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Hydronic Heating System Bypass

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]Not sure whether this question belongs here or under "Power..."  It is not electronic; although all of the controls are and additional ones may be added.

Problem:  System heats slowly and cannot maintain an inside/outside temperature differential much above 40°F.

System (source: Weil-McLain installation manual):

My system was installed in 1993 and has had several modifications since.  The boiler is a Weil-McLain.  Specifically, I am questioning whether the highlighted connection between return and supply is needed and/or whether it should be a valve?  There is already an automatic bypass valve shown just below and labeled system temp control.  Boiler is bit shown but would be at the bottom of image.  Only its intake ("From System") and output ("To System") connections are shown.  That highlighted bypass is 1-1/4" copper pipe.  The To System is 1" pipe.  There is a circulator in the boiler and a second circulator  as shown (upper left) for the system.

What seems to be happening is the bypass creates its own loop.  Boiler fires, output is limited to 180°F, and shuts down, but the loop limits the temperature to the house circulator and radiators.  The boiler then cycles quite frequently (every 5 minutes or so).  Water to the house gets warm (about 60 to 70°F) but not hot.  Return water is is about 10°F cooler.

The system circulator is a 1/4HP Armstrong.  I have a replacement Taco 0011 pump handy, which should be more than adequate for the house, but it is not a simple bolt in job.

I am contemplating changing that direct connection with a ball valve that will allow adjusting the mix.  However, is that bypass even needed?  I would be easier just to plug it.   What are  your thoughts on that?

Hmm, VERY strange.  I've never seen a setup like that.  It seems like this pipe would reduce flow through the boiler, and prevent it from delivering maximum heat transfer to the house loops.  My system certainly does not do that. the combination of all loops flow directly through the boiler.  The manufacturer may have had some reason to not want the full circulator flow to go through the boiler, but I can't imagine why.  It almost has to waste the heat generated and send it up the chimney.  OH, the note at the bottom right seems to imply there is ANOTHER circulator inside the boiler.  Then, it does make some sense.

As for your problem, there is something SERIOUSLY wrong, if the loop temp is only 60-70F!!!  Maybe the internal circulator pump has failed or the mixing valve has gotten stuck.  You might have to get a tech out who knows this manufacturer's system.


That loop temp was wrong.  It was a reading from a  HF IR cooking thermometer.  I thought about as soon as I left for Cleveland.  (I'm back now.)  Actually, it's between 115° to 125°F (guesstimate by feel).  That is, definitely warm, but not hot.  No where near 140°F (60°C).

The funny thing is, the US DOE ( shows a similar cross-connection:

But, there is only one cross-connection (my system has 2 in parallel), and the ostensible reason for that is to reduce condensation in the combustion chamber by increasing the temperature of the return water.  I understand what they are saying, but there is a 105,000 btu flame there too producing lots of water vapor.


--- Quote from: jpanhalt on December 07, 2021, 07:28:46 pm ---That loop temp was wrong.  It was a reading from a  HF IR cooking thermometer.  I thought about as soon as I left for Cleveland.  (I'm back now.)  Actually, it's between 115° to 125°F (guesstimate by feel).  That is, definitely warm, but not hot.  No where near 140°F (60°C).Weaaa

--- End quote ---
Well, I can tell you I can't hold my hand on the loop pipes for even a fraction of a second when the system is running.  That has to be WAY over 125F.
So, your earlier diagram had a notation about a circulator pump INSIDE the furnace.  If that failed, it would certainly cause the problems you mention.  It could be just a controls issue, where that circulator is not being turned on.  If that pump is not working and you remived the bypass, it would turn it into a system just like I have here.  I have a 1975 system that is pretty original, 3 zone valves, one circulator.  I've rebuilt the circulator a couple times, replaced a few zone valves.


Thank  you.  When I first bought the house in 2010, there was quite a differential between heated water from the house circulator and return water.  It is much less today.

Last weekend, I removed the Taco 007 circulator that was inside the boiler and replaced it with a new Taco 0010, which has a slightly larger capacity.  The 007 was OK.  My rationale was that a larger capacity pump from the boiler would shift the ratio of return to heated water to the house circulator in favor of heated water.

I got a little more heat, but then weather got cold. So, I left everything else alone.  Tomorrow is supposed to be above 50°F, and I plan to remove the larger Armstrong circulator and check it out.  I have a Taco 0011 (1/8 HP) to replace it with, if defective.  I may replace it anyway as the Armstrong is not maintenance free like the Taco.  I also have a ball valve to insert in the crossover, but it will be a tight fit with 1-1/4" copper pipe.  If that pump is defective, I will probably leave the ball valve installation until Spring. 

I did some playing with disconnecting one or the other pumps and looking for a temp differential across the pump.  In each case, the differential increased significantly across the non-running pump.  I interpreted that to mean the pump was working at least somewhat.   Both pump motors run and turn off and on simultaneously. 

I have also thought about getting one of the cheaper ultrasonic flow meters (see: ) to assess relative flow, but have decided not to jump into that.


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