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"Incremental" hydronic radiant floor + baseboard project

Hi all,

I hope this is an interesting topic.

The goal is to increase the comfort of the key living areas in my house through the installation of a hydronic, natural gas system. Currently I have a functioning forced air nat gas furnace. I want to hear opinions on the design feasibility of installing this hydronic system that would run concurrently with forced air. Further below I will lay out the 'why' I want to do it this way.

Here are some the facts:

1. In Denver, Colorado
2. 2002 Construction
3. 2400 sq feet of living space plus 'half conditioned' unfinished basement
4. House design is two story with significant double pane windows, most of which have a south east exposure
5. House design has a large great room such that the kitchen + downstairs living room + upstair loft and catwalk are all 'one room' from an air perspective (e.g. there are no doors to separate).
6. The highest use room is the kitchen/breakfast room which also has the most windows and least doors. It is situated in the SE corner of the building

The problem with the forced air system is that the system has to be at a higher temperature to make that SE breakfast nook with all the windows be comfortable when there is no solar gain. Additionally, when it is sunny in the winter, this room also gets the most heat gain in morning. This is great from a comfort perspective, but means often in Colorado there is demand for heat in this room before sunrise in winter, the demand drops depending on solar gain, and then the demand increases after sunset.

The breakfast nook is hardwood floors, and the joist are accessible from the basement.

I can imagine a hyrdonic floor system whereby a boiler supplies radiant floor heat to the breakfast nook, twice a day on sunny winter days, all day on 'arctic vortex' days, no days in the summer, some days in the spring and fall.

Here is where I am at from an experience and research perspective:

Experience: I sourced, designed, and installed an evacuated tube solar water heater that heats both domestic water as well as a small green house and garden beds (not all at the same time!). It has been working well for 7 years, surviving both a deep freeze where the glycol mix got slushy, as well as a corroded failed thermosensor that kept the pump from circulating resulting in extreme hot conditions.

Research: I have spent a lot of time researching this forum, the "DIY" websites, the forum's opinions on those websites, modulating condensing boilers, and I purchased and am working through 'Modern Hydronic Heating'.

Here are some of the 'best practices' that seem hard to integrate:

1. Doing a heat load calculation by room given this house is mostly one giant room. Also the breakfast nook is the 'worst' in this regard with two external walls, no internal walls and being adjacent to a living room with a two story profile. Would doing a 'building envelope' heat load instead provide better results?

2. Design day criteria. This concept I understand. I also have seen experienced people talk about the need to eliminate short cycling. I have read this one case as the 'ideal' is boiler comes on 'once' in the fall, and turns off once in the spring. Colorado can have 65 or - 15 days in February. Would a design criteria of 'no more than 2 on/off cycles per day during cold sunny winter days' be acceptable from a boiler functioning for a long time perspective?

3. Integrating the fluxuating solar gain. With the SE facet of the building being a 'wall of windows' there can be 10,000+ BTU/hr times in the winter

Here is my 'imagined' system, and the point at which I 'lose the math'

Closed primary /secondary with a modulation condensing boiler (navien NHB-80, HTP-UFT-80). These units go down to 8,000 BTUs.

Pex radiant 1/2 under joist for the 80 sq feet of nook and 40 sq feet of galley kitchen.

Radiant baseboard in the great room and adjacent 'dining room' to 'make up the difference' of the 8,000 BTU min burn of those mod con boilers. Enough linear footage of the right selection of product (would work with pex under floor) for Designing the system from the boiler's perspective: it needs to be able to 'dump' at least 8,000 BTU/hr when there is demand for heat from the breakfast nook.

So: 120 sq feet of 3/4 in oak on subfloor with radiant pex underneath. 120 x 30 BTU's per sq ft ( I have read the 'heated' discussion on this forum, going with 30 from a conservative side) = 3600 BTU

Slantfin multi/pak 80: 180 --> 460 BTU / foot depending on supply water temp ranges of 110 --> 140 and product selection. So with supply temperature of 140 and the right product and air temp of 65 with 20 feet of this board >= 8000 BTUs (rounding down for conservatism).

8,000 + 3600 = 11,600 BTUS 'heat sink potential' should give enough 'heat dump' with margin for error of one of these boilers running at full turn down (8,000 BTUs).

I have a novice understanding of the condensing principal: efficiency begins at return temperatures of 130 and increases the colder it goes.

So the design challenge is to get it hot enough for the radiant baseboard, not too hot for the oak, and return temps to condense. So 140 supply temperatures, a 20 degree drop = 120 return, which based on this chart I am looking is about 90% efficiency.

One of the benefits of the 'incremental' approach I believe is that I can test the system out over a winter and see how close I am to my calculations. This data will make the rest of the build out more likely to be accurate.

Any takers?


  • hot_rod
    hot_rod Member Posts: 16,304
    A load calc first and foremost. Below are 3 different free load calc programs. Room by room if possible, certainly the bed rooms are walled off :)

    Passive rooms are tough to control, more from an over-heating perspective. A small floor sq footage with a lot of glass will be a room to look at carefully.

    Hydronic for all the heating, forced air for AC. It's not so easy blending forced air and hydronics in one space.

    A mod con with a small buffer is an option, it replaces P/S piping and helps with low load conditions. Or a tank style mod con, or Viessmann high mass 300

    Panel rads might be a better option, easy to add and subtract and have room by room even radiator by radiator zoning. You van blend panel rads with radiant areas. The panel rads get large when you use a low SWT, below 120F for example.

    More realistically 25 btu/ sq ft for a comfortable floor.

    Some good output comparisons of the various emitters here.


    Radiant walls and ceilings are another option, higher output than floors.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Zman
    Zman Member Posts: 7,330
    edited September 2018
    Get your room by room heat loss done first. Do you have any plans for the house?

    Size the emitters to all run at the same water temperature. You will likely need a buffer tank to get anywhere near your boiler cycle goals.

    I would not even consider traditional baseboards. If you cannot get radiant in the floors,walls or ceiling, I would go with panel radiators. I would be very surprised if you need to get 30 BTU/ft out of the floor.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • ekubec
    ekubec Member Posts: 4
    @hotrod, thank you. I will do the heat load calc. On the cooling side. I have 4 minisplits in the upstairs rooms (3 bedrooms + loft which is open to the great-room-breakfast nook-dining room-entry way). Also I have a large window evap cooler in the greatroom, which does most of the cooling during the day.

    Countering the potential for over heating is the fact that this is one giant space that is open to the second floor: 15' x 30' = 450 sq feet of 'no second floor'. The heat can 'bleed off' through that 450 sqfoot 'open space'.

    Is a buffer tank (non-integrated) controlled via a mixing valve on the return side? E.g. as the mass of the radiant heats up and the day heats up, the return temps heat up, and the boiler can draw cooler water in from buffer (and put some heat into the buffer) to keep the return water temp at optimal for condensing / no cycling)

    Here is a pic of the rooms. I am facing directly East. The 'wall of windows' faces southeast.

  • ekubec
    ekubec Member Posts: 4
    @zman, thank you. I do not have plans for the house. Its about 40' wide, 30 - 42' long. 2 story.
  • hot_rod
    hot_rod Member Posts: 16,304
    Wow! mostly glass, not much wall space for radiators. The carpeted great room would be tough to radiantize.

    Some insulated window blinds would help a bit.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream