Heat not keeping up

DBSerink

We have hydronic floor heating in our basement (cast-in-slab) and on the main floor (staple up). The house was just completed last year and this is our first winter. We're very disappointed. The air temperature in our house is 65f and we can't get it too increase.

We have a Navien NFB-175 boiler and 3 zones, 1 for the basement and 2 for the main floor. The main floor tubing is 1/2"" and runs up then down each joist spacing (16"). There are transfer plates every 2'. The floor temperature runs at 80f-85f but we can't get the air temperature above 65f when the outside temp is -35f.

My contractor says 85f is the maximum the floor temp should be and we need to install a supplementary forced air system ($$$). Is he correct? Or am I being hosed?

Unfortunately we are the general contractor (self managed) and we hired one company to design the system (mechanical engineers) and another to install it. My installer says the engineers designed the hydronic system correctly because it is achieving the maximum floor temp. But the engineers should have known that with our house design (lots of windows) supplementary heating would be necessary and proposed a backup system. They didn't.

I am hoping to get alternatives to ripping the floor and walls apart to install a forced air system and also opinions on whether we should pursue court action with the engineers for under-designing our heating system. As an engineer myself I know that its hard to successfully sue for failure to identify a problem unless we can prove the engineer knew about the problem and "neglected" to inform us (ie: we can prove negligence).

pecmsg

What's the surface of the floor wood, carpeting? You don't want the surface much above 100 - 110°F so the 85°f # seems really low.
What has the Mechanical Engineer said? His design his problem!

With what you describe supplemental MAY be required. Slowly raise the supply temp and see how it performs.

hot_rod

You are correct so far. 82°F is about as warm as you want a surface temperature. If the entire floor surface is that warm and you cannot cover the heat load, the design should indicate that. In some rooms, high ceilings, large glass, radiant floors alone are not capable of covering the load on cold days.

Most designs include a disclaimer indicating the design acknowledgment and requirements for supplemental heat. Those should always be read and signed by whoever is responsible.

An experienced radiant installer can usually know just walking into a room if the radiant floor will cover the loads.

Is the the installer or the GC responsible for the final product?

Got a copy of the load calc and design. If it was done on any of the common software programs LoopCAD fo example, it spells out clearly, maybe in red print, the need for supplemental heat.

Panel radiators, kickspace heaters under cabinets are a couple possibilities.

GGross

deleting my original comment, was redundant

If you have a portion of the design documents that you are comfortable sharing here I would think that may shed some light on the situation.

HVACNUT

I don't know much about radiant design but I do know a BTU is a BTU. 
Maybe it's not temperature. Maybe it's piping and/or controls.
Do you know the supply and return temperatures at the manifold(s)? Each loop?
Can you post pics of the boiler piping and describe the operating controls? Is there Outdoor Reset involved? Is it a boiler setting?
What's under the basement slab?

WMno57

Agree with HVACNUT, a BTU is a BTU.
What is the heat loss of the house on a negative 35 F day? That number should have been calculated somewhere along the way.
How does that compare to the output on a Navian 175? 175,000 btu Input. Guessing around 150,000 BTU output. If the homes heat loss at -35 F is 125,000, then you need more radiators. If the heat loss of the home at -35 is 175,000 btu, then your boiler is undersized for the location and home
Negative 35 F straight temperature or windchill?
How often does it get this cold?
My shoot from the hip thought is to add some electric resistance heating. Baseboard or space heaters. But we need more details.
Where is this located?

mattmia2

Was a heat loss calculation done?

Oh, and did you change anything about the envelope after you give the ME the design documentation?

MaxMercy

Normally, the floor temp is about 8 degrees on average higher than the air temp, so you should be OK, but on the other hand, minus 35 F? Wow, that's friggin cold.

EDIT: Curious, I did some checking on radiant floors because I have no experience with them. This is what I found regarding radiant floors in very cold climates:

"I specify radiant heat as a complimentary heat in certain areas of very few designs.

In some of the rooms with more open areas and large windows, if you were to use only radiant heat in our cold climate, you wouldn't be able to walk on the floor without having the feeling that you're doing a fire walk. Radiant heating is regulated to prevent the temperature getting to that level so on a small footprint, it's insufficient to do the job alone."


If you're only having issues on a hopefully rare day of -35F, then would be a supplemental electric room heater be a solution? How often are you seeing temperatures that cold?

From what I've read, the radiant floors are slow to respond to temperature changes, is it possible to raise the room temperature a day before bitterly cold weather is forecasted?

WMno57

Is there glycol in the system? If there is, that will reduce the output.

Jamie Hall

Helpful comments up there, but a summary: first, 85 is as warm as you want the floor. Anything higher and you are going to be uncomfortable unless you always wear shoes with insulated soles. So that's a real limitation. That also means, though, that there is a maximum output which the radiant floor can manage -- and unless your structure is heavily insulated, at your design temperatures I would be mightily surprised if a radiant floor -- even if all of the floor area were effective (no rugs, no couches, no easy chairs, no cabinets etc.) that you could reach a comfortable air temperature.

Supplemental heat is needed... lots of ways to do it.

ethicalpaul

-35 is so cold that Celsius and Farenheit get together and agree

bburd

If the boiler capacity is adequate to cover the heat loss, supplemental panel radiators or hydronic baseboard on a separate zone or two would be far less disruptive from a construction standpoint than adding forced air.

lobuc

My experience has been that I would agree with most of the above contributors on the slab but the staple up is a different beast. We get similar temp extremes and colder, I have had to run high temps in staple up also need to insulate under the staple up a detail that is often skipped but makes a world of difference. The loop temp is not the finished flooring temp. Multiple homes no complaints of damaged flooring or burnt feet yet.

hot_rod

I haven't found any below the floor method that gets the entire surface to a consistent temperature. You will always see some stripe. The type of floor covering has an effect. I'm not a fan or carpet, but it does spread the heat out a little more evenly across the floor surface. The back 1/2 of this ThermoFin and WarmBoard demo has carpet.

The carpet (insulation) reduces the floor output, as you can see. Even throw rugs with thick pads below really limit the output.

Of all the systems copper tubing in the heavy gauge ThermoFin, shown below, transfers heat the best. The two outer plates were not in tight contact as I pulled them down to make the pex crimp. Plates must be in good contact to get the powerful conduction transfer.

If you get over 82F surface, even just over the tube, it is not comfortable to stand on. On hard surface floors anyways.

Warmboard solid aluminum surface does a good job of moving the heat, but the 12" OC does still stripe a bit.

TonKa

FWIW, if you do end up needing supplemental heat, there are electric and hydronic options which are nowhere near as disruptive to install (and probably significantly cheaper, too) as forced hot air.