Converting from baseboard to in-floor

Discussion in 'Boiler Forum' started by seattle_steve, Aug 6, 2012.

  1. seattle_steve

    seattle_steve New Member

    Messages:
    28
    Location:
    Seattle, WA
    I had a Munchkin boiler installed a couple years ago with 3 zone valves. Only one of the zones was initially connected -- to baseboards on my main floor. The other 2 zones were intended for a future 2nd floor addition and finished basement.

    I've now had the 2nd floor addition framed (plumbing and wiring finished, insulation next) so want to heat it now. I'll have access to the 2nd story subfloor (from below) and would like to install radiant tracks and tubing to the underside of the subfloor to heat the 2nd floor (instead of installing baseboards). I plan to convert the main floor zone from baseboards to in-floor as well.

    My main question is: can I use one continuous loop (1/2" tubing?) for the 2nd floor zone? Or do I need to split up that loop into 2 or more sub loops? If the latter, is it as simple as adding wyes/tees to the start and end of the zone loop? (I'm concerned whether that will ensure even heat flow through the sub loops?) The second floor is approximately 800 sq feet and consists of 3 bedrooms and one bath.

    I am aware that I will also need to reduce the boiler temp for the conversion from baseboard to radiant.

    Anything else I need to know or other recommendations for this conversion?

    Thanks.
  2. Dana

    Dana In the trades

    Messages:
    3,032
    Location:
    01609
    You basically have to stay within a 300' limit on any radiant loops or you won't get enough flow. In most cases that will mean running 3/4" to purpose-made manifolds, and splitting out the multiple 1/2" loops. In an 800 square foot zone that will be at least 1200' of half-inch PEX, and an absolute minimum of four sub-loops on the manifolds, but going with somewhat shorter loops but more of them reduces the pump power necessary to make it all work. Keeping the lengths of the sub-loops identical is critical for getting the same flow through each loop too.

    Read up on it before diving in- most radiant heating designers are using application-specific software tools to get it right, including pump specs, water temp, etc. Getting it "almost right" or "good enough" is certainly possible when hacking at it, but it won't always.

    Every good heating design starts with a room-by-room heat loss calculation, from which you can work out the radiation/output required. Odds are you've been able to run your baseboards at 140F-150F and still have enough heat getting into the room at an outside design temp of ~ 20F(?)

    Unless you want to become a junior radiant heating designer, it's probably better to PAY A PRO to do the radiant design, specifying everything from the heat transfer plates to the manifolds to the pumps, mixers, hydraulic separators,etc, even if you do the installation as sweat-equity. Do not substitute freebie radiant designs from web-vendors- they end up being more expensive and less efficient than radiant done right by a competent designer. A grand or so spent on a good design will save that much in excess/inappropriate hardware and a lot more in frustration level from having to re-plumb stuff to get it to work well.

    Is your boiler one of the Contender models (sloping domed top) or one of the re-skinned Peerless Pinnacles (cube-shaped)? IRRC with the Contender there is no outdoor reset built in to the controls, whereas the li'l cubes do, which can make things easier to tweak in sometimes.
  3. seattle_steve

    seattle_steve New Member

    Messages:
    28
    Location:
    Seattle, WA
    Thanks Dana. I guess this is a little more involved than I had hoped... Maybe I will get some bids on design/install of in-floor and then weigh that against just putting in baseboards, which I think I can do myself. Only problem is the bathroom, where there's no good place for a baseboard...

    Yeah, I have a Contender. No outdoor reset, although it is available as an add on.
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,352
    Location:
    New England
    With a good design, it should be detailed enough for you to do the actual install. A good one won't be free.
  5. Dana

    Dana In the trades

    Messages:
    3,032
    Location:
    01609
    From a cost point of view there's nothing to compare- a baseboard system will be a small fraction of the cost of even a DIY radiant floor (unless you go with a high-temp staple-up, which means limited condensing efficiency and slow response time on the radiant floor.)

    You can go one better on efficiency with a baseboard system by ensuring there is sufficient linear feet to get design-day heat into the rooms with only 140F water, which means you'll be in condensing mode 100% of the time.

    You can go one better on efficiency by using low-temp panel radiators or cast-iron baseboard, or low-temp radiant baseboard rather than fin-tube convectors. (I sometimes can get reclaimed cast iron baseboard for new-fin-tube prices on craiglist, but new it's $50/foot.) Unlike fin-tube, cast iron baseboard panel radiators/radiant baseboard has predictable well-characterized output even with water temps <100F, which can improve both comfort and efficiency when used with an outdoor-reset control scheme.

