Balancing Rads and In-Floor Heating

[Davie]

Looking at a system with a combination of rads and in-floor heat. Some rooms will have both, and some will have only rads.

For the rooms with both heat sources, how can I set it up so the rads only come on when the floor isn't sufficient to provide the necessary heat? Can I use TRVs?

 

[Dana]

Setting it up the rads as the second stage of a 2-stage heating approach in the rooms with both radiant floor radiators can work fairly well. The reaction time of radiant floors to changes in load or recovery from setbacks are pretty slow, but if the second stage only comes on after a time-out of say, 15-20 minutes from the initial call for heat the primary radiation will still be the radiant floor, but the recovery ramps after the time-out are much quicker. During periods of lower load there will be a bit more cycling than if controlling the room temp via floor alone since the radiators come on and satisfy the thermostat quickly after the time-out.

Another approach would be to control the floor's temperature with a thermostat with both floor & room temperature sensors, and the rads controlled by a separate room air thermostat. Whenever the load is fully covered by the floor's output the radiators are never in use, as long as the radiator's thermostat is set a degree or so below the floor's thermostat.

The intended zoning the system and the boiler control strategies will also play into what works best, but there are several ways to get there.

 

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[Davie]

Thanks, Dana.

Your second suggestion, where the radiators and floor are on separate thermostats is very intriguing. Near design temperatures, would that lead to a situation where the floors are always circulating and only the rads are "cycling"? That would mean that (provided the floor load is above the min. fire of the boiler) the boiler is running constantly, right? My max floor output on the first floor is 11,500 btu/h, and I need an extra 6,400 btu/h at design temps., so I should be okay to find a boiler that fires below that. I've seen a few that you have pointed out to others.

Is there an advantage to having a smaller radiators at a higher heat? Or could I run the floor and rads at the same temp, and just upsize the radiators a bit?

 

[Dana]

Are you running a fixed temperature out of the boiler, or is it on outdoor reset, responsive the outdoor temp?

 

[Davie]

I'm thinking outdoor reset, but could go either way.

 

[Dana]

At what water temperature does the floor emit 11,500 BTU/hr?

 

[Davie]

I'm looking over those numbers, and I must have mixed the values up when I stated 11,500 btu/h floor output and 6,400 btu/h radiators.

In fact, the total floor output is 14,620 btu/h at an Average Water Temperature of 145F, if my calculations are correct.

I see some differing opinions about what maximum radiation per sq. ft. is practical. I went with 34 btu/h/sq. ft. based on a maximum comfortable floor surface temperature of 85F and a room temperature of 68F.

 

[Dana]

A 145F AWT would be above the condensing zone, but close enough to make outdoor reset more appropriate if you really need temps that high at design temperature.

Most of the time the load would be well below that, and the water temperature requirements much lower, making a condensing boiler under outdoor reset control an attractive option.

 

[Davie]

The other option, I guess, would be to keep the AWT lower at design temps, and make up more of the load with the rads.

14,620 btu/h should match the heat loss at an outdoor temperature of 22F (design is 1F). Which would mean that anytime it's below 22F outside, the boiler isn't condensing, right?

If I dropped the AWT to 125F, the floor would emit 10,750 btu/h, which would match the heat loss at 35F outdoor temperature. Which would leave 10,500 btu/h to make up with the rads.

 

[Dana]

The entering water temp at the boiler has to be somewhere around 125F -127F to hit the beginning of condensing at ~90% efficiency, which depending on flow, etc is an AWT of about 130F (135F-out, 125F-return). To edge up into the mid-90s for combustion efficiency is typically an EWT of about 115F , or an AWT of about 120F.

A room load of 14.6K @ +22F implies a fairly large or fairly lossy room (lots of windows, perhaps?) That's a bit more than half my whole-house loss @ 22F. (A ~2400' 2x4 framed 1920s bungalow w/ clear glass storms over wood sash single panes.)

 

[Davie]

The house is very lossy. The r-value of the walls is only R3.9. (8 inches of structural double brick with no air gaps; 1 inch air gap; 5/8" lathe and plaster.)

And it's attached on one side, so I'm only counting losses on three walls.

Edit:
Some of the walls have been drywalled with 1/2", and that's what I used as typical for the calculations, as seen in the image.

 

[Dana]

That is indeed a lossy way to build a wall! Seems like there is a lot of legacy construction of that description in urban Ontario.

Insulating it from the interior risks freeze/thaw spalling. Those risks can be managed, but it requires a bit of analysis. If the plaster and lath start to give up at some point it's worth doing some of that analysis before putting a new finish wall up. Insulating it with rigid foam on the exterior is pretty safe, but often has the ae$thetic issues to deal with, as well as window flashing details to work out.

 

[Davie]

Yeah. It's a shame. The reality is I'm not going to do any of that work before the new heating system goes in.

I was hoping that adding a couple of inches of rigid foam on the largest exterior wall (34' x 19' with only 2 windows) might bring the overall design load into the range where the floors could handle it, but it won't. Running that math, we'd still be about 8500 btu/h short overall, and those deficits are over 1000 btu/h each in the kitchen, living, hall, and two bedrooms. Couldn't solve it with just an extra rad or two.

