Primary Secondary Flow Direction

OLmako20 · January 24

Good afternoon, great resources on this site!

I am a homeowner/DIY and had an installation of a Vitodens 100 B1KA w/on demand DHW a few years ago. The installed didn't go that well as the contractor hooked the boiler up in reverse. This was very noticeable as DHW had uncontrolled temperature as it was flowing in reverse so the outlet temperature sensor always thought it was cold because cold water was flowing in. They corrected that and the DHW temp was resolved.

The boiler has been in service for a couple of years and working OK however I noticed that the primary and secondary loops are also reversed. As I stated the house has been heating OK, but I am sure the boiler is not operating at its peak efficiency. Basically the boiler gets pre-warmed water as it mixes at the closely spaced Ts with the return water from the radiators. I know there is mixing here based on the demand of the system, but I would think this would really raise the return temperatures to the boiler unnecessarily.

The fix would be to re-plumb, which I plan to do at some point, but would switching the direction of the secondary loop circulator work? this is a quick unbolt flip over and re-bolt. This would mean the zone valves and pump are on the return side, not sure if there is any issues with that.

Below is the schematic of how the system currently is plumbed.

Thoughts?

EdTheHeaterMan · January 24

That will work only if you also reverse the zone valves. If you look closely at the zone valves there is an inlet and outlet. When the zone valve is installed backwards, you can get other problems.

another point is that the hottest water will go the what used to be the coldest part of the system. That may or may not be a problem, depending on how the system is designed.

hot_rod · January 24

Most zone valves are directional, they will have flow arrows on them. Reverse flow will often cause them to bang or hammer.

Reversing at the tees may be the easiest, best fix.

EBEBRATT-Ed · January 24

Its piped wrong the supply and return at the two closely spaced tees need to be swapped.

I don't get what the zone valves have to do with this. If you reverse the zone pump you would have to reverse the zone valves. That is more work than swapping the two pipes.

OLmako20 · January 25

Thanks for the feedback. Yes I agree probably just best to swap the Ts. Now my next question, do I need to swap the spriovent? And water makeup to the supply side of the primary loop? Since i will be opening up the system anyway is a low loss header worth the extra effort/money over closely spaced Ts?

hot_rod · January 25

Is the spirovent on the hot supply out of the boiler? If so it looks like the tees are correct.

OLmako20 · January 25

no it’s on return side

EdTheHeaterMan · January 25

I just answered a question with a Similar problem where the return and the supply was reversed. His was trying to solve a different problem but offered the diagram in this post.

EdTheHeaterMan

Jan 25, 2025

I want to make a few corrections on your drawing here.
The primary loop is the loop with the expansion tank
In your drawing the expansion tank is located on the system side of the primary secondary piping, so the primary loop is the zone valve system.
That would make the boiler loop the Secondary in your drawing.
The closely spaced tees are connected in reverse. the hot water from the boiler supply is being put in just before the return piping to the boiler, So the installer got it backwards and your system is preheating the return to the boiler and the return from the system is cooling off the heated water to the radiators.
I believe that your bang may come from water expanding or contracting as a result of all three zone valves closing against the check valve in the Zone Circulator. See the Red shaded section of the diagram below
https://us.v-cdn.net/5021738/uploads/2ZLJJ7D8FGU1/screenshot-2025-01-24-at-6-41-19-e2-80-afpm.png
Once all the calls for heat are satisfied, the hot water in the baseboard radiators will cool off and contract however that water has no connection to the expansion tank, so Hydraulic pressure (actually Hydraulic Vacuum) is put on the circulator check valve and the zone valves. As soon as one of the zone valves opens, that Vacuum will be released rapidly and that may be your BANG.
Since you need to make the repair to the reversed supply and return from the boiler to the system anyway, you can make several other corrections at the same time.
Move the expansion tank to the supply pipe from the boiler
Remove the check valve from the Zone Circulator
Put the supply from the boiler ahead of the return to the boiler in the system (ZONE) loop so you are not mixing the hotter water with the colder water.
https://us.v-cdn.net/5021738/uploads/4TDSZC9YQYS2/screenshot-2025-01-24-at-7-34-23-e2-80-afpm.png
This way the DHW circulator and the System circulator will be in the hottest portion of the system and pumping away from the point of no pressure change. (the expansion tank)
All the piping will have access to the expansion tank from one direction or the other even when all the check valves and zone valves are closed.
Leave the boiler pump on the return to increase the boiler pressure going into the boiler as many manufacturers recommend, in order to keep water in the heat exchanger from flashing and making a different noise.

Here is the diagram as his system is installed. See the closely spaced tees and how they are reversed. I don't believe that he knows that reversing the supply and return like this is a problem. All he wanted is to solve is the noise issue.

First Diagram reposted here

Take a look at the second diagram and see how easy it would be to make yours look like that one. Disregard the DHW loop if you are not using an Indirect Water Heater.

Second Diagram reposted here

I offered the second diagram to resolve his noise problem and to correct the reversed piping on the closely spaced tees. See if your system can be repiped this way where the Expansion tank is located on the boiler loop supply side I understand that your boiler pump is internal to the boiler unlike this diagram, but all the parts are in the right order for noise free and trouble free operation

OLmako20 · January 25

thanks for the advice. I think once the winter heating season is over I’ll take a crack at repiping the system.

Is a hydraulic separator vs closely spaced Ts a good improvement, like the caleffi hydrosep? It would be more of a project, but since the system will be open anyway might be a good time to.

PC7060 · January 25

simplest thing is to swap the connections at the boiler, leave T as is, and see how it performs for a year. With exception of the input/output piping mistake at boiler, the installed did nice job. You’ve got isolation valves in the secondary loop so won’t have to drain down system.

hot_rod · January 25

it becomes a little weird when you have two expansion tanks in a system. Two PONPC locations

Your boiler has a pancake expansion, and you have added a second. The one in the boiler, probably a liters, size may be close in capacity to a #30 an be adequate on its own? You would need to know or guesstimate the water volume of the system

Those metric tanks have a small BSP thread, so it not easy to adapt a larger external tank into that connection point.

Some mod cons have high head circs, that one may as it looks to be a coil type boiler HX. the correct expansion tank connection can be more critical, so you don’t pop relief valves with the added delta p.

PC7060 · January 25

@hot_rod - is the expansion tank the vertical rectangular object at upper left of boiler?

hot_rod · January 25

PC7060

Jan 25, 2025

@hot_rod - is the expansion tank the vertical rectangular object at upper left of boiler?

yes

IMG_9536.png

OLmako20 · February 4

any benefit from a low loss header similar to sep4 by caleffi? Or will closely spaced Tees do the job?

hot_rod · February 4

low loss header, closely spaced tees and hydraulic separator, all three properly installed will provide hydraulic separation.

Some of the hydraulic separators provide multiple, important functions .

The Caleffi Sep 4 has a micro bubble air purger built in the top

On the bottom it has a dirt separation function that will remove particles as small as 5 micron.

Additionally it has a magnet to remove small metallic particles, mainly magnetite, which is like a powdered graphite consistency .

It also has an easily serviced auto air vent on the top.

For installers it saves time when installing all those components as separate pieces.

With todays high efficiency boilers and pumps you need all those functions, to keep efficiencies up, possibly provide warranty coverage.

A circulator that fails due to magnetite accumulation in nor a warrantable failure. ECM circs need magnetic protection!

Primary Secondary Flow Direction

Welcome

Comments

Welcome

Welcome

Quick Links

Categories