Low pressure in DHW loop of hydronic heating system


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Old 02-14-15, 08:19 AM
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Low pressure in DHW loop of hydronic heating system

Hi and thanks in advance for any help.

I have a Weil-McLain Ultra that does both FinTubeBase and Domestic Hot Water. I recently had to change the pump for the DHW. I replaced it with the WML recommended model. While dealing with an unrelated heating issue in one of the zones I noticed that while it was in the DHW loop the pressure dropped from the normal 14psi to about 4 or 5 psi.

What would cause this?
 
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Old 02-14-15, 09:26 AM
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I replaced it with the WML recommended model
What model was that?

Is it the same one that was there?

It sounds like you are seeing the DIFFERENTIAL PRESSURE developed across the pump being subtracted from the gauge.

Is the DHW pump pumping AWAY from the boiler, into the DHW tank?

Can you show us some pics?
 
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Old 02-14-15, 09:41 AM
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The Weil-McLain recommended is variable speed B&G which I have have set too the highest speed as per the manual for my system. That setting would be about 14 gpm at that head. Of course, if I set it for a lower speed the pressure in the DHW loop will go up higher, but then the water in the loop heats more because it is moving through the heat exchanger slower and I'm unsure of what issues that may cause.

Yes the pump is pumping the water away from the boiler to the tank and back.

Pics are not possible at this time, sry.

Edit: ok, I think I now understand what you mean. Yes... the pump for the DHW pumps away from the boiler, unlike the Taco 0014 that moves water between my boiler and heating loop which pushes water TO the boiler. So the difference that I am seeing is the pressure lost at the gauge do to the pump pulling water away from it.

Correct me if I'm wrong here. =)
 
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Old 02-14-15, 10:09 AM
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I have have set too the highest speed as per the manual for my system.
I doubt if that's necessary. 14 GPM? Ya think? I don't. If it's 1" pipe to and from DHW, you shouldn't be moving more than EIGHT anyway.

What model boiler? Don't mind me if I want to read it for myself!

if I set it for a lower speed the pressure in the DHW loop will go up higher, but then the water in the loop heats more because it is moving through the heat exchanger slower and I'm unsure of what issues that may cause
Yes, lower speed will allow more 'dwell time' for the water in the heating loop inside the water tank to transfer it's heat to the tank water. The temp coming back to the boiler will be a few degrees cooler... MAYBE... but there should be no issue with that at all...

the Taco 0014
Why was the 014 chosen? Seems inappropriate at first glance. But not knowing the particulars, I can only guess...

So the difference that I am seeing is the pressure lost at the gauge do to the pump pulling water away from it.
I think you have the right idea.

When a pump runs, there is a pressure difference between the suction and discharge side. Depending on it's location relative to the point that the expansion tank is connected to the system, this differential pressure will either ADD or SUBTRACT from the STATIC pressure.

The STATIC pressure is that which exists in the system when no water is moving.

If the theory is correct, you should see a difference in the difference when you run the pump on medium or low.
 
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Old 02-14-15, 12:28 PM
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I have 2 Weil-McLain Ultra 235's used in tandem heating 16 separate zones each with their own T-stat and a 75 gal Weil-McLain Indirect Fired Water Heater attached to one and a 150 gal attached to the other one.

My pumps are sized correctly.

I have a Grundfos P/N: 96406965 Model:E 125v circulator pump running the FTB.

The 0014's only pull water from the main loop through the boilers and back to the main loop.
 
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Old 02-14-15, 12:44 PM
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Must be one giant McMansion!

My pumps are sized correctly.
Sounds as if you're convinced... can't argue with that!

The 0014's only pull water from the main loop through the boilers and back to the main loop.
So you're running PRIMARY/SECONDARY, and the two boilers are on secondary loops, and EACH of them has a 014?
 
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Old 02-14-15, 01:36 PM
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So you're running PRIMARY/SECONDARY, and the two boilers are on secondary loops, and EACH of them has a 014?
Yes, I have a Tekmar Controller that times how long each one is used and switches them back and forth on 48 hour cycles as Primary and Secondary so they "age" the same.

Yep, they each have their own Taco 0014. One is a series 2 but the other had to be replaced a few years ago so it is a series 3. I once used a 007 as a stand in for the 0014 because I had it available but it caused all sorts of issues, hence my trepidation a dialing back the variable pumps on the indirect fired water heaters.

With the weather like it is right now they are both running full steam all night long. In fact yesterday the primary was all the way up to 186 target. I imagine Sunday will be the most taxing day in the system's history with the cold and wind we're supposed to have here in Jersey.
 

Last edited by NJT; 02-14-15 at 01:58 PM.
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Old 02-14-15, 02:01 PM
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Sounds like a pretty nice system...

I imagine Sunday will be the most taxing day in the system's history with the cold and wind we're supposed to have here in Jersey.
Yeah, not looking forward to that.

It wasn't supposed to snow until after sunset, but it's been snowing here for about the past hour.

I just saw where they are upping the forecast from 2-4 up to 4-7 depending on where the so-called 'inverted trough' sets up. I don't think they know WHAT is going on!
 
 

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