Pitfalls of using PEX to connect replacement radiators?

[Veronica F]

Hoping to get some advice regarding connecting new (steel, not cast iron) radiators to existing piping. Contemplating oxygen-barrier, up to 200F pex. Unsure of what size PEX. Unsure if it is even a good idea.

My home: built 1941, one story with basement, a bit over 1000 sq ft, closed loop hot water heating system, all radiators gone when I purchased it. The rooms each have two pipes (currently capped) protruding through their floors. The floors are wood, except in the bathroom, which is concrete over wood.

I replaced the old dead boiler and the old dying water heater with a combi boiler/tankless. Max temp 180F. No added circulating pump. The old expansion tank is still hooked up. I had one new steel column radiator installed in one room. The installer kept the existing piping that came through the floor for one valve connection, but had to alter the piping below the floor and add a new hole to the floor for the other valve connection. This was apparently fairly labor intensive. And therefore expensive.

I need to add more replacement radiators. Because there will not be a match between the width (distance between) of the existing piping coming up through the floors and the width of the new radiators, I'm looking for the simplest, quickest solution to connect the existing piping to the new valves/new radiators.

I realize that short runs of pex above the floor won't be beautiful. I can live with that, or fabricate some sort of covering/sleeve.

Are the short runs of PEX I'm considering feasible? Code compliance issues? How do I determine the correct size - do I match the existing pipe size or the valve size (if they are different)?

Thank you

 

[Dana]

Odds are the old system was over piped and over-pumped, so there isn't really a need to match it. If the radiation pump is giving you reasonable (25F or less) round trip temperature drops, adding more length to the loop isn't going to be a deal breaker. If the delta-T is under 10F adding the extra radiation is going to improve efficiency slightly.

Most combi-boilers big enough to deliver decent hot water at Ohio wintertime incoming wter temps are going to be oversized for a 1000' house with insulation in the walls & attic, and at least storm windows over wood sash single panes, and it may be short cycling, especially if running it at condensing water temperatures. Even a pretty leaky 1000' house will have a heat load of about 20-25,000 BTU/hr, which isn't much above the minimum fire output of many wall hung combi-boilers. But given enough radiator to emit the heat it'll still do OK efficiency wise, even if it's less than ideal, and won't be modulating much.

I don't believe there are any code issues with using PEX above the floor, but the aesthetics leave a lot to be desired, as noted.

If you had the luxury of time to run load calculations to get the radiator sizing right it would be possible to max out both efficiency in comfort, but it sounds like you're in a scramble to get the system up and fully functional in every room, so you may have to live with some room to room temperature imbalances for awhile.

 

[Sponsor]

 

[Jadnashua]

I don't know of any pex that can stand UV...most rooms will have a little bit of that through the windows. If you do use it, cover it. Some radiators seem to be only anchored in place by their supply pipes...pex won't do that, so you'd need to devise some other way to hold them in place. The ID of pex is smaller than the equivalent iron piping, so you may need a larger diameter for best flow (which, you may not need).

 

[Veronica F]

Thank you, Dana.
My new boiler is 28k-140k - I was trying not to go too big, even taking into account the lack of wall insulation. The windows were all replaced some years back with double-glazed vinyl. I'm in SW Ohio, which is balmy compared to most of the rest of the state.
I'm thinking that I will have imbalances and modulating until I get all the radiators installed and find the right temp setting. The info you gave about the delta-T should help me. If I can't figure it out, I will at least be able to have a better conversation with a professional.
My installer recommended not using the outdoor sensor - can you think why?

 

[Veronica F]

Thank you, Jim.
Yet another reason to cover the pex! I was thinking of stitching some FR fabric and velcro tubes. And you've given me the simplest description of the difference in size - now it clicks that of course pex has a smaller ID!
The radiators I'm looking at buying are wall mount (with optional feet to have them sit on the floor).

 

[Cacher_Chick]

Another option you might look at is installing hydronic baseboard heaters to which you can run the PEX in such a way that is is all within the units.

 

[Dana]

Thank you, Dana.
My new boiler is 28k-140k - I was trying not to go too big, even taking into account the lack of wall insulation. The windows were all replaced some years back with double-glazed vinyl. I'm in SW Ohio, which is balmy compared to most of the rest of the state.
I'm thinking that I will have imbalances and modulating until I get all the radiators installed and find the right temp setting. The info you gave about the delta-T should help me. If I can't figure it out, I will at least be able to have a better conversation with a professional.
My installer recommended not using the outdoor sensor - can you think why?

The min-fire input of 28K means it's min-fire output is probably higher than than your design heat load, and an outdoor sensor and outdoor reset curve would probably just induce short cycling at lower boiler tempratures with your limited radiation. Without knowing the size of the radiation you're going to have to just experiment with setting the fixed temperature.

Turn the thermostat WAY up so that there will be a very long call for heat. The burner will most likely be turing on and off before the thermostat is satisfied. Time the burns, and count the burns. If the burns are under 100 seconds and is cycling a rate of 10 burns or more per hour its' really hard on the boiler. Raise the boiler's temperature setting to where o where the burn time is more than 200 seconds &/or the number of burns per hour are five or fewer during a continuous call for heat. If that temperature is above 140F it won't be condensing, so you'll be limited to ~87% combustion efficiency, but it'll still use less gas than if it's condensing but short-cycling.