 

[Davie]

When I realized just how much heat the house loses, and that I can't make it up by the floors alone, I scuttled my plan for radiant floors on both levels. Now I'm aiming to do radiant floors on the main level with rads to makeup the extra when needed, but not doing the whole second level, just to save on the labour of ripping down all the ceilings.

The kitchen ceiling is coming down anyways, so I was thinking of putting in one radiant loop upstairs under the bathroom and bedroom, but I'm unclear on how to zone it all. Originally I had planned 3 zones: 2 radiant floor zones with 3 loops each, and one basement radiator zone. But now it's looking more complicated.


Now I'm wondering about 3 zones:
-basement: rads only
-main floor: radiant floors with two radiators as second stage heat
-upstairs: front two rooms: rads only; back 2 rooms radiant floor with rads as second stage

This is basically what I'm wondering about, but I'm not sure how the different temperatures will work. I'd like to keep the EWT as low as possible, of course.

 

[Dana]

Run the room by room heat load numbers and find the radiation that fills the bill to make it all work at your target temperature. Most flat-panel radiators are specified at both 180F and 140F average water temperatures, or some just at 140F, but not all specify the output at 120F, (which is where you want to be most of the time over the whole heating season).

As a general rule, the output at 140F is about half what it is at 180F, and the output at 120F is about half what it is at 140F. If you set up the room radiation such the room's heat load is covered at the outside design temp with 140F water (which is is needed only 1-2% of the time), more than 90% of the time the water temperatures will be in the condensing zone.

Getting it to where it needs only 120F water at design condition takes about twice the amount of radiator that it takes with 140F water at design condition, which gets to be expensive, and takes up a lot of wall area.

 

[Davie]

That's smart. I've done a room by room heat load calculation, so I'll look at what size of radiators will supply what I need at 140F AWT.

Would you then run the floors and radiators at the same temperature (based on the outdoor reset)?

Another approach would be to control the floor's temperature with a thermostat with both floor & room temperature sensors, and the rads controlled by a separate room air thermostat. Whenever the load is fully covered by the floor's output the radiators are never in use, as long as the radiator's thermostat is set a degree or so below the floor's thermostat.

The intended zoning the system and the boiler control strategies will also play into what works best, but there are several ways to get there.

Back to our original discussion, that would mean that the floors aren't supplying their full capacity until approaching design conditions. So, less comfortable, perhaps, but more condensing efficiency.

 

[Dana]

Yes, run the floors at the same temps as the rads. That may mean running your 'second stage' rads in the radiant floor zones a bigger fraction of the time, or simply designing those rooms to run them in parallel with the floor's loop, tweaking the radiator flows to where it just keeps up with the load on design day.

 

[Davie]

Right on.

I'll have to see how much cost the 'second stage' would add, and balance that against the possible extra comfort.

 

[Davie]

Run the room by room heat load numbers and find the radiation that fills the bill to make it all work at your target temperature. Most flat-panel radiators are specified at both 180F and 140F average water temperatures, or some just at 140F, but not all specify the output at 120F, (which is where you want to be most of the time over the whole heating season).

As a general rule, the output at 140F is about half what it is at 180F, and the output at 120F is about half what it is at 140F. If you set up the room radiation such the room's heat load is covered at the outside design temp with 140F water (which is is needed only 1-2% of the time), more than 90% of the time the water temperatures will be in the condensing zone.

Getting it to where it needs only 120F water at design condition takes about twice the amount of radiator that it takes with 140F water at design condition, which gets to be expensive, and takes up a lot of wall area.

Another thought/question about this:

In the master bedroom, it's hard to find a single radiator that will fit in a reasonable space in the room, and provide enough heat at 140F. I would have a 1300 BTU/h deficit. Of course, I realize I could put in two separate rads, but I would rather try and avoid this.

(One exterior wall has a decommissioned fireplace along it, and the other has a bay window; the interior walls have the bed, the doorway, and the closet. The best radiator options are long and low under the window or tall and narrow elsewhere.)

If I design the system at 150F AWT, a rad under the window will have enough for that room on design day. My concern is that if my whole system runs at 150F, my floors will be too hot underfoot. And, of course, 150F is out of the condensing range, so even with outdoor reset the efficiency of the system will drop.

 

[Dana]

Radiant floors (other than concrete slabs) won't get too hot with 150F water on design day. Even under the subfloor systems with extruded aluminum heat exchangers can usually run 180F water without cooking anybody's feet.

Low and under the window is going to be the best radiator option, since the rising heat off the radiator will counteract the motion o the cool air falling down the face of the cold window.

A 1300 BTU/hr shortfall for a larger room isn't usually a big deal. How big is that room? Remember that every watt of lighting or plug load in the room will be emitting 3.412 BTU/hr, so 150 watts of fluorescent lighting would be emitting over 500 BTU/hr, a standing conscious human is good for at least 325 BTU/hr etc. It all adds up. If the shortfall only occurs under design condition it'll still be keeping up at least 98% of the time. If that room loses ground by a degree or two at 6AM on the coldest day of the year do you really care?