Replacing Some Radiators with In-Floor Radiant Heat?


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Old 01-13-08, 06:59 AM
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Replacing Some Radiators with In-Floor Radiant Heat?

Hi Folks:

In my farm kitchen, I have two smallish iron radiators (hot water system) that I would like to replace with something else to free up the wall space and, of course, improve my heating process if possible. I am thinking about radiant in-floor with pex under the joists. The kitchen is 15 x 12 roughly.

My dumb question: Is it possible to simply replace the radiator(s) with an equivalent length(s) of radiant (i.e. pex) piping without adding manifolds, pumps, mixing valves, etc.? Can you "in theory" go from the feed-side black radiator pipe to the radiant system, back to the outlet-side of the pipe system without alot of other valves or pumps? Again, assuming the radiant loop volume is equivalent to the radiator volume. Am I missing some plumbing and/or heating system magic here?

I'm sure it's not that simple of an answer so here are more specifics if they help... I have a brand new Weil McClain CGI-4 nat gas boiler with new circ pump, expansion tank, air elimination valve, etc. sized properly for my house and heating it very well. Water temp is always between 90 and 135, pressure around 18-20 psi (two-story 1500 sf house). A recent energy audit by the gas company said we're operating pretty efficiently. No problems with the system - just the inefficiency of old-fashioned radiators taking up wall space in the room.

Thanks for your help, Gurus!
 
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Old 01-13-08, 08:43 AM
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Not a radiant guru here by any means, but I can make a few comments...

Do you have access to the underside of the floor ?

There are some "IF's" to be considered. IF you can match the BTU output of the radiators with the BTU output of the radiant floor, then you can do BASICALLY what you propose, with the following caveats:

The water in the floor tubing should be controlled at a lower temperature, usually around 100*F . You don't want the floor surface itself above around 85*F , it will get uncomfortable to the feets pretty quickly. So this means some sort of mixing valve in order to temper the water to acceptable limits.

You said the water in the boiler runs 90-135* ???? That's MIGHTY low temps. I'd be VERY surprised if you don't have problems with flue gas condensation in the boiler. This is something that you may want to have looked at carefully by a real heating contractor. If you've got cast iron radiators, and a converted gravity hot water, or steam system, there should be some BOILER PROTECTION scheme installed in the piping. Like a BYPASS valve ...

Personally, before you consider upgrading the system to a radiant floor, I think you should make sure that brand new boiler isn't going to be ruined in a few short years by flue gas condensation woes.
 
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Old 01-13-08, 08:47 AM
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Let me add this: It IS possible to do a radiant floor without tempering the water. I think they call this a 'staple up' installation. I don't think it's nearly as good as using the heat transfer plates, etc, but won't require the mixing valves.

If you've got a camera, post some pics of the boiler and associated piping around it on www.photobucket.com (free) and provide a link to them here. You will get much better advice if we can see what you are working with. Let's see the radiators you propose to replace also.
 
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Old 01-13-08, 10:16 AM
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Here's the paradox. Simple and cheap to do is very difficult to calculate and control, and easy to calculate and control is harder to install and much costlier.

As NJ Trooper stated, your using condensing water temps. How is your boiler protected? It sounds like you'd have been better off installing a condensing boiler if there isn't adequate protection, so protection from condensing may need to take precedence over RFH for the kitchen, or at least be done as part of the same project.

BTW, I'm in the process of doing the same thing in my kitchen as what you are proposing. I have plates installed under the flooring - the extruded ones are extremely expensive, so be warned. My calculations showed that with my boiler supply temps being in the 90-140 range (condensing boiler with outdoor reset) using 4" aluminum plates mounted 8" on center with 100 exposed floor coverage would get pretty well what my baseboard rad supplied. I'll actually be only doing about 75% coverage. I don't want heating under the fridge or under the recycling and animal water dishes. Still close enough. The one thing I am doing is splitting the loop into 2 to minimize the pressure drop. Instead of a single 200' loop, I'll be installing 2 80' loops so that the current pump can handle it. The one end of the loop should be supplied where the rest of the system is supplied and returned where it all returns. There likely isn't much pressure differential where the rad was to do many fit in a narrow diameter.

If you end up needing significantly lower temps for the floor temps a Taco RMB is a nice way to do it. Nice compact control but it's close to $1K. Includes a separate ODR, 2 pumps etc. Nice stuff...
 
