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questions on staple up radiant

Hello, I had some questions on a radiant installation I am about to do. I am a homeowner doing a renovation on a small bathroom and kitchen in Somerville, MA. I would like to take out the standing cast iron radiators (much as I like the way they look) in order to create some more useful space in these little rooms. So I'm thinking of installing radiant floor loops in these rooms. The boiler is a Vitodens 200 which I had installed last year. It is installed using the low loss header with a circulator on the secondary loop, and all the radiators are now piped with pex back to a manifold near the boiler. There is only one thermostat for the system, and balancing is done with the valves and flowmeters on the manifold. The hottest I have ever seen the boiler run was ~130F, when it was really cold out (something like -5 or -10).

The kitchen is about 120ft^2 with about 90ft^2 floor not covered by cabinets or appliances. When I had the Vitodens put in last year I did a big spreadsheet for heat loss for the entire apartment room by room, and from this I did some specific work for the kitchen. After adjusting for the post-construction wall materials (adding insulation), I end up with just under 2000 BTU/hr heat loss with a delta T to outside of 70 degrees F. Before construction I had a HL of about 5000 BTU and the cast iron radiator in there was around 7200 BTU (EDR * 150). I would like to use 3/4 solid wood floor in there, either refinishing what is already there if it doesn't look too bad after peeling up the vinyl and masonite, or by putting new stuff in. Anyway, this leads me to a requirement of something like 22 BTU/hr per ft^2 in the open floor space, which seems reasonable. If I use an R of 3 for the upwards heat flow to a room at 65F, this means I am going to need ~130 degree water. That is about the hottest I have ever noticed the boiler running too, not having adjusted the reset curve that the installer put in to the Vitodens. I seem to be getting close to marginal in terms of heat input to the room though, and I don't think I would want to use any hotter water for fear that it won't be good for the floor. Wouldn't need to worry about that if I used engineered flooring, but I don't like that stuff.

The bathroom should be no problem for getting enough heat into it.

I just had a few questions on this project I am considering:

- I am thinking about putting the tubing under the subfloor, as I can access this easily from the basement and I don't particularly want to deal with the height mismatch issue if I put the tubes on top of the subfloor. However, I don't know whether to attach the tubes directly to the subfloor with clips, staple up heat spreader plates, suspend the tubes under the floor, etc. I was thinking of using foil faced fiberglass under the tubes (with an airspace of 2") to fill the rest of the joist cavity. Any opinions on the best way to go?

- The heat input situation seems a bit close to the edge in the kitchen. Should I put some tubes in the wall or ceiling to augment the heat input?

- Do you think I will have an issue with the radiators and floor heat being in the same system using the same temp water? If anything, it seems like the radiant floor would want higher temp water than the radiators. But maybe this could be compensated by using a greater area filled with tubes (i.e. walls or ceilings).

- Does anything sound crazy here?

Thanks for any help!

-Holly Gates


  • Ted_9
    Ted_9 Member Posts: 1,718

    Without know much more about the project and doing some calcs, I am concerned with you the method(stapleup) which you have chosen. Typiclly, in kitchens and baths with hardwood, you may need supplemental heat.

    And I recommend if you go with staple up, to use aluminum plates and insulation right up to the plates. Dont bother with the foil.


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  • Paul Pollets
    Paul Pollets Member Posts: 3,656
    Calcs and 3 way mixing

    Staple up with plates will work the best, and give you significantly lower design temps for the staple-up zone when compared to a suspended application without plates. I use the Wirsbo 3/8" double aluminum transfer plates (3/8" hePEX is easier to negotiate through the bays)then, R19 batts under the plates. or the Joist trac plates. Add a Viessmann 3 way valve and motor actuator for the 2nd mixed temperature. I'd also check the loads with appropriate software to make sure supplemental heating is not required.

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  • Dale Pickard
    Dale Pickard Member Posts: 231


    Your knowledge of heating is impressive. I have to wonder how you came it? I think that you are applying that knowledge well to your project. I don't think that you need to use any special software to inform your project. It sounds to me like you have a pretty good handle on it.

    Our extruded aluminum heat transfer plates could help you with your radiant floor retrofit whether you want to work above or below your floor. The ThermoFin extruded plates are of a much heavier gauge than than the formed sheet metal plates to which Paul refers. They also grip the tubing with a strong snap grip which facilitates good heat transfer while eliminating expansion noises which are so common with these plate type installations.

