Retrofititng radiant floor heating to old boiler system

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Old 03-21-07, 06:37 AM
J
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Retrofititng radiant floor heating to old boiler system

I'm researching how to install a radiant floor heating system in my kitchen. If its feasible, I might want to do the rest of the lower level.

Here's the kicker, though: I want to use the current hot water heating system, which employs standard cast iron radiators whose piping tees off from two main lines.

So, I guess the main question is, can I divert the hot water that loops through the radiator into the radiant tubing that snakes back and forth under the floor?

I worry some sort of adjustments to my boiler need to be made in order to accommodate a change in the system. Would narrow tubes necessarily carry more water than the radiator because they must travel so far?

Another problem might be the water temp itself. I think its set at 160 degrees, which might be too hot for plastic tubing.

J
 
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Old 03-21-07, 10:02 AM
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Primary?Secondary Radiant Circuits

Yes, this is very widely & rather easily done with existing hot water systems; one of the great advantages of having hydronic is that all kinds of circuits can be designed.

For an easy to understand article with clearly drawn, simple diagrams, go to the Plumbing & Mechanics site below & enter "a little floor warming please", into their search box; this is an article by John Siegenthaler, a well-known author/engineer on hot water heating.

Most of these piping circuits are tied right into the existing piping with "closely spaced tees" & require little more than a 3-way mixing valve & small circulator; they are designed to heat a small area such as a bathroom or kitchen, as you mentioned in your post; the mixing valve reduces the water temp to ~100 degrees.

In other diagrams he shows how to use a zone valve or zone circulator with a mixing valve to create a radiant circuit using the same boiler.

This can be PEX tubing or copper tubing; PEX can stand temps up to at least 200 degrees.

The tubing is usually stapled to the floor underside between the existing floor joists, if access is available, with aluminum heat reflectors and/or insulation to direct the heat upwards.

If you DON'T have access to the underside kitchen floor it becomes more complicated & expensive; you would have to apply sheets of material over the floor that have the tubing embedded, or pour light weight concrete to embed the tubing, usually a job for experts.

There are even systems that use normal boiler temps of 180 degrees "the plain vanilla system", that may or may not work in your application.

Once you have read the "floor warming" article, scroll down to read the numerous other radiant articles, among which are "the plain vanilla system", "less is more", "dashed expectations", "a signature system", "da fundamentals".

Siegenthaler's book "Modern Hydronic Heating" Vol. 2 is avaialble free of charge in most public libraries, & is a very easy read, rare for a hydronic engineer; his book has a list of the symbols he uses in his circuit drawings, along with their meanings.

http://www.pmmag.com
 

Last edited by jack horner; 03-21-07 at 10:16 AM.
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Old 03-22-07, 02:44 PM
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Thanks for all the info. I'll check out Siegenthaler's book and see what its about.

Regarding my kitchen, I do have access from the underside; it simply involves tearing out a lot of sheetrock. There are crazy networks of pipes in every direction, as befits an older home, but fortunately most of these run beneath the joists, and therefore wouldn't block a the hydronic tubing.

My kitchen would be suitable for this project because it currently has a 1-inch thick cement tile floor, which is supported by another inch of crushed pumice stone. I figure, if anything, this layer of stone would act as a good absorber and dissipator of rising heat from the tubing beneath it.

The rest of the first floor has regular flooring strips covered by hardwood. The enclosed porch is much like the kitchen: cement tile.

Only issue lies with the heating pipes themselves, which are thickly wrapped with asbestos insulation. This insulation stops at the sheetrock. This theoretically means I could tie into the circuit above the insulation, without disturbing the asbestos.

As for including a mixer and circulator, well, that's good information that I did not even consider (and a complication I was hoping to avoid).

Anyway, I'll see what Siegenthaler has to say.

J
 
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Old 03-22-07, 06:43 PM
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Under Floor Radiant Zone

I'm sure the other contributors will add additonal valuable info on which way you should go.

One of the first steps is to do a heat loss calculation of the kitchen to see how many btu's/hour are bleeding out of the room on a cold day; some free HLC sites are below; under floor radiant heat supplies ~25btu/hr per sq. foot of kitchen space, so you will have to match this figure with your HLC.

If your current boiler is doing the job of heating the house, it can probably take a small radiant zone with no problems.

