Ideal water temperature for baseboard heat.
Last Post 24 Oct 2011 11:28 AM by Dana1. 20 Replies.
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24 Oct 2011 11:28 AM
Posted By ctech99 on 22 Oct 2011 12:23 PM
Hello,

I found this thread on a google search, and while it may not be on the same topic as the original post, it does pertain some similar information.

Here is my question:

I purchased a home that was built in 1979, it has an older traditional boiler system (is that a condensing or non-condensing?). It has baseboard radiant heat that is plumbed with copper lines throughout, the house is divided up into 3 zones.

As we slowly remodel rooms, I have debated running some underfloor heat with some pex tubing. 3/4" with some transfer plates fastened to the bottom of the subfloor, and tying that into the existing copper lines that are plumbed to that room.

Now, the issue here is the water temp. I've read/been told that your baseboard heat is around 180F+, and your underfloor heat should be around 140F.

Can I make this work? Can I say turn down my boiler to maybe 160 to make the whole system work, with a mix of underfloor and baseboard heating systems? Can I run everything at 180F and be safe? Will that get to hot and buckle the subfloor? If I turn the boiler temp down, do I risk the colder water return cracking the boiler?

I've only had one person tell me that I can't easily replace 1 baseboard piece with some underflood pex tubing, I just want to get a few more opinions before I try this.

Thanks

If the boiler is circa 1979 it is not a condensing boiler and is not tolerant of temps lower than ~130F (if gas) or ~140F (if oil) coming back from radiation entering the boiler.  If you run the radiant floors as indivitual zones with a thermostatic mixing valve (as Blueridge recommended) that isn't much of an issue.  But this really needs to be designed, not hacked to get best results.  If you're doing it room-by-room you'll end up with a very micro-zoned heating system with micro-loads, and keeping the boiler from short-cycling itself into low efficiency and an early grave may become in issue.

But if the boiler really is 30 years old it's probably time to bite the bullet and buy a condensing boiler anyway. This is even more true if the boiler is more than 2x oversized for the load at outside design temperature, which is common.   Gas-fired oilers of the late-'70s ran steady-state efficiencies  of  ~80%, and 75-78% AFUE when brand new, and 30+ years hence with normal corrosion, liming & wear & tear on the heat exchangers it's probably not doing better than 75% combustion efficiency and 65-70% AFUE.  If it's 3x oversized for the load, knock that down to ~60% AFUE.  With three zones it has to be more than 3x oversized for the smaller zones, and it may in fact be short-cyling on zone calls, cutting further into efficiency.  Replacing it with a "right-sized" modulating condensing boiler it wouldn't be surprising to see a 35-50% reduction in fuel use.

180F is an arbitrary number- fin-tube's output is predictable down to ~120F.  Most older systems were designed with both the boiler and the fin-tube lengths oversized for the actual heat loads.  If the house has been tightened up since it was built and it has storm-windows or insulated-glass windows in decent shape the probability that the baseboards can deliver sufficient heat at outdoor design temps is quite high, but running it there with the existing boiler without some modification to the near-boiler plumbing to feed in some direct boiler output water to keep the return temps up would wreck the boiler in short-years from condensation. But running it at 140F or lower is ideal for getting the condensing-efficiency out of the boiler, since the return temps would almost always be in the condensing zone.
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