    With these extrusions you will be able to easily meet your design loads in those zones with water temperatures no greater than the 130°f water temp that you describe. If you put the plates under the floor, you could likely use the same reset curve as the radiators.

    Alternatively, you could use the VitoDens control feature that allows the mixing of a lower water temperature on a separate reset curve (the "B circuit" curve"), but this approach requires a motorized 3 way mix valve and a separate circulator. It's cool, but expensive and a little overkill for a kitchen.

    A more appropriate alternative might be to use a non-motorized "dumb" mix valve which mixes return water from the kitchen zone with hot ("radiator" temp) water from the low loss header. This would let you mix a lower water temp for the kitchen vs the radiators but still modulate the temp on the same reset curve ast the radiators. The response is the same, but at a lower water temp. You don't need a separate circulator and likely you would zone the kitchen with a zone valve. If the radiators are already placed in separate zones, then you probably already have the requisite differential pressure valve for this kind of zoning. If not, one could be installed. This delta p valve allows the circulator to "slip: when all the zone valves are closed.

    The point is to encourage more constant circulation, but this goal is often thwarted in kitchens anyway because of the internal gains caused by cooking activities.....especially bread baking .....hmmmmmmm!d

    Others on the Wall here can provide you with information regarding their experience with ThermoFin.

    If you were to contact us with your street address, we can provide you with samples and more information. We work with do it your selfers and we with professional assistance in your area.

    We have been a Viessmann distributor for over 10 years and we now have a great deal of experience with the VitoDens 200....what a sweet machine. You chose well.

    Good luck with your project,

    Dale Pickard
    Radiant Engineering Inc.
    501 East Peach
    Bozeman Montana
    406.587.6036 ext 102
    [email protected]
  • John McArthur_2
    John McArthur_2 Member Posts: 157

    We went to radiant heat in Somerville a couple of years ago. It has to be well insulated.We did staple up and use a separate zone run from boiler through a flatpack exchange heater where we throttle down temp. We could not maintain temp last year in the sub zero cold snap and wound up with a couple of electric heaters.

    If you think that you need supplimental heat you can go to a kick heater under kitchen cabinet. Do not toss the cast rads.

  • If you have a viessmann, do some thermofin or thinfin under that floor and you'll have the sweetest system you can get in your situation. I bet the viessmann will run better than ever.

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  • Holly Gates
    Holly Gates Member Posts: 7
    Thanks for the advice

    Hello Dale, thanks for the helpful suggestions, and the compliments. My heating knowledge comes from maintaining a huge hot water radiator system for 4 years at my co-op in college, and from lots of reading. I am an engineer, and was originally materials science at MIT before I switched to EE, so I took classes like thermodynamics, etc. I've read a lot on the internet, and found "Modern Hydronic Heating" to be an excellent read. A poor substitute for real world knowledge, but luckily I can ask silly questions to people like you through the internet!

    Thermofin looks pretty good, from the information posted on your website. Could you give me some idea of how many BTU/hr per ft^2 output you anticipate with 3/4 wood subfloor and 3/4 hardwood on top? Maybe there is a curve or table for output vs. water temp? Also, I would be interested in some pricing information and samples; I'll send you my address offline.

    I think your idea for a dumb valve is good. I think I might just try to use it on the same curve with the same temp water as the radiators, then add a mixer or something else if it isn't working out. If anything, I think the radiators may need lower temp water than the floor because they are so huge. I think the issue might be that the radiator heated rooms could be at temp, and the radiant heated room will be cold. In that case I suppose I could do a mixer to send cooler water to the radiators, but that seems backward from the normal situation...

    I don't have a delta P valve on my secondary loop, simply because there are no zone valves or TRVs so any time the circulator is running water is flowing through all the zones. There are manual valves and flow meters on each zone, which I can close partially to give a rough balance to the heat output.