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Old 01-13-08, 11:32 AM
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First thing to do is a heat loss calculation for the room and make sure radiant can handle to the load. Also that your subfloor and floor coverings can deal with modestly warm water temps.

You don't have to go nuts with controls. Some somewhat simpler strategies are here:

http://journals2.iranscience.net:800...,62497,00.html

Troop, "staple up" generally refers to heat transfer plates applied to a subfloor from below. As opposed to in-slab (tubing embedded in concrete or gypcrete) or warmboard/climate-panel (tubing put into machined grooves in a plywood-ish medium below the finish floor, typically above the subfloor).

MN, if you are really running 90-135F, then unless you have specific piping/valving to protect the boiler, you have a potentially serious issue with condensation. Think acid rain in your flue and inside the cast iron heat exchanger.
 
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Old 01-13-08, 12:07 PM
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Originally Posted by xiphias View Post
Troop, "staple up" generally refers to heat transfer plates applied to a subfloor from below.
What do they call the type I'm thinking of, where the tubing is either stapled to the joist, below the subfloor, or installed on hangers, also below the subfloor, with an air gap, and insulation beneath ? This is usually run at boiler temps IIRC, no need for mixing valves...
 
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Old 01-13-08, 12:49 PM
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Originally Posted by NJ Trooper View Post
What do they call the type I'm thinking of, where the tubing is either stapled to the joist, below the subfloor, or installed on hangers, also below the subfloor, with an air gap, and insulation beneath ? This is usually run at boiler temps IIRC, no need for mixing valves...
Maybe Watts Onix tubing?

Ultra-Fin is good for even higher temps and is hung in the joist cavities rather than stapled or installed in plates.
 
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Old 02-11-08, 03:15 PM
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Yikes! Flue Gas Condensation? Don't see anything 'bout this...

Thanks for your replies about the radiant heat options for radiator replacement - but the flue gas condensation issue stopped me cold - - - to think a brand new $5,000 installed top-name boiler installed by a reputable heating company could be subject to this life-shortening thing without a mention of it is incomprehensible to me.
This is an induced-draft unit which pulls/blows air into the boiler flue before ignition. Does that info help? I have asked the contractor about what the circulating water temp should be (it's 135 after completing a heating call cycle) and he said that's correct...
To me, it seems "efficient" if a system can satisfy the heating needs of the house without burning alot of gas to heat the water more than necessary, but I'm not a thermodynamics expert. I'll tell you that my radiators are nearly too hot to touch when heating - the cat gets up and moves pretty quick when the heat kicks on!
How can I tell if flue gas condensation is happening and what SHOULD the water temps be for this boiler/furnace? The literature says nothing about how to control this BTW and I've read the install manual four times.
Thanks again, folks!
 
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Old 02-11-08, 04:13 PM
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Normally, the high limit on a boiler will be set at or around 180 ... When the water gets to that temp, the burner will shut off until the water cools ...

You say the temperature varies from 90 to 135 ?

Where and how are you measuring this temperature ?

The flue gas condensation issue rears it's ugly head when the water RETURNING to the boiler spends a lot of time at temps below around 135 ... You've heard of "Dew Point" as it relates to the weather ? Well, the Dew Point of flue gases in a gas boiler is around 135 , which means that if those gases are in contact with a surface at or below that temperature, dew will form. That dew is acidic and corrosive.

So, if your return water is very cool, there's a good chance that the surfaces on the inside of the boiler are also, and that nasty acid dew will form on them.

You are absolutely correct that if you didn't have to heat the water as much, you could save some fuel, and that is why there are systems known as "CONDENSING" boilers. They are designed to run at lower temperatures, with lower exhaust temperatures, so they can extract that extra 10% or so of heat from the flue gases. But, they need to be DESIGNED to not be damaged by the corrosive condensate (which is trapped and pumped to a suitable drain after being 'neutralized')

Whether or not all this is an issue with your system depends a great deal on how it is piped, and how much time your return water spends below 135.

Some 'reputable' installers aren't, or may be simply ignorant of the issue... it happens...

Is there a bypass ? or a thermostatic valve ? ...
 
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Old 02-11-08, 04:48 PM
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Which manual were you reading ? Start in this one at page 21 ... chapter 4 ... read to like page 29 or so ... pay attention to the "WARNINGS" that are printed in there...

