Posted By losttrail on 11/09/2009 8:44 AM
Thanks for the info. Looks like an interesting product. I'll get more info the closer we get.
Has anyone done a conversion from baseboards to floor?
It's a fairly common retrofit. The above-subfloor solution you're proposing has been commercialized by folks like Warmboard ( http://www.warmboard.com/
) and is fairly responsive compared to slabs & staple-ups (under-subfloor installation.) And above the subfloor is the preferable retrofit if you're ripping up the rugs to install hardwood/tile/etc.
You'll still need to insulate under the subfloor to keep heat losses from radiating downward, and don't get sucked into thinking reflective insulation (radiant-barrier) is either necessary or sufficient for that function (most of the time it's neither.) R15-R19 is usually sufficient between conditioned spaces, R19+ would usually be called for above unconditioned space.
The hydronic water temps of radiant are dramatically lower than most baseboard designs call for (you'll likely be looking at 110-140F vs. 160-200F), and if you don't have a high-efficiency condensing boiler you'll need to protect the boiler from excessively cool water or it's lifespan will be drastically reduced. Return-water temps entering oil-fired boilers needs to be above 140F, but with gas/propane you can usually tolerate return temps as low as 130F, provided the flues are lined & properly sized. (Flue condensation/corrosion can be an issue otherwise.) But since that's often above the output temps delivered to the floor, there will be some plumbing tweaks necessary (primary/secondary loops, boiler-bypass, etc) to keep both the radiant floor and the boiler happy. The specifics will vary depending on the particulars at hand.
You may need to add water-mass to the system to keep an old-skool boiler from efficiency-robbing short-cycle burns as well (particularly if the system is broken up into several micro-zones.) Since most are 2-4x oversized for the coldest-hours-of-the-year heat load anyway, micro-loads start to look RIDICULOUSLY undersized for the heat output.
Done right, dropping from 180F temps to 130F temps will by-itself yield ~15% in fuel savings, assuming you stick with the same boiler. But with a modulating or modulating-condensing boiler utilizing "outdoor reset" control (changing the boiler's water temp in response to outdoor temps) you'll reap much more (30-50% savings isn't unusual.)
It's a hefty chunk o' change up front, but with radiant you can typically run much of the season well below 120F water temps, where real 90%+ efficiencies kick in with mod-con boilers:
If your system design indicates mid-winter water temps predominantly higher than 130F it's harder to rationalize the cost of retiring functional boiler for a mod-con at current fuel prices.
Note, if you're running a 4x oversized short-cycling high-mass boiler your true AFUE is going to be in the 55-65% range, even if the specs say 78-83%. This can sometimes be mitigated by adding thermal mass (big buffer tanks) to the system, but for the money you may be better off retiring the beast early and installing a "right-sized" mod-con or modulating low-mass copper-tube boiler.
In the end, it's usually more than just swapping baseboards for radiant- there is some system design to be done, but if you plan to live there awhile it's worth it from a comfort & cush-factor point of view. There may be some home resale-value aspects, but don't count on ever making it back in fuel cost savings. (It's easier to calculate/rationalize for new-construction than with retrofits.)