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P/S configuration and multiple TermUnits on one loop.

CTG Member Posts: 14

Starting new thread so as to not hi-jack the RWT -Too Low thread further.
CTG said:

Hi Heatinghelp,
Is it proper to bump an older thread? Should I start a new one?

I learned over the long holiday weekend I have the same issue with low return water temperature to the boiler and found this thread very helpful in explaining some of the causes and solutions.

The low RWT had led to P/S and TermUnit concerns...

How's the overall performance, comfort?
The one thing I may change is not taking the indirect off of the primary loop with closely spaced tees, and take it directly off of the boiler outlet and return on the inlet, rather than have it run all the way around the loop, past the zone heating, then back to the boiler.
But if the indirect tank is recovering fast enough (or it's on priority) I guess it doesn't matter.
It also looks like your injection piping is incorrect.

Watching this for a bit now, it is clear the Indirect “secondary loop” is not isolated from the primary boiler loop. I have installed temperature sensors at the inlet (SWT) and outlet (RWT) of the boiler connections to the Indirect tank that are several feet from the “not so closely placed Tees”. As the primary loop temperature rises and falls the loop to the indirect will as well. So when the primary loop circulator is on for heating, the loop to the indirect rises and falls along with it and “passively” adds or extracts heat to/from the indirect. Less than ideal it seems!

In watching the mix loop when DHW calls at the same time a zone(s) is calling for heat, there is a little push into the the mix loop showing on the RWT sensor I installed, but both SWT & RWT on the mix loop seem to stabilize in a minute or two. I have a relay now interrupting the call for heat to the 356 when DHW calls, so zone valve(s) will be open in this case but the mixing loop circulator and injector are off.

One thing I see on the mix loop is that as the injector pump increases then the change across the boiler rises. Seeing ranges of 3-15 degrees delta. Any chance it is by design? Would seem to make sense that as the injector calls for more to the mix loop the boiler delta increase would be desirable. Clearly this loop is not designed as outlined in the P/S documents you linked.
Zman said:

Dipping into condensing temps at startup is normal. It is important that the boiler runs hot long enough to dry the condensate on the boiler and vent. When you drive around you can see rust on the vent terminations that have oversized atmospheric boilers. Some are like yours where they are running low temps, others are boilers that short cycle and never dry out.

I setup so the boiler would run for a while yesterday morning. We have 4 upstairs zones that we generally keep at 55* so I reset the 356 to TermUnit 2 for those and bumped the thermostats to approximately 70, then observed the boiler cycling flame on to flame out and kept track using stop watch & lap timer for about 45 minutes. Outside temperature was 19 initially and came up to 21 during this time and the target mix temperature started at 120 and dropped to 118. The boiler minimum is 120 in the manual and that is what the 356 is set for and it maintained the temp above this for this period. High limit on boiler shuts flame off at approximately 180 and boiler would re-fire between 155-160.

The initial cycle the boiler was on for 10m49s then off for 5m18s, then the on times gradually shortened and off times lengthened:
On 10:49
Off 05:18
On 01:58
Off 02:41
On 01:29
Off 02:59
On 02:11
Off 02:20
On 01:24
Off 03:46
On 01:19
Off 03:22
On 01:16
Off 03:46
On 01:18

Last night I seemingly created a combination that could not be satisfied. I had returned the TermUnit setting to 1 then set the upstairs zones to approximately 70* or so - simple thermostats that are not very precise. The thermostats were never satisfied overnight and it was not terribly cold by our standards: 25-30 degrees. Watched a couple cycles this morning. Mix target was 96 or 97. Boiler would run 2m15s - 2m45s, then be off for 25 to 30 minutes. The primary loop would go to approximately190 (flame out at 180 or so) then over the next 1/2 hour drop to 107 at which time the boiler would turn on again for 2-3 minutes.

When the primary loop was dropping to 107 so was the SWT to the Indirect hot water tank and RWT was picking up 8-9 degrees and exiting at 115/116.

Are those burn times sufficient to avoid condensation? Seems short, but I don’t know what to expect at this point.

Should a bypass circuit with the thermostatic mixing valve @hot_rod linked be installed to protect against these drops in RWT? Or are they short enough not to worry about?

I was able to get into a light fixture as you suggested and determine there are no plates on the underfloor install for the upstairs zones. Just staples periodically holding poly tubing to the underside of the flooring. Appears that I have two distinctly different TermUnits being served by a single mixing loop. Wondering if I should return this back to 2 where it was in order to match the upstair for now?

I am reading the idronics documents and watching some videos hot_rod has produced as I have time and bandwidth to absorb/digest. I have ordered a couple books on the subject as well.

I know simply by square footage of the zones I must have considerable variance in the size of the zones. Is there a way to determine the size of the zones (in gallons) and the overall size of the system for a system that is now buried/covered and inaccessible?

Not in a hurry to install a new system, but the existing unit is 16 years and and the day will come eventually. Seems maybe the last 16 years have not been easy on this unit! What other data/information can/should be measured/learned with the existing system to develop a plan for the next system?

The house is staying warm and with Zman’s tweak we have DHW priority now. So, no emergency at the moment, but does not seem the system is optimal and would like to learn what can be done better moving forward.

Any tweaks to this system that would carry over to a new system? For example, would a variable speed circulator for the mixing loop benefit and carry over? Clean up the loop to the Indirect DHW? ??

I have attached again the drawing of the existing system and photos.

Have learned a lot already. Thank you for all the assistance!

Best regards,



  • hot_rod
    hot_rod Member Posts: 16,287
    micro zoning is a tough go on a large low mass copper tube boiler like that. If one to two zones call it will be hard to eliminate the short cycles, as that boiler has only one speed. full speed ahead.

    Getting accurate temperature control is also tough with a boiler that can accelerate so quickly under low load conditions..

    Some of those copper tube boilers had modulating gas valves to help with zoned systems.

    A mod con would eliminate a bunch of piping and pumping, help with modulation to all those zones, if that is in the cards some day.

    Back in the 90's that was a common piping layout.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • CTG
    CTG Member Posts: 14
    hot_rod said:

    ...A mod con would eliminate a bunch of piping and pumping, help with modulation to all those zones, if that is in the cards some day.

    Hi hot_rod,

    Yes, I think the future will have a mod/con. No modulating capability on the existing unit.

    Would like to begin sorting out the future plan for that and understand what the simpler piping and plumbing layout is.

    Also, if there is any measuring/monitoring of the existing system/zones that can be done to provide valuable information to the process of laying out the next system?

    Thank you!