Basement reno - switching from cast iron loop to home run PEX

[JohnBoston]

Hello,
In early planning stages of a basement reno. and would like to get some input.

First off, I should say that I am very happy with my current heating system, although it is outdated and less efficient, and the only reason I am taking on this project is to gain headspace in the basement.

Also, I don't have the $ to do a full system upgrade (mod con direct vent etc etc) so it is something like this approach or not at all.

Some details about the house and current heating system:
- 100 yr old, 2000 sq ft, brick exterior colonial, outside of Boston. We have been living here 5 years.
- 2 floors living area with unfinished attic and basement.
- Gas heated Utica boiler from 1996 (150K input), and Ruud 50 gallon hot water heater from 2015 (40K input). We also have a gas dryer and stove.
- One zone/single thermostat.
- The boiler is serviced every year and they always say it is in great condition, I know as well that it is 25 yrs old. If/when it goes I would prefer another low maintenance, longer life cast iron replacement.
- 2 original gravity 3" cast iron loops (feed and return) supplied from 1 1/4" copper from the boiler. The 3" loop is supplying 12 cast iron radiators (8 on first floor).
- Single Taco 007-F4 circulating pump
- The radiator feed lines from the 3" loop are a mix of 1" iron (mostly), and 3/4" copper. There does not seem to be any reason as to why some are 1" and some are 3/4 - not radiator size related and the all heat equally well.
- I have not run a heat calc on the radiators but the largest is 5ftL x 20" H x 8" D (and has 26 sections)

The general plan:
- Remove the 3" cast iron loops and replace with home run 1/2" pex-al-pex direct to each radiator to replace the current 1" and 3/4 inch supply.
- Feed and return manifolds tied in to the current 1 1/4" copper from the boiler.
- Keep everything else the same.

Are there obvious red flags in this plan? Things that I have not considered that I should? I can upload pics if helpful.

Thanks

 

[Fitter30]

Read this article
https://www.phcppros.com/articles/7804-when-old-tech-meets-new-tech

 

[Sponsor]

 

[JohnBoston]

Thanks - that was a good read and an optimal design. Trust me, I would love to do that - new mod/con lochinvar knight with ODR, TRVs etc. but it is just not in the budget. I had some early discussions with local reputable heating guys for something similar and the back of the envelope all in price is $25-$30K. We have saved some money for when the current CI boiler finally gives up but nothing like that.

Assuming that that is the Cadillac option, is my Honda option really not an option? - I don't want a Ford Pinto either. I know that I am adding more head to the system and may need to change circulating pump, use low pressure drop manifold etc ..... but is it doomed to failure even outside of that? Why?

 

[Fitter30]

In the article using a pex manifolds might be a easier way to pipe the system but need idea how much water flow to each heat emitter would need.

 

[JohnBoston]

I calculated the output for each radiator using this table from Utica
https://uticaboilers.com/sites/default/files/Radiation Tables.pdf

The radiators are the 5 tube type. There are also 3 in wall radiators that were not an exact match but I'm assuming the #'s are close enough.

This is what I got: 89,000 BTUs total

1st Floor
1. Living room 1 - 22 sections x 2.66 sq ft/section = 59 sq ft x 150 = 8,778 BTUs
2. Living room 2 - 22 sections x 2.66 sq ft/section = 59 sq ft x 150 = 8,778 BTUs
3. Family room 1 - 22 sections x 2.66 sq ft/section = 59 sq ft x 150 = 8,778 BTUs
4. Family room 2 - 24 sections x 2.66 sq ft/section = 64 sq ft x 150 = 9,576 BTUs
5. Dining room - 26 sections x 2.66 sq ft/section = 69 sq ft x 150 = 10,374 BTUs
6. Hall - 14 sections x 4.33 sq ft/section = 61 sq ft x 150 = 9,093 BTUs
7. Kitchen - in wall - 9 sq ft x 150 = 1,350 BTUs
8. Bath - in wall - 7 sq ft x 150 = 1,050 BTUs

2nd Floor
9. Bedroom 1 - 14 sections x 3.5 sq ft/section = 49 sq ft x 150 = 7,350 BTUs
10. Bedroom 2 - 18 sections x 3.5 sq ft/section = 63 sq ft x 150 = 9,450 BTUs
11. Bedroom 3 - 25 sections x 3.5 sq ft/section = 88 sq ft x 150 = 13,125 BTUs
12. Bath - in wall - 9 sq ft x 150 = 1,350 BTUs

I'm not really sure where to go from here but will give it a shot based on what I could find.

Assuming 20 degree design temperature (?), and at 4 fps (?), 1/2" pipe would flow at 1.5 gpm and 3/4" would be at 4 gpm. 1 gpm = 10,000btus, 2 gpm = 20,000btus, 3 gpm = 30,000 btus, 4 gpm = 40,000 btus

My largest radiator is 13,000 btus and the majority are <10,000 so 1/2" tubing would work?

Am I completely wrong/off base here?

Thanks

 

[Fitter30]

59 sq ft x 150 btu's @170* water =8850 btu's
[PDF] Cast Iron Radiator Sizing Guide - Express Radiant
www.expressradiant.ca › pdfs › pro...

 

[JohnBoston]

59 sq ft x 150 btu's @170* water =8850 btu's
[PDF] Cast Iron Radiator Sizing Guide - Express Radiant
www.expressradiant.ca › pdfs › pro...

Thanks

I checked the actual temp. setting on the boiler and it is at 160. Seems low but that is what it is set at. This reduces the btu/hr to 130 which reduces the EDR as shown below.

Should I use less than 160 supply temp.? In the doc that you linked it also says "Please note that a house heated with all cast iron radiators, the supply temperature generally does not exceed 140F or 90Btu’s per square foot output for the cast iron radiator."

1st Floor
1. Living room 1 = 7,608 BTUs
2. Living room 2 = 7,608 BTUs
3. Family room 1 = 7,608 BTUs
4. Family room 2 = 8,299 BTUs
5. Dining room = 8,991 BTUs
6. Hall = 7,881 BTUs
7. Kitchen - in wall = 1,170 BTUs
8. Bath - in wall = 910 BTUs
2nd Floor
9. Bedroom 1 = 6,370 BTUs
10. Bedroom 2 = 8,190 BTUs
11. Bedroom 3 = 11,375 BTUs
12. Bath - in wall = 1,170 BTUs

Either way this seems to be supporting that 1/2" pex would be enough. Thoughts?
Thanks

 

[Fitter30]

1/2" should be fine look at manifolds for radiate heat for piping some have flow.meters built in