Add loop to existing hydronic baseboard system (sunroom)

[Sycamore Farm]

We bought a home with hydronic baseboard heat and love it. The previous owner (builder) added a sunroom later but never added heat. It is currently a 3 season room off the master bedroom connected by a doorwall. Leaving it open usually keeps the room fairly close to the bedroom temperature because they insulated it really well and there are south and west facing large windows. That's fine if it's not super cold (we are in Michigan, so it does get really cold sometimes). I would like to add hydronic heat to the room. I have a couple concerns. While it is insulated in all walls, ceiling, and even below the floor, there is no basement below the adding. It's just a crawl space (dirt). We could not run a loop around the whole room because eventually there is an exterior door. I will be hiring a professional to do the work but would like to be educated a bit before engaging. Is there a practical way to take the loop into the crawl space to make a loop, or could we perhaps just have a larger wall radiator for the room? The room is 16' x 16'. It would be possible to bring plumbing up from the original exterior wall (where the bedroom is) to supply the room. I've considered just using a good space heater or doing a mini-split, but I'd prefer to just keep the hydronic heat throughout. I love not having forced air. Thanks for suggestions!

 

[Dana]

We bought a home with hydronic baseboard heat and love it. The previous owner (builder) added a sunroom later but never added heat. It is currently a 3 season room off the master bedroom connected by a doorwall. Leaving it open usually keeps the room fairly close to the bedroom temperature because they insulated it really well and there are south and west facing large windows. That's fine if it's not super cold (we are in Michigan, so it does get really cold sometimes). I would like to add hydronic heat to the room. I have a couple concerns. While it is insulated in all walls, ceiling, and even below the floor, there is no basement below the adding. It's just a crawl space (dirt).

Is the crawl space vented to the outdoors?

Are the crawl space walls insulated? (R15 continuous insulation on the exterior walls would meet current IRC code minimums, and would be a good idea no matter how you're heating the room above.)

Is there currently any insulation between the floor joists? What type of subfloor?

Finding the optimal heating solution starts with a heat loss/heat load calculation. Take your pick- none are perfect, but any will be way better than a WAG:

IBR method (using a spreadsheet)

LoadCalc

CoolCalc

When using an online tool be AGGRESSIVE rather than conservative on air tighness & R value assumptions or they will oversize (by a LOT). An IBR will oversize too if using old-school standards for air leakage assumptions. Houses in MI are tighter than that, some a LOT tighter. It's not crazy to assume zero ventilation & infiltration (for any of these) when making these calculations.

With an insulated unvented crawl space it can be pretty cheap & easy to DIY install a finned convector type subfloor radiant, which can operate a the same water temps as a baseboard system, and would not interfere with later modifications of the conditioned space walls for doors, windows, cabinets, etc.

 

[Sponsor]

 

[Sycamore Farm]

Hi Terry. Thanks for the reply. The crawl space is actually just covered by aluminum siding, so not meeting code. I need to look further into this because the this crawl space walls do not have 16" on center studs. I'll add that to my list of activities. There are no vents, but seems like a moot point without an insulated crawl space. The joists however are completely filled with rolled insulation. The subfloor is 3/4" plywood and seams are sealed.

One other note. I'm new to boiler heating, but it looks to me like we have three loops. There is no manifold, so I think just means we have a single zone. Just wanted to mention in case that plays a part in your suggestion.

 

[Dana]

Hi Terry. Thanks for the reply. The crawl space is actually just covered by aluminum siding, so not meeting code. I need to look further into this because the this crawl space walls do not have 16" on center studs. I'll add that to my list of activities. There are no vents, but seems like a moot point without an insulated crawl space.

Vents are about purging moisture, and has nothing to do with insulation. The IRC calls out standards for the sizes of vent openings, but if it's not moldy on the bottom edges of the floor joists the aluminum siding is "leaky enough" to be the moral equivalent of a vented crawl space.

The joists however are completely filled with rolled insulation. The subfloor is 3/4" plywood and seams are sealed.

How deep are the joists? Current IRC code minimum is R30, which is about where an low-density R38 fiberglass designed for 2x 12s performs when stuffed into a 9.25" deep 2x10 bay. It's possible to get R30 into a 2x8 bay using rigid rock wool.

But if the joists aren't on standard spacings it's nearly impossible to install batts with a decent enough fit to work well.

Without more info on the joist depth & spacings I'll withhold going into detail on radiant options, but you DEFINITELY don't want to be running your heating system lines in a parasitically-vented uninsulated crawl space below the floor insulation. If the power goes out during a cold snap the lines can and will freeze. Adding antifreeze could work, but it complicates the hydronic heating design issues.

One other note. I'm new to boiler heating, but it looks to me like we have three loops. There is no manifold, so I think just means we have a single zone. Just wanted to mention in case that plays a part in your suggestion.

So it's just one pump, and the loops split off close to the pump without zone valves or anything?

Some pictures of the heating system plumbing would be useful here.

 

[Sycamore Farm]

I replied a couple of days ago, but I must have forgotten to post. Doh!

Looks like they did not build the side walls of the crawlspace to accept insulation. I need to add this to my project list to improve. The joists are 2" x 12", 16" on center. They put visqueen over the entire bottom of that structure. Based on all of this, I think the underfloor option would be a lot of work.

I should mention that the room has an unfinished floor currently (just sub-floor), so if that helps with other options we can consider that. We will be installing a commercial grade vinyl plank with its recommended underlayment.

Here is a shot of our boiler. Note: when my boiler guy put in a new expansion tank at the start of the season, he pointed out that the hot water heater vent is under-sized. I also have this on my list. Would love to switch to a combi-boiler in the future.