Your system would be characterized as "hydronic","radiant floor heating", "under floor system","staple-up", "dry system"; if you Google any and all of those terms you will get a lot of sites specific to your application; you certainly lucked out by having access to your under floor joists; this makes it so much easier to put in a zone.

Dan Holihan of Heatinghelp.com also has a basic radiant floor heat installation manual for $20; a free design & installation manual from radiantcompany.com is avaialble at their website.

A very basic radiant zone (if the kitchen is not too large) is sometimes installed by simply extending the supply/return piping going to a radiator to first snake thru the kitchen floor joists, or using a monoflo (venturi) valve to do a take-off of the main supply line to then snake the tubing thru the joists & back; this would use boiler water @ ~160 degrees, so the piping would have to be spaced several inches from the sub-floor to prevent the floor from overheating.

This could be loosely construed as "radiant floor heat", but the basic concept of radiant heat is that the FLOOR ITSELF is a very large thermal mass radiator, which is constantly heated to ~100 degrees & subsequently continually exudes radiant heat throughout the kitchen; the lower radiant tube water temp is also much more economical to maintain.

Google "the plain vanilla system" for a description of this procedure; this would eliminate the need for a 3-way mixing valve & a circulator.

This would also depend on the type of piping distribution system you have now going to the radiators.

The Burnham site below specifies HLC data input for the entire house, but it will also accept data for a single kitchen.

http://www.burnham.com/heatloss1.cfm
http://www.heatload.com
http://wwwslantfin.com/he2/
 

Last edited by jack horner; 03-22-07 at 07:19 PM.
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Old 03-22-07, 08:59 PM
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Nothing to add here. Jack's suggestions and insight are spot on. There's a bunch of simple, straightforward ways to do what you're after, and the suggested resources are all there.

Read up on the way(s) to do it right, and go for it.
 
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Old 03-23-07, 08:11 AM
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Only thing that still nags me is the economy of the project. I mean, there's no question that having a warm tile floor underfoot is much preferable to the 45 degree foot freezer we currently have.

But the question lies in the mixing of cool water with the boiler's 160 degree water in order to keep the floor at a reasonable temperature.

Does this cause the furnace to work harder because its internal thermostat is registering much colder water returning from the circuit? Siegenthaler's article suggested it might.

The room thermostat will not be affected because the air temperautre will be just fine.

I'm jsut confused as to how the boiler's thermostat and the air temp thermostat upstairs reconcile one another...

J
 
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Old 03-23-07, 09:48 AM
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Radiant Zone Addition

It would depend on the boiler equipment you have (especially the aquastat) & the way it is set to operate.

Some boilers have an internal domestic coil that requires the boiler to constantly heat the water to ~140 degrees for the taps; others are cold start boilers which allow the boiler water temp to drop considerably lower.

Other than that, the room t-stat really controls the boiler water temp; when it gets a call for heat, it turns the burner on; when the water temp reaches the temp pre-set in the aquastat, the circulator kicks in to pump the hot water to the rads, & return the cold water sitting in the rads back to the boiler to be heated; there is also a differential control in the aquastat that will continue to fire the burner to heat the returning cold water up to a pre-set "high limit" in the aquastat of ~180 degrees.

This pumping of cold water back to the boiler to be heated & the pumping of heated water out to the rads & radiant zone will continue until the room t-stat reaches the temperature set by the homeowner, at which time the burner will shut down; the circulator will continue to circulate the water until the water temp drops to the "low limit" of the aquastat, usually ~160 degrees.

Adding a sub-floor radiant zone will add an additional load onto the boiler, depending on the square footage of the kitchen & the amount of tubing you use, but this will also be partially offset by the floors being heated by the room radiators.

There is always some standby heat loss from all supply/return heating pipes as they sit there in a partially cold basement & inside exterior walls, waiting for the t-stat to make another call for heat; given the relatively small amount of water contained in the proposed radiant tubing zone, from the boiler's point of view, it is having a much harder time time heating all that water in the rads & their associated piping.

If you have a very large kitchen then it's possible that the added water volume from a radiant zone will reuire a boiler main bypass or other measure to prevent boiler consensation due to low water temps.

But I don't think the added water volume of a small radiant zone will dramatically affect the amount of fuel used.

Many boiler mfgs have a technical dept. that will design a radiant zone on their dedicated software free of charge; it's well worth a toll-free call; just make sure you mention that you own their particular brand of boiler.
 

Last edited by jack horner; 03-23-07 at 10:00 AM.
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