    Maybe it would be better if I went to zone valves for each zone; the guy I had put in the Vitodens said there were little zone valves that could mount on the manifold. I have a 9x Rehau Pro-balance manifold. However, with all zones valved I am not sure how to do the control. Clearly with 8 thermostats (one in each zone), each thermostat would directly control its corresponding valve. But for controlling the secondary circulator, I should think it ought to be a logical OR of the 8 thermostat signals, so that if any zone valve opens, the circulator will turn on. This could maybe be acheived just using a diode OR circuit to drive the circulator relay. In this scheme the Vitodens would only serve to keep the water in the header at a constant temp (which of course would vary with outside temperature according to the reset curve). Is there a better way to do this? Maybe the Vitodens has inputs for multiple thermostats and can control the zone valves and circulator relay itself... I haven't read the controls manual yet.

  • Holly Gates
    Holly Gates Member Posts: 7

    I am not going to toss the radiators. I think cast iron radiators are beautiful, and I think they are fantastic to use where one has enough space for them. I've called two places to see if they are interested in buying the radiators (plus a claw foot tub and some other excess plumbing stuff), but no one has gotten back to me with a definitive answer yet. I live right down the street from A1 plumbing, which as I understand it is one of the premier sources for old radiators in the entire country. I ride my bicycle past there all the time, and have cruised their yard on several occasions. If no one wants to buy them I'll stick them in the basement for future use as shop heaters someday down the road.
  • Dale Pickard
    Dale Pickard Member Posts: 231

    Ok Holly,

    Now I know with whom I am dealing! ; -)
    I had to chuckle over your over analysis of the zone valve circuits. It can be complicated and confusing over how to make the most appropriate set of trade offs.

    Back to the beginning,

    I understand that you have 8 radiators plumbed to a 9 loop manifold the circuit from which are driven by the distribution circulator on the distribution side of the LLH. The distribution circulator should now powered by the Vitodens controller.

    One proposal would be to plumb the new radiant zone to the 9th circuit of the manifold and provide the manifold with 9 zone valve motors.

    If you do this you will need the delta P valve to allow the circulator to slip when all the zone valves are closed.

    8 of those motors would have to be wired in parallel so that a single representative thermostat would open them all simultaneously. The 9th zone motor gets it's own thermostat, located in the kitchen/bath. I don't like this option because of the redundant zone valve motors, even though they are cheap, reliable and low power draw, it's too much stuff.

    Another proposal would be to do the same thing as above, with two zone valves external to the manifold. One on the manifold return and one on return from the kitchen zone.

    Either of these two proposals would send the same water temp to the two different devices.

    Introduce the dumb, non motorized 3 way valve to lower the water temp to the kitchen zone. This design will require a 2nd circulator on the distribution side of the 3 way valve. Because you need this circulator anyway, the simplest way to control the zone and power the pump is with an external relay and thermostat and transformer to power the circuit.

    As long as you only anticipate one mixed zone than this is fine.
    If you went to two mixed zones, it would be preferable to wire the mixed circulator to the same boiler circuit that powers the other "hot" zone distribution pump and use zone valves to segregate the two mixed zones. You will need the delta P valve to allow the pump to slip.

    Alternatively, you could wire the mixed pump to a relay that is tripped using the end switches in the zone valves but I see this is as a lousy design. It requires the transformer and relay and it discourages constant circulation. And I've come to hate end switches....I know I'm not alone.

    The mixing devices give you control over energy input to the spaces as a function of the load. As you lower the energy input to the load so as to just meet it, the circulator times will increase. The point is to encourage more constant circulation to encourage efficiency on the part of the boiler system and the use of energy by the building.....while maintaining comfort.

    Any of these scenarios will allow the two zones to operate independently and put enough heat into the two zones over time to satisfy them. It's hard to imagine that the water temp appropriate for the rads is too low for the radiant floor. If you went with the dumb, slave valve approach, and you opened the valve to it's maximum, the radiant floor would get the same water temp as the rads. If you really anticipated that this is how you might end up running it, then drop the mix valve altogther and go back to proposal 1 or 2.

    Good luck with the Vitodens control manual.....It's an experience in
    itself. You can program the control to do a lot of things.

  • Weezbo
    Weezbo Member Posts: 6,232
    Hello. Holly. May i suggest that you take a look at a Taco

    modulating mixing 2 port zone valve.it has outside re set of its own.and its own sensors,with re set ratio...i think it could be used as a stand alone without any other pumps or communication other than thermal requirements.i can e-mail some thing so you may read just how it would work and you can do the math:)
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