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Mixing radiators and radiant floor heat


I asked this over on Gardenweb but realized I would probably get more detailed info on efficiency, etc. here.

We are just starting work on a 800sf addition (2 floors @ 400 sf each) on a 1938 brick colonial in the Washington, DC area. Existing home is 2 floors on a 26x30 footprint so I think about 1500sf finished area.  We will also finish the basement (unfinished now) and do some interior remodeling along the way, but I'm not going to worry about heating and cooling for that at this point.  The digging for the crawl space for the addition began today and I know I should have already made these decisions.

Currently we have an older gas boiler feeding traditional cast iron radiators for heat. We love the quality of radiant heat (we have central air for the cooling side) and have made the expensive decision to keep radiant heat instead of just toss it all and put in heat pumps. Along the way, we decided to use Runtals for the radiators in the new addition and will probably replace the radiators in the main home with new runtals as well since we have to remove a lot of the existing pipes to clear room for the basement remodel and they take up less space in what are already relatively small rooms. Our HVAC guy also recommended going to all Runtals to keep it simple in the long run and free up space.

One of my questions relates to water temperature for the old rads and the new runtals, which is something I meant to ask the HVAC guy next time we get together but I would love input from all you experts as well. Even though the existing walls in our home are probably only something like R3 (brick, block and plaster) the old radiators only got hot in extremely cold weather and even then you could still put a hand on them and not feel they were too hot to lean on, etc. So maybe they could be run at the same temps since the Runtals will be in rooms with R21 in the walls and R38 in the ceiling. Any thoughts on the value of running Runtals mixed with traditional radiators vs. just Runtals are welcome.

In the last few days, we've also begun considering radiant floors in the addition instead of Runtals and then either Runtals in the main house or just use the existing traditional radiators in the main house. I expect the floors and rads (either type) would definitely have significantly different temperatures. The new boiler spec'd at this point would be a Lochinvar Knight high efficency boiler with an indirect 65 gallon hot water heater.

My questions about radiant floors relate to complexity and efficiency in a mixed system, since there is no way we would take on putting in radiant in the existing house. How much complexity (and cost) does running different temperatures incur? I understand there would need to be a mixing valve. Does this require a lot of fine tunning and expertise?

In a hybrid system of radiant floors and radiators, do you reduce the efficiency of the lower temperature need of the floors since you need to heat the water to the temp for the radiators and then are just "watering it down" for the floors?

Any thoughts are welcome, especially since I know I'll need to make a decision on radiant floors very quickly because that needs to be considered before framing.



  • ZmanZman Member Posts: 3,478
    Heat loss

    Someone needs to do a heat loss calc on your home. This will tell you how much radiation you need in each room. Anyone that does not do this is guessing.

    Your existing brick walls may not perform as badly as you think. Because of their mass, they tend to store energy during one part of they day and radiate it later. This works for both heating and cooling.

    The existing radiators will likely perform quite nicely at lower temps. In a perfect world the new heaters or floor radiant will be sized to operate at the same low temps.

    If you do need mixing, many new boilers are capable of this as well. This allows the boiler to run at as low a temp as possible and maximize efficiency.Aftermarket controllers like tekmar also do an excellent job.

    Do not rush this part of the job. The extra time spent calculating now will guarantee a comfortable system with no compromises.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • RobGRobG Member Posts: 1,850
    Heating Dilema

    Have you settled on your HVAC contractor? You are very lucky to be in the territory of Foley Mechanical. I would highly suggest that you contact them sooner rather than later, meaning, if they were to do the work you will not have problems down the road, if someone else does the work (it sounds from your description that you are doing the contractors foot work already) you will probably end up calling Foley Mechanical in after the fact to fix the errors.

    I am not affiliated with Foley, I just know their work and reputation. I am sure that others here will verify what I have just told you,

  • milhousebarksmilhousebarks Member Posts: 7
    Keeping the old rads


    Thanks for taking the time to give us your thoughts on keeping the existing radiators.  It makes a lot of sense from a cost/character and resource use point of view.  I really hate throwing anything away that is useful and keeps in character with the home. 

    A few challenges would be they really all need to be stripped and repainted because of the many years of paint (and some cracking paint).  I've tried to paint a few of them when painting a room and it is a nightmare trying to clean out all the dust, dirt and dog hair in all those recesses, let alone smooth out any chips and cracks.  Seems like this would be the time to have them refinished off site.  The other challenge is a design challenge since we've been thinking of the Runtals and in a few places in the remodeled areas used their narrow profile to slot them in under the front of a new window seat, etc. which is harder to do with the big old rads.  Hmmm....

    A heat calc was done by our HVAC contractor (sub to my GC) so he could spec the appropriate runtals and boiler to give us a budget.  I just started thinking again about the radiant floor option which brought all our old decisions back into the crosshairs.

