[Greenbuilding] suspended slab

[Shawna Henderson]

Most definitely you want to isolate the DHW from the floor. The two 
systems should never be fed from the same tank. The reasons are twofold:

1. If you run the floor off the DHW tank without a step-down or mixing 
valve, you are going to be running the floor too hot, and you will end 
up with extremely high bills. Also, the tank has a longer recovery time 
than a boiler, so you will potentlally suffer cool showers if there is 
too much demand on the tank at one time (think cold winter morning, high 
heatling load).

2. And more important than #1 -- the cooler water running through the 
floor and back into your DHW tank can drop the temperature of the water 
to a point where salmonella and other greeblies can flourish. Then your 
whole family's sick and you might not have enough hot water to clean up 
after them...

There are DHW tanks out there that have an integral heat exchanger in 
them, allowing you to store hot water iin the tank for DHW, but run the 
floor (with its lower temperature demand) through the heat exchanger. No 
cross-contamination. Loverly. Only one heat exchanger. Works divinely 
with a direct-vent propane unit tied to a solar thermal system and a 
second tank in parallel.

Also, with the slab, definitely insulate under it. And don't be swayed 
by bubble pack insulation, 'radiant' or not. There's a Canadian 
manufacturer (truefoam) that puts out a product that is specifically 
made for hydronic floors. It's a 1" or so rigid foam that has grooves 
cut into it at 6" centres, so you just fit the PEX into it. I wouldn't 
recommend it for new slab construction, but for a retrofit situation, 
its great. The reason I wouldn't recommend it for new constr. is that it 
puts the heating tubes in the lower 1/3 of a 3"or better slab, reducing 
the efficiency of the heat delivered and also increasing the lag time. 
With a retrofit 1.5" overpour (or suspended slab, a term that's new to 
me), this is not so much of an issue.

Hope this helps

Shawna H.

rigaziodesigns wrote:

>George,
>
>When you say "The big question is do you need to isolate the radiant floor
>from the domestic hot water, with a heat exchanger and 2 pumps." do you
>mean he may be required by code or is this a functionality question?
>
>Thanks all!
>Lisa
>
>On 9/25/06, George J. Nesbitt <geoedb at idiom.com> wrote:
>
>>   Uninsulated slabs can be a real comfort problem. A well designed
>>forced air heating system can overcome this, although not as easily as
>>heating the floor directly.
>>   Ideally you would add insulation on top of the existing slab and
>>then install the radiant floor. If you don't you will get more heat loss
>>through the slab. If you use the existing water heater it needs to have
>>enough capacity to heat both the domestic hot water and the floor. The
>>Gas Research Institute has guidelines on sizing the water heater, I
>>don't know if it is available on the Internet. You can possibly turn of
>>the radiant floor when there is domestic hot water draw to prevent
>>running out of hot water sooner. The big question is do you need to
>>isolate the radiant floor from the domestic hot water, with a heat
>>exchanger and 2 pumps. Taco makes an integrated exchanger and pump system.
>>   Don't forget that you have to cut down any doors within the area
>>being raised.
>>
>>
>>rigaziodesigns wrote:
>>
>>>A friend of mine recently moved into an existing house with slab on grade
>>>floors in the main living space... and of course the floors are cold. He
>>>looked at the Radiant Floor website (see below) and thinks the suspended
>>>slab installation is for him since if he uses the new concrete as his
>>>finished floor he will only loose 1 1/2" head room. He also assumes that
>>>
>>he
>>
>>>will be able to heat the space off of his domestic hot water heater. BTW,
>>>
>>He
>>
>>>lives in the washington DC area. Does anyone have experience with this
>>>
>>type
>>
>>>of system or know if they work well and are worth the investment?
>>>
>>>http://www.radiantcompany.com/details/suspendednotes.shtml
>>>
>>>Thanks,
>>>Lisa
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>>
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