise is not with residential system but with commercial/industrial systems.

Assuming that your boiler is large enough to supply both the space heating load and the water heater load you will probably need to install the dedicated circulator to the water heater. If the boiler output is borderline or undersized for the entire load then you are correct in needing a priority control, I just don't know if this Taco unit is the correct application.
 
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Old 08-14-06, 06:58 PM
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Zvc404

The ZVC404 should do exactly what you want as long as the heat transfer rate is high enough to prevent the other zones from being deprived of heat for too long. I expect an indirect to be satisfied within 1/2 hr.
You probably already have looked at this but here's a link to the wiring diagram on the ZVC404.
http://www.taco-hvac.com/uploads/Fil...ry/102-109.pdf
 
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Old 08-14-06, 07:30 PM
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Thanks!

Furd, you are correct. The total load is probably more than even this fairly grossly oversized 118k IBR output boiler can handle, which makes priority the way to go. The indirect wants ~105k BTU/hr input for rated recovery, and the space heating is ~64k BTU/hr of emitter (slightly oversized for the 47k BTU/hr design day heat loss). So a total load of ~170k BTU/hr cries for a priority control. FWIW, the indirect has a really poor manual with no head loss or other useful info. It should probably be pumped with an 0010 or equivalent.

Grady, thanks. I was probably reading that as you were typing. I somehow missed that at first in rummaging around the Taco site. Found the product line brochure first. It doesn't say what the cutoff priority time is. Will dig further. Experience suggests (from non-heating season) that the indirect is indeed satisfied in under 30 min. Probably 15-20. As much boiler as we've got, I again suspect that a bigger, dedicated circ (on priority) might improve it?
 
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Old 08-14-06, 07:46 PM
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Circulator

If you blow water thru the loop too fast, you won't get any better heat transfer. What kind of temperature drop are you getting between boiler supply & return on the indirect? If the boiler is steadily firing, a circulator won't do you much good because heat is being transfered nearly as fast as the boiler can make it.
Pardon my lack of memory on exactly what boiler you have but if it has a large water volume, you might want to install a three way zone valve to provide by-pass until the boiler reaches at least 120. Energy Kinetics does this on a timer which keeps the domestic circulator off for a given amount of time thus allowing the boiler to heat.
 
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Old 08-14-06, 08:02 PM
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Good point. Good questions. Boiler is a Burnham 206. Water content 5.5 gallons. That large or small? So what am I bypassing with said valve? Just keep looping the water back to the boiler until it's hot enough to do some good in the indirect?
 
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Old 08-15-06, 01:58 PM
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Care to speculate about this? What if I added a PC700 outdoor reset control? This would control the space heating zones with partial outdoor reset. The indirect on priority would fire to the boiler high limit on a call for DHW. Two add-on cards for the ZVC could also control: 1) time limit for priority call (PC605) at 60 minutes, and 2) do a short (couple minutes) post-purge into the indirect before returning to space heating (PC600). The first prevents underheating and freeze-ups in the space heating zones. The second would get the supply temp down a few degrees before going back to space heating. That would help reduce expansion noise, etc. that might result from introduction of DHW-temp water (190F) into the space heating loop, which might only want 150-160F supply, right after a DHW call.

Any concerns with all this mucking around with the system temps? We're only talking 30-40F, and that seems like it's already in the range (or smaller) of what the boiler experiences anyway, particularly in shoulder seasons when it's doing basically a cold start once in the morning to get the house up to temp, which it can generally hold through to the evening setback with only a couple more firings during the afternoon.

Note with our cast iron boiler and no protection strategy, this would be partial outdoor reset down to about 150F supply which should give 135-140F return, i.e., above condensing temps that would rot the boiler.

Thanks.
 
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Old 08-15-06, 04:32 PM
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Add-ons

Personally, I think it is overkill. The time limit on the DHW priority is a good idea but beyond that, I dunno.

With only 5.5 gallons in the boiler & a priority on the DHW, I really don't see the need for a by-pass. From a cold start, I suspect you would reach a temperature at which you are actually transfering heat to the indirect in only a couple of minutes.
 
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Old 08-15-06, 05:20 PM
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Advice gratefully received and well-taken. Thanks.

Yes, you are correct that from a cold start the boiler gets up to temp pretty rapidly.

I do think the PC700 is worth a shot. I did the heat loss calcs thoroughly, and even playing it pretty conservative I get design day supply temps around 165F for the coldest room.

Potential energy savings and the potential relief from a lot of the expansion noise from going bang-bang to 180F all the time makes me think it's a worthwhile experiment. We'll see.
 
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Old 08-15-06, 05:28 PM
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Pc700

If the control doesn't work out as expected, you can always disconnect it. The upfront cost & work involved in installing/disconnecting (if need be) certainly doesn't begin to approach that of a new boiler.
Let me know how things work out.
 
 

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