CI Radiator Re-pipe - Primary/Secondary vs Boiler Bypass

GRIZ · October 2018

I'm in the middle of a radiator re-pipe of a converted gravity system. Cut out the supply and returns and replaced with home runs of Pex-Al-Pex to a manifold. I had originally planned to keep all the near boiler piping the same as it was since it had been working well to this point. I didn't see any reason to change anything. Had a Taco 007 on the return side to run the whole system (1 zone, 8 rads). However, by the looks of the flue, there was some pretty good condensation going on. I need to remedy that moving forward. Looking pros and cons for Primary/Secondary vs Boiler Bypass arrangements. If I go with Primary/Secondary do I need to add additional controls to run the additional pump?

Zman · October 2018

Primary secondary will not fix a condensation issue. The water will still come back too cold. A boiler bypass with a thermostatic valve would do the trick.

GRIZ · October 2018

Gotcha. Now that I look a little closer the primary/secondary arrangement I was looking at was actually also piped as a system bypass. There are several arrangements detailed in my Weil Mclain boiler manual, but rather than thermostatic valves they detail a process to using mechanical valves to control the difference in temperature between the system return water and the boiler exit water.

GRIZ · October 2018

So I guess I am actually looking for pros and cons for P/S with System Bypass vs Single Pump setup with Boiler Bypass?

hot_rod · October 2018

A cast iron boiler with radiators? Really no good reason to go with P/S and two pumps if you are just looking for return temperature protection, A simple thermostatic 3 way boiler protection valve will guarantee protection. A by pass, even a pumped bypass does not have a temperature response mechanism to assure 100% protection. If that is your goal.

Our next Coffee with Caleffi webinar is on boiler protection options if you want to tune in Oct 18, noon central.

GRIZ · October 2018

Thanks for the input guys! Yup cast and cast. Weil McClain CG Series 11. This is the schematic I'm leaning towards:

Boiler protection was my main goal, but it would be nice to also keep the supply water at the rads at a lower temp rather than putting 180 degree water in them and really cooking. In the previous arrangement they were warm to the touch when heating, but never uncomfortably hot to touch. It's my understanding that in the P/S arrangement the secondary loop typically runs at a lower temp than the primary most of the time. Would you agree with that concept? This also appeals to me because Pex-AL-Pex is only rated to 180-200 degrees, so I'd like to stay away from that high limit if possible (without adding a lot more unnecessary complexity).

I'm looking at using a Caleffi 521609A or a Taco 5004-HX-C3
for the thermostatic valve to set the boiler return water temp to 140. And a single zone switching relay to run the system pump.

Is this still overkill? Should I just keep the thermostatic valve and ditch the extra pump and P/S arrangement? That would be close to the original configuration, but adding boiler protection.

hot_rod · October 2018

skip the PS, use a boiler specific protection valve like Caleffi 280 or Danfoss, Paxton. You want a high Cv valve. Put the boiler on a reset control, limit to 160 if that is adequate.

The PS is going to give you varying temperature blends, that may work against your goal.

GRIZ · October 2018

Thanks again for the input! I will tune in to the Coffee with Caleffi webinar. I am looking at the Caleff 280966A now. I had to get heat going due to outdoor temps to keep the house comfortable.
I have included a photo of my current arrangement. With the shutoffs at the mainfold I can drain the near boiler without draining the rads and add the thermostatic valve fairly easily. When I add the outdoor reset controller (picked up a Taco SR501-or-4 for this), the SWT sensor for the OR controller should be just outside the Boiler bypass so it's measuring the system temp and not the boiler bypass loop temp, correct ? I'm going to pipe the bypass between the horizontal sections about where the barcode sticker is on the supply line.

I have attached a photo of my current boiler setup and a picture of the heat exchanger. Is this typical, or is this a sign that it has been condensing excessively. Much of the single wall flue vent pipe was corroded to the point of having large holes in it. (Yikes!!) That's why I really want to add the boiler protection. I actually expected the heat exchanger to look worse. Thanks again for your input.

hot_rod · October 2018

we will show six or more piping options in the webinar this Thursday

GRIZ · October 2018

Very informative webinar Hot Rod. Really appreciated the comment about the location of the fill in relation to the thermostatic valve. A couple questions:

You commented on the different temp ratings, of these valves and how you had a 130 but determined that you only needed the 115. I was originally thinking I would go with the 140 deg valve, but maybe the 130 would suffice? I'm not really sure how to determine which one I would need?

I think my question about location of the SWT probe from the OR was answered. It should essentially be right at the supply manifold.

hot_rod · October 2018

Here is how the valve works and a graphic on the flows at various temperatures.

https://www.caleffi.com/sites/default/files/file/01223_17_na.pdf

So a 130° plus 18° would = 148° by pass off completely. A 130 would be fine for your boiler, a 122 would work fine for an oil fired based on the difference between gas and oil dew points.

Wood or bio-fueled boilers a a bit more of a wild card depending on fuel and actual operating condition.

You'd need a combustion analyzer to determine exactly the CO2 in your flue.

CI Radiator Re-pipe - Primary/Secondary vs Boiler Bypass

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