Weil-McLain CGi boiler installation manual
 
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Old 02-11-08, 05:41 PM
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Originally Posted by MNFarmBoy View Post
Thanks for your replies about the radiant heat options for radiator replacement - but the flue gas condensation issue stopped me cold - - - to think a brand new $5,000 installed top-name boiler installed by a reputable heating company could be subject to this life-shortening thing without a mention of it is incomprehensible to me.
Let's cut the reputable company a bit of slack for a moment. If they designed and installed your system to operate using the radiators at the standard 180F, then flue gas condensation shouldn't be an issue, and it's quite comprehensible to me that they wouldn't tell you about it. It's not an issue with the system as designed and installed. By introducing radiant, you have changed the game without telling them. Perhaps the reputable company, had they known of the intention for radiant, would have designed and controlled the system differently.

That said, the Siegenthaler article that I linked above provides some general guidance for boiler protection when mixing high and low temp heat emitters. If you are committed to getting radiant in the kitchen, study up, then either DIY or call that reputable company and see what they propose.

Originally Posted by MNFarmBoy View Post
To me, it seems "efficient" if a system can satisfy the heating needs of the house without burning alot of gas to heat the water more than necessary,...
Welcome to the world of "advanced" hydronics. (In quotes because it's incomprehensible to me that certain basics in hydronics are still considered "advanced.")

With proper piping and controls you could probably run that same boiler at much better overall system efficiency using outdoor reset that would supply a water temperature more closely matched to the outdoor temperature. On page 28, your manual suggests injection pumping for boiler protection. They also suggest contacting W-M if you're planning a low-temp system using this boiler. Sound advice, both.

If you stepped up to higher-tech, you could have a modulating/condensing boiler that would not only match the supply water temperature, it would also modulate the burner so it would 'simmer' on warmer days, and 'boil' on the coldest days.

It is unfortunate that many installers, reputable companies included, are not pushing more efficient systems. I suspect the major impediment is their reluctance and/or experience with peddling higher efficiency boilers, piping and controls to a public that is largely concerned with initial costs. It's not worth wasting their breath. Too many times people want the less expensive option. Better boilers, better controls, and more piping add to the cost of an already expensive project. What homeowners don't often hear is that the extra expense can often be recouped in just a few years. With rising energy prices, it should pay for both the installer to be persistent and the homeowner to do more homework.
 
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Old 02-11-08, 08:05 PM
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Getting back to your question about radiant. Do a heat loss if it is more than about 4200 btu/h it would require secondary heat. As a rule to detemine if radiant will work by itself most guys use 25-35 btu's per sq ft of usuable floor. When the heat loss is done detemine if the numbers above work. If not you will have to add heat above the floor.

Trooper, the radiant hanging in the joist space is actually called joist space heating. This never went over very well. The insulation was impotant and no leaks from the joist space. It usually worked in systems with 160f or above water temperatures.
 
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Old 02-01-09, 02:40 PM
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Who,

Did you ever finish your install?

Also who's extruded plates did you use
 
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Old 02-01-09, 08:23 PM
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I did... I used Wirsbo plates and Kitec XPA. It worked great. I'm in the process of adding spot floor heat to 2 baths with a few strips.
 
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Old 02-04-09, 03:50 PM
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who any pictures??? How did you calculate the btu output of the pex?
 
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Old 02-04-09, 10:39 PM
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I think I might have some pix at the boilerbuff web site.

As for the calcs I used SlantFin and the Iggyware flow calculator that is in the attached CD from the Modern Hydronics book SWMBO bought me one fine occasion.

Anyway, I basically just used the Slant Fin numbers. I switched from fin-tube to underfloor.

It showed me that to match the BTU of the 9' of slant fin only 75% of the floor should have plates. That worked out great. There is no heat under the fridge, recycling garbage areas and where the animal water dish was kept.

I used the flow calculator because I wanted to see if I could get the underfloor heat off of the built in pump.

I have no idea what the flow is with the monoflo and could never figure it out but I grabbed a number like 10 gpm or so based on how it works fine on Grundfos 15-58 at either of the upper speeds but not a the lowest speed. Then I realized that 2 80' loops would work better for flow than a single long one since it had to share speed 2 of the 15-58 pump.

Anyway, the math used with those 2 tools worked, although the temps of everything are lower because the SlantFin program overstates heatloss by about 50%. Your true heatloss might only be 2/3rds of the number it tells you but as long as everything is proportional the house stays balanced for heat.

I do fintube and radiant and my electric and my total electric for the boiler stays under 100 watts total. Those tools were great!
 
 

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