 

[Dana]

So it looks like a mini-manifold with three loops directly above the boiler, one of which tees off to a fourth?

Are those gate valves or globe valves? From the appearance they could be either, but gate valves will wear out if used for flow balance/control. I don't see any other flow-control valving on those branches, but the flow balance could be located on either supply or or return. Mark the valve handle position, and see if they are cranked all the way to the counter-clockwise (open) position, or somewhere in the middle. If in the middle we can hope that they are globe valves. You may have other hints from part numbers /names in the casting, etc.

Where is the pump?

In your climate (US climate zone 5A) a true vapor barrier such as polyethylene on the UNDERSIDE of the floor joists is a significant mold hazard, only mitigated by the fact that the subfloor above is vapor retardent. During the winter the cold bottom edges of the joists are loading up with moisture that is entering the assembly via vapor diffusion through the subfloor. What you have now is a moisture trap. At some point (before the joists develop rot) installing something more vapor open than the subfloor (such as half-inch OSB or CDX) should take the place of the polyethylene sheeting. If the house is air conditioned the cladding on the bottom side has to be at least somewhat vapor retardent to keep the subfloor from loading up with moisture during the peak humidity of summer. Half inch CDX or OSB runs about 1 perm or a bit less when dry, but becomes more vapor open rising to 4-5 perms or more when the humidity level is high enough to support mold, which offers a path for the assembly to dry:

Assuming the batts are reasonably well installed with minimal compressions or voids and the cavities are completely filled it's fine to just keep them.

The Ultra-Fin solution is surprisingly cheap & easy to install. If going that route it would be best to replace the fiberglass with R30 rock wool, which would leave ~4" of space between the insulation & subfloor. The biggest line-item expense would be the ~$400 of R30 rock wool (if making the entire floor radiant, which may not be needed.)

If the poly sheeting can be removed intact, installing it on the dirt floor of the crawlspace with liner staples would be an excellent re-use. Ground moisture and other soil gases are constantly being emitted into the crawlspace, and polyethylene sheeting does a great job of blocking most of it, even if there are a bunch of old staple holes in it.

If there is going to be a finish floor installed, a 3/8" PEX Roth Panel radiant solution above the subfloor would add only 3/4" of additional height. But that is going to require lower temp water than the other zones which makes it a more complicated & expensive design than an Ultra-Fin solution.

Have you run the load calculation numbers for both the master bedroom and the sunroom (separately)? With both numbers in hand and the length of the baseboard in the master bedroom it's easy to estimate the amount of radiation needed for the sunroom to keep the temperatures reasonably balanced. Without the load calculation numbers it's a WAG. It isn't rocket science- no hard math required, (a smarter than average 5th grader can handle it), but essential to getting it right.

 

[Sycamore Farm]

Thanks, Dana. A lot to digest here. Let me at least answer a couple of things. On each of those three lines, the bottom valves are completely open and the top ones are completely open. My boiler guy showed me how they would empty the lines for winterizing. They would close the bottom ones, attach a hose to the top, and empty the line one at a time. In fact we did this because the lines were filled with a very old anti-freeze. He suggested we go with water because we don't winterize. It definitely helped the radiators heat up more efficiently.

I really appreciate the suggestions about the vapor barrier. To be honest I hadn't even considered that. It seems like the guy skipped some important steps thinking it was fine because being just a three-season room.

I've attached a picture of the pump. It's inside the boiler chamber. I will have to do the calculations when I have more time. I really like the idea of an above floor radiant. I originally wondered about electric (not my preference) but also some some PEX solutions. With my system at 160+, there must be a solution to step down the temperature (some sort of mixing device that brings the temp down with cold water).

 

[Dana]

Building in moisture traps is a bad idea even with a 3 season room. Most people don't really "get" what vapor barriers are for, and how to design them properly into a building assembly. Most people also don't get that at about 0.5 perms a 3/4" plywood or OSB subfloor is a Class-II vapor retarder, sufficiently vapor tight that under Canada's National Building Code it qualifies as a "vapour barrier". That's still an order of magnitude more vapor open than 6 mil polyethylene, but with a finish floor above it there isn't a sufficient drying rate toward the interior to protect the joists or to keep the fluff between the joists from getting soggy.

In a not too cold climate such US climate zone 5 (like Lansing) improper use of vapor barriers has caused at least as many mold/rot problems since the 1980s than they have ever saved.

WarmBoard or EcoWarm systems are something of the gold/platinum standard for above the subfloor system. Roth Panels cheaper and still pretty good. RHT panels w/sheet metal heat spreaders is probably cheaper still.

But any above the subfloor solution would still need to be mixed down to avoid temperature striping and/or hot-spots too hot for barefoot comfort. This requires a thermostatic mixing valve and a pump on a secondary loop teed-in to the existing heating loop for that zone. See the "low temperature secondary circuit" portion of the diagram below. The radiant pump would be powered off the same relay as the main pump so that it only runs when there is a call for heat.

It's still quite a bit cheaper & easier to go with Ultra-Fin and R30 rock wool plumbed in series (or parallel, with some flow balancing valves) with the loop serving the bedroom using 3/4" PEX and run it at high temp. Teeing into the loop near the bedroom with "purge tee valve" would let you do a crude flow control splitting off a portion of the flow to the radiant floor without adding a second pump or mixing valve. With some trial & error tweaking of how far the valve is cracked open a reasonable balance can be found.

 

[Fitter30]

Purge tee might work all depends how many feet of head and the pump you have. Three pumps would give you better temp cont controll.