    With any mod-con boiler running at low temp it's important to have sufficient radiation in each zone that it won't short-cycle even at minimum-fire on the small zones when running at low temp. The more zones you have the more significant this issue is. If the baseboard design was set up to supply design-day heat at 180F and the smallest zone only emits the minimum-fire output BTU/hr into the room at 180F, it will short-cycle like crazy with 100F water. Look up the specs, and do at least the napkin-math on it.

    Hopefully yours is one of the smaller Contenders, the MC-50 or MC-80, since the ~44KBTU/hr minimum-fire of the MC-99 is more than the whole-house design condition load for most homes in Seattle, and WAY above what a single zone would be in a 3-zone house and it will have orders of magnitude more burn cycles, cutting into both efficiency and boiler life. If yours is one of the bigger Contenders you can still get there, but it will affect the system design- you may need to increase the thermal mass in the system to extend minimum burn times and bring the number of burns/hour down. (For reference, the min-fire output of the MC-99 is more than the whole-house heat load @ 0F of my ~2400' circa 1923 bungalow with known gaps in the insulation. Unless your house is extremely leaky or very large, at your design temp of 20-24F you'd be unlikely to have a heat load greater than 30-35KBTU/hr, and if it's pretty tight it could be under 25K.)
  6. seattle_steve

    seattle_steve New Member

    Messages:
    28
    Location:
    Seattle, WA
    Hi Dana,

    Thanks for the additional info. When I said weigh the in-floor estimate against baseboards, I wasn't strictly talking about dollars. I understand that baseboards would be cheaper. Honestly, I find baseboard heating comfortable. (OTOH, I've never lived with in-floor radiant, so maybe I don't know what I'm missing.) The main thing I don't like about baseboards is the aesthetics of them. So, the calculation I would be doing is: how much is it worth to have completely hidden heating (and possibly more comfortable heating)?

    Looks like I have a MC-99. I live in a 1908 Craftsman (2 x 4 exterior walls on main floor, that have blown-in insulation).

    Steve
  7. Dana

    Dana In the trades

    Messages:
    3,032
    Location:
    01609
    Using cast-iron baseboard is a architecturally-appropriate & period look for a craftsman:

    [​IMG]

    [​IMG]

    [​IMG]
    (^^^from a used-goods $7/foot listing^^^)

    The stuff is indestructible, cleans-up and repaints pretty easily. With 180F water they're ~600BTU/foot like fin-tube, but unlike fin-tube they still give reliable output with 120F water ( but only ~200 BTU/ft ) or 100F water (~125 BTU/ft), which works really comfortably & efficiently with with outdoor reset control on a mod-con.

    Modern radiant baseboard radiators like the Runtal UF-2 series has comparable BTU/ft numbers to typical fin-tube or cast-iron baseboards, but have a more modern, slicker look:

    [​IMG]

    [​IMG]

    The minimum-fire output of the MC-99 at the lowest condensing efficiency is about 39KBTU/hr, and it would surprise me if it didn't short-cycle on you existing baseboard zones if you run it in condensing mode. At 140F out/120F return (~90% combustion efficiency, and 130F average water temp) a typical 600BTU/foot @ 180F fin-tube baseboard delivers ~ 260BTU/ foot. To be perfectly balanced at min-fire in condensing mode would take a zone with (39,000/260 =) 150feet of baseboard. If the smallest zone has less than half that (< 75' of baseboard) the burn times become pretty short.

    At 180F out/ 150F return (average temp=165F) they emit ~480BTU/foot into the zone, and even a 50' zone would be putting out about half the min-mod output for reasonable-length burns, but with 150F return water you're only looking at ~85% combustion efficiency (out of a boiler that has the potential for 95%, if you run it cool enough.)

    Replacing the fin-tube with cast iron or radiant-baseboard would add some thermal mass, enough to have stabler room temps, but not enough to prevent the short-cycle with a boiler so oversized for the smallest-zone load.

    A common solution is to use a cheapo electric HW heater (no power hooked up) either as the hydraulic separator in the system, or plumbed in-series with the boiler, between the radiation-returns and the boiler. Since there is typically 3-7F of hysteresis in the boiler controls (varies with model & manufacturer, and not always specified) raising 40 gallons of water 3F takes (40 x 8.34lbs/gal x 3=) 1000 BTU. So even if the fin-tube were emitting zero BTU, at 39,000 BTU/hr or (39,000/60 minutes per hour =) 650BTU/ minute, you get a minimum burn time of 1.5 minutes out of it. If the hysteresis is higher, the burn times are even longer, but ideally you'd be getting well over 2 minutes/burn even at minimal heat loads & the smallest, lowest-mass zones.

    Adding a buffer tank like this is something of a band-aid solution, (but a reliable band-aid), and it's much cheaper than replacing the boiler with a more appropriately sized one. It allows for lower operating temps (= higher efficiency) & smaller zones without short-cycling the boiler into an early grave.

Share This Page