High Temp return for Radiant?

....jc

I am having a baseboard system retrofitted because it doesn't work well. The installation has 1/2" pex & Slant Fin 15's. We will be tearing out some baseboards and reusing them in different places. I have some cool spots where there isn't a place for a baseboard, and these spots have stone tile & vinyl flooring.

Q1? Given 180* water temp, and after running thru appox 28' of baseboard(560*/ft @ 180*), the water temp s/b down to what, about 150*? Would it be feasible to use this return to feed psuedo radiant floor heating from under the subfloor? The subfloor is about 1" thick OSB. I am thinking that 150*(or whatever) water is too hot for the pex to be stapled to the subfloor, but how about hanging it an 1" or 2" below? The joist bays are 16" wide and 16" high, and there will be R-19 insulation below the tubing. I was thinking of running 2 tubes about 4" apart length wise down the bays. Will this work?

Q2? Is there any way to determine, given the water temp, how many btu's a raw 1/2" pex pipe will 'throw off'?

Thanks Much,
....jc

Weezbo

Hello...

let me ask...what are some of the dimensions,and what are some of the lengths of pipe you are dealing with? what type of btu requirement do you have to attain in the rooms with the stone faced floors? most manufactures have designe data for thier pex...

....jc

The area is 32' x 24' over an unheated garage. Currently 1/2" pex pipe runs from the boiler room into and around the perimeter of this area. The entry way and a bath room are at the near end of this area, and they are the areas that I would like to use the 'return' pex under. These 2 rooms are actually fed from another zone, which I would like to change.

Treated as 1 large room, this area needs 19,500 btu's, and is supplied with 19,000, but a room by room heat loss shows a different picture. The 2 rooms on the far end are both short of heat, while the rooms in the center have a little more than is needed. This is why the overall heat loss doesn't look too bad. The entry way is short 500 btu's while the bath is about right.

The upper level of the house has 3 zones and they are all lacking enough heat emitters to handle the heat loss. So, we are embarking on a retrofit to fix this mess. The largest area, LR/DR/KIT/NOOK has about 1,200sqf, 23' high ceilings, and lots of glass. We are looking to remove the baseboards and replace them with radiators. Then using the removed baseboards, place them into the other 2 zones to 'beef them up'. We will most likely be making changes in the actual runs of pex where we can.

S Davis

Flow

One thing you will have to watch is your flow on your loops,your baseboard or Rad's needs 1 GPM per 10,000 BTU's with a 20 degree delta T, so depending on your heat requirement in a given baseboard zone you will have to figure out how long the loops are you will be adding and how much head loss you will have to make sure your pump will give you enough flow to move the heat required.

S Davis


Apex Radiant Heating

....jc

Flow Indeed!

Figuring that flow was the reason the system performed so poorly, we upgraded the pump twice. Right now we have a Taco 11, and that s/b more than enough. However, as we tear this thing apart, we'll be checking the flow to see if we find any problems.

Now with the added baseboard in the zone and piping changes, I'd have 53' of baseboard. Now, if my math(and fomula) is correct, at 1 gpm with 560* baseboard(@180*) I'd have a Delta T of 59.36. Not too good.

What I'd like to do is split the zone in 1/2 by putting in a 3/4" pipe to the rear of the zone(from the boiler), and feed each 1/2 from there. The existing supply & return for the zone would then both become returns, and I'd use those returns to create the psuedo radiant heating in the entry way and bath. Now calculating the longest new 1/2 of the zone, with 28' of baseboard @ 1 gpm my Delta T would be: 31.36. Now if I could get the flow up to 1.5 gpm, my Delta T would be 20.91. More reasonable.

Anyone see any problems with that scenario?

In replacing the existing BB with the BB removed from the LR area, I now have approx 60% more btu's than the heat loss calls for(calc'd in the room that I have the least choice of the BB to replace). By using the left over BB, I can keep every room in that zone at the same 60% more than the heat loss. I've heard about 'short cycling', and it would have to be pretty cold outside for that to happen. However, if it's that cold out, chances are that 1 of the other 4 zones would also be calling for heat, so I would think that I would be ok.

Any opinions?

Thanx,
....jc

Paul Pollets

Radiant zone of high temp

If you're trying to add a radiant zone using high temperature supply, try the Oventrop "Uni-Box". The device is designed to make a radiant zone from radiator (or BB temps) without worrying about mixing valves or VSI. The built-in TRV valve will control room setpoint. This device can deliver 13-15K BTU's to the floor. The units require a reverse-return or direct return piping strategy, and it sounds like you're BB is piped without a separate, dedicated return. BTW, The pipe sizing for 28' of BB should have been 3/4", not 1/2"

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