Hi everyone,

We recently bought a house that is heating by hot water radiant baseboard heat. We are ripping up most of the flooring in the house. (It is a ranch) The basement is completely unfinished. We will be doing kitchen, and dining room in tile, living room in hardwood, bedrooms in carpet, bathrooms in tile.

We would like to remove the baseboards from the kitchen, dining room, and living room, and replace them with pex stapled up from underneith the sub floor, between the joists. Those areas are all pretty much open so I am thinking one zone there to keep it simple. I understand, and have confirmed that my baseboards run at 180 degrees, this will be too hot for staple up application. So I think what I need to do is install a mixing valve. From what I gather the mixing valve will take cooler return water and mix it in to the loop to lower the temperature? I am going to go get some exact measurements of the rooms today. 

Also if this is done, can I also run a couple small loops for the two bathrooms?

It seems to me that I should staple up the pex piping with light weight aluminum plates, then insulate with... I am not entirely sure yet... From what I have read reflective doesn't provide much use, your better to insulate with some sort of fiberglass, or foam boards?

I was trying to figure out what type of boiler I have, it is a Hydrotherm NG that outputs 86400 BTU/hr   Our house is about 1300 sqft
 

Some more insight would be great..


Thanks,
Doug

Since you're ripping up the finish floors you might want to consider putting the PEX above-the-subfloor rather than a thin-plate staple-up. Above the subfloor it'll respond quicker, and you'll be able to run cooler water, for higher efficiency.

Getting the balance right on a single zone single-water temp approach with different types of emitters and floor finishes is not a trivial design problem. Bathrooms run out of floor space for radiant floor- you may need to supplement with panel radiator or radiant baseboard.

You may be running 180F water to the baseboards, but it's highly unlikely that 180F is necessary for getting design-day heat out of them. Most baseboard systems are overdesigned for the actual loads and many can deliver the heat just fine at 140F (especially if the house was tightened up and better insulated, or newer windows installed after the initial heating system was installed).

Whether staple-up or above-the-subfloor PEX, cheap unfaced R19 batts are enough over a semi-conditioned basement. If it's above a breezy open air pier foundation it's probably worth screwing OSB to the underside of the joists and making the OSB an air-barrier (mastic sealed seams, etc), then blowing full joist depth with cellulose.

86K of boiler output is a lot of boiler for a 1300' house, unless its literally uninsulated and located in central Manitoba or something (in which case it's just-right, or maybe not enough. :-) ) It would literally just the R value of an uninsulated wood-sided wood frame building, and a design temp is above -25F for the boiler to be right-sized. With insulation in the walls & attic, decent windows (or tight storm windows), and reasonble air-tightness, odds are your actual design temp heat load is well under 40KBTU/hr. If the baseboard is similarly over-designed you could probably run it at 130F and be fine.

All good heating system designs start with a room-by-room heat loss calculation. There's a pretty-good freebie heat loss tool out there you might start with:

http://www.taco-hvac.com/en/products/products.html?current_category=370

The instruction vidi lives here:

http://www.taco-hvac.com/products.html?current_category=370&show_vid=1

When you have that all roughed in you'll have a better shot at figuring out what you need for emitters in those rooms for whatever fixed temp you think you'll be able to run, and have at least a shot at having it work.

Dana,

I am in South Western Ontario. So we do get cold here but the house does have newer windows ect. So it sounds like I might be able to get away with turning the temp down to 140 degrees for the baseboards. I will make some measurements tonight.

I will also start looking at the calcuations in the near future too. We do get -13F here for some nights in the winter, but it is not a solid winter like that.



I will do some more hunting.    With a boiler older like that do I have to worry about damaging it with too cold of return water

What town? (punch it in to weatherspark: http://weatherspark.com/#!dashboard;a=Canada/ON/Thunder_Bay )

Boilers can be protected from cold water slugs when driving low-temp heating systems in a number of ways- don't let that be a driving factor. (

http://weatherspark.com/#!dashboard;a=Canada/ON/London


Looks like -9 is the coldest average. I measured the baseboards that are exsisting..... Living room = 13 foot section, for a 13x14 room. The kitchen\diving room is slightly larger and has 14 feet of baseboard heater....

So now I just need to figure out how much pipe and how many runs I should do. Dana1 your suggesting I just turn the boiler down to 140 and see what happens with whole house temperature come the cooler months. See if the other baseboards can handle it

According to ASHRAE charts the 97.5th percentile design temperature for London Ontario is 0F, the 99th percentile design temp is -4F. Even though it gets colder than -4F, it's duration is but for a very few hours, not long enough to matter except during a once in a century type of cold snap.

With decent windows and insulated 2x4 construction, I'd be surprised if your heat load at -10F was more than 35KBTU/hr, and it might be as low as 25K, but using a standard heat loss calculator like the Taco would tell you to within 15-20% or so.

Are you saying you have only 27 feet of baseboard in the whole house?!! Most ~2" square fin-tube puts out ~600BTU/foot at 180F, so if you only have 27' you're looking at only ~16 KBTU of output on a 86.5K boiler, which is a recipe for short-cycling the boiler like crazy! Even at 200F you're looking at only ~20K of emitter (and your 0F heat load may be about that.) I was thinking there was probably twice that much. At 140F average water temperature you can probably count on about 300BTU/foot, maybe a bit more.

You can't just crank down the output temp of a boiler to 140F without some risk, since on a gas boiler if the water entering the boiler coming back from the radiation drops below 130F in continuous operation you'll have condensation in the flue, potentially ruining the chimney if it's a terra cotta or masonry liner. If the return water stays below 125F you'll have condensation on the heat exchanger plates in the boiler, shortening it's lifespan by years/decades. If you want to play with it, look up how the controls work (its probably a standard Honeywell). Some allow independent high & low limits, others allow a high limit with a fixed hysteresis, others with an adjustable hysteris- it just depends. If you set it up so that the return water is 125F-130F or higher when it re-fires you'll be OK, since with a boiler so oversized for the radiation it'll spend the bulk of the burn at temps well above condensing temps. If it can be set up with a very large hysteresis you can lengthen the burns, limiting short-cycling losses.

When setting up for lower temp operation there are several ways to deal with low return-water temps by the configuration of the near-boiler and loop plumbing. (Primary/secondary, boiler-bypass w/thermostatic valve, etc.) But what makes sense in your case will depend on the emitters you have in each room, and the temperature(s) they need to run, and it's not a 1-line equation.

Start by determining the heat load of each individual room at -5F or -10F using that downloadable Taco heat loss calculator, (they have 99% design temp & weather averages in there for the London AP- I just checked) then you can work out what it takes to deliver that heat with the various emitter options, and at what temp. If you can figure out a way to get done with 140F or even 160F water, all at one temp, there are various options for getting it to run the radiation at or under 140F, most of the time to maximize efficiency and comfort.

The answers to how much pipe and how many runs, even the type of heat spreaders it takes all depend on the room to room requirements, and how that works out in BTU per square foot of radiant floor you need. It remains to be seen if that can all run as a single-zone single temp system with the other rooms, other emitters. This level of design isn't well suited to hacking or "design by internet forum", but with some room by room numbers, perhaps some the pros here might be able to tell you what's possible with what approach. It's probably something you'd be better off paying someone to design, even if most of implementation is DIY.

In the meantime, if you're planning to heat with the baseboard for the upcoming season, time the burn cycles of the boiler going into the shoulder season. If they're under 5 minutes with multiple cycles in an hour, it's taking a serious hit in efficiency. If they're under 3 minutes it's a huge hit. You may be able to fix that somewhat tweaking the existing aquastats on the boiler, but not by much. For low-mass baseboard emitters to be in good balance & long burns with 86.4K of boiler output takes over 100' of combined length, and a high-limit setting of 200F+. (And 100+ feet of baseboard would also likely be able to deliver design-day heat with 140F water, which is what I was thinking.)

Perhaps you should start by having a professional do a room by room Manual J heat load and specify, tubing, emission plates and design temperatures. On line heat load programs are of little to no use to the novice.

The boiler is too big, but could be improved with outdoor reset mixing controls.

Some of our systems run with floor and baseboard at the same temperature. The basement temperature has to be considered before the sub-floor insulation is chosen.

You might also consider using panel radiators as they are more DIY friendly, can operate at lower operating temperatures, if properly designed, more efficient and true radiators compared to fin-tube baseboard.

DIY radiant floor design is always the first mistake.

Agreed, depending on the basement situation, you may not need insulation below the tubing at all.

Sorry guys, only 27 feet total in the living room, kitchen, and dining room. the other rooms can stay baseboard I don't mind.

Only a proper heat load can answer your questions.

Posted By BadgerBoilerMN on 26 Aug 2011 10:18 AM
Only a proper heat load can answer your questions.

Yup!

That downloadable Taco freebie tool isn't bad if you don't want to pay the pros.  At the moment there isn't enough info to take a WAG at what the heat load is in the rooms that will remain baseboard and what water temp would be needed to support that load with the existing baseboard.

For yuks, how much baseboard is currently (or had been) in the rooms that you're converting to radiant? (Which may relieve some of the short-cycling concerns on the existing, likely oversized system.)

But like I said- design by web-forum isn't very likely to succeed here.  If you're willing to read up (a lot) and do the math, you'll have a shot at it.  Start with the room by room heat load calc (the tools make that part pretty easy)- shooting in the dark doesn't cut it.

How do I find out how many BTU's underfloor piping puts out? I am playing around the the TacoLoad tool now, it is pretty cool

Btus are determined from temperature of fluid, flow rate thru piping and density of fluid(glycol mixture).

Or if working on a design (vs measurement), the R value between the fluid and the room, the horizontal distribution of the heat, the room and fluid temps, etc.