    Any thought on relative efficiency of mixing temperature requirements in the different zones.  It makes sense to me that you would be wasting energy when you cool down the water heated by the boiler to radiator temperature by mixing it with cooler water, but maybe it is a wash since you have to heat less water overall...
  • SWEISWEI Member Posts: 7,356
    edited May 2013
    Room by room heat loss calcs

    will tell you what temperature water will be required for the existing radiators at design conditions.  Size the Runtals using that temperature and their derating chart and they will play nice with the boiler's outdoor reset curve. 
  • milhousebarksmilhousebarks Member Posts: 7
    Selecting a HVAC contractor

    Hi Rob,

    Thanks for the suggestion of Foley.  At this point we do have a HVAC contractor who comes to us as a sub to our GC.  He seems knowledgeable and the reviews I could find of his work were positive.  He has already given us a budget for the boiler/runtal/water heater, but I just started thinking about the radiant in the floors option and will need to get back to him if we decide to explore that option.  And if we decide to use something like Warmboard we'll need to do that ASAP so the correct floor height is maintained.

    While it may seem like I'm doing the footwork myself it is something I want to do.  I have found I am a much better customer if I understand the basics of the products and processes and can ask the right questions.  If I ask questions and am given reason to doubt I may well decide I need to push for a different HVAC person, possibly Foley. 

    I am also considering hiring someone to give me some time as a consultant once we decide which system to use so we can have another set of eyes double check heat loss calcs, etc. 


  • milhousebarksmilhousebarks Member Posts: 7
    Makes sense to me


    That seems like a key piece of the puzzle.  That way no mixing valves required.  Does this lead to better overall efficiency?
  • RobGRobG Member Posts: 1,850
    An educated consumer

    An educated consumer makes for a good customer. If you know what you want and can let your contractor know what you expect, the project should move smoothly (provided your con-tractor is not a con-artist). I am not implying that your contractor is not top notch, I just don't know him/her.

    However, type "Dan Foley" in the site search block at top of the page, there are not many people in the industry that have his credentials.

  • SWEISWEI Member Posts: 7,356
    In-floor tubing

    even without Warmboard will require much lower water temps than the existing radiators, which would require two different water temps, preferably on separate reset curves.  You could size the plate radiators for either one, though they would have to be quite large if you're trying to match them to Warmboard.
  • ZmanZman Member Posts: 3,478
    Check out...

    This post on proper piping and flow rate for cast iron rads

    A favorite for sure.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • milhousebarksmilhousebarks Member Posts: 7
    Is mixing different temp emitters inherenty inefficient?


    That makes sense, but I think that making the rads big enough to emit the same heat with water temps set for the floors would lead to monsterous rads, so that doesn't seem viable.

    I'm still trying to figure out if using a mixing valve to drop the temp for the floor zones would give me higher gas bills than if I had all the zones using rads that all used the same temp water.

  • milhousebarksmilhousebarks Member Posts: 7
    More complicated than I thought

    Hi Rob,

    Back when we decided to keep radiant heat as part of our addition/remodel I didn't realize how sophisticated modern systems had become.  If I had known I would probably have insisted on pulling the HVAC part out of the GC's realm and making it something we would contract directly.  I'll keep Foley in mind if it seems like our guy is over his head.  Don't know at this point.

    Thanks again for your input.
  • milhousebarksmilhousebarks Member Posts: 7
    So even the classics are complicated

    Interesting reading and a good reminder that the system is more than the boiler and radiators (or floors).
  • SWEISWEI Member Posts: 7,356
    Mixing down

    will not harm efficiency, in fact with proper design it may actually increase efficiency.  Depending on flow rates, it may be possible to use the returning 'cold' water from the radiators to heat the radiant floor.  This would allow a mod/con boiler to run at a higher thermal efficiency. 
  • Paul PolletsPaul Pollets Member Posts: 3,017
    Design Quandry

    The contractor should hopefully have a skillset that recommends a two temp system for the RFH and radiators. There should be A/C as well, for an integrated system.

    Foley Mechanical is the best in the DC area, hands down. Don't hesitate to contact him if you can't get what you want, or want local advice.

    The Viessmann Vitodens 200 system pictured below is making 4 different temperatures that include RFH, HW Coil on air handler, radiators and a DHW Tank.
  • RichRich Member Posts: 2,224
    different temps

    Millhouse ,

         The referral of many to Foley Mechanical is the best advice you could get ! Dan is quite simply the best in your area and probably any area .

        2 Temps or 3 for that matter should not be a problem .  Taco makes a really nice  mixing valve with Outdoor reset , the I Series . Will work independent of boiler reset so all parts of the system can enjoy the same features and play nice together . 

       Referrals from others .  These don't always mean much , just because someone was personable and made the people referring them comfortable is not always what it seems . Heating systems in general have not performed like the systems we design and install today . If the referrers are comfortable and save a bit of money they will give a positive review , just means it is better than what they are used to . It seems that you want the best by all the effort you are putting forth , there are not that many contractors that can deliver what you are wanting . If you are getting the idea that this current contractor is in over his head he very well may be . This is your home and you are not obligated to use the G.C's choice of contractors .

       My concern is for my industry and defending it from receiving negative publicity and making sure people who decide to go hydronic have a positive experience . If you decide to reach out to another save your time and sanity and call 703-339-8030 and ask for Dan and relax .
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC 732-751-1560
    Serving most of New Jersey , Eastern Pa .
    Consultation , Design & Installation
    Rich McGrath 732-581-3833
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