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[Ash]

What size pump do I need / would you recommend?

350' total loop, with 20+40 plate exchangers.

Previously had a taco 007, wondering if it was too small?

[slimjim]

It all depends on what your restrictions are, 1 inch nominal pipe? 90's? Primary secondary on your loops and temperature differential across the heat exchangers!

[RSI]

What kind of temperature drop were you getting with a load on both plates?

an 007 is not a good choice for that setup. Since you already have used it, I would start by going off the temperature drop to estimate correct pump size. An 0015 is probably going to get you close but you may need to go up to an 0011.

If you want to go with another brand, there are a lot of options but not much difference in price unless you go with a Chinese pump.

[Ash]

The pex is 1", w/4 90's in the loop. 

I don't have accurate statistics on temperature drop other than when under a full load, the furnace has a real hard time keeping up.  With a cheap IR gun i've seen temp drops of 30* or more. Under a full load my water jacket (107gal) temps drop considerably, anywhere from 20* to 40* 

Would a Taco 2400-20-WB be overkill?

[RSI]

Here are some estimates for your system. It looks like the 0011 and 2400-20-wb would pump the same.

3 gpm 5.7'
4 gpm 9.5'
5 gpm 14.5'
6 gpm 20'
7 gpm 26.5'

[slimjim]

WOW 20-40 degree drop under a load at the boiler with just a 007, does the boiler catch back up to temp, you really should check the heat loss on your pipe, I'm willing to bet that it is part of your issue, I would like to know what it is without a load?

[Sprinter]

Could yo imagine feeding, or if it can keep up with a bigger circulator with more flow........uh oh

[LittleJohn]

350' loop is alot of pipe, and depending on what your velocity/GPMs are your head pressure could be quite high.

If you are running 1" PEX @ 2ft/sec (or about 3.5 gpm) head pressure is about 2 feet of head/100' -> in your 350' loop, about 7' of head
Now if you are running 1" PEX @ 8ft/sec (or about 14 gpm) head pressure is about 22 feet of head/100' ->or about 77' of head 
***DOES NOT INCLUDE HEAD LOSS FROM HEAT EXCHANGERS (HX)***

...you will want to do the math and figure out what velocity/GPM you need, so you have a delta T of about 20F across your HX

[JDfarmer]

What's the maximum tolerable/ideal temperature drop across the water / air exchanger with forced air central system?
I have no heat loss between the house and the boiler at all. I have a 3 speed water pump; at low the temp across the exchanger is 172/142 30F, at mid II speed, about 15F drop. Is 15F drop ok 175 to 160, or do I need to go the highest speed III?
Are there disadvantages just keeping it at high? Would that make the Pressure too high?
 
thanks

[LittleJohn]

You are talking about a Water to Air Heat Exchanger so I think your delta t can be higher.

I know for applications like radiant floors you want to keep return temperatures at about 20f; it gets to be more of a comfort thing.  If delta T is higher you can walk across floor and feel different temperatures with your feet.

IMO, I think as long as the water going back to the OWB is above 140 and you are comfortable in the house its all good.  Just realizing that if it gets any colder outside you may need to turn the pump up.