Mass concrete radiant heat

MrPumpit

Hi, I recently built a new home and I’m seeking advice on “fine tuning” my radiant system. The main level has a suspended 6” reinforced concrete slab with nine runs of 1/2”pex at 240’ per at 9” o.c.. The area is open concept using one thermostat, cathedral ceilings, spray foam walls & ceilings. Heat source, Burnham Alpine 210 running on propane.
Last heating season i used 2400 gallons of fuel, much higher than I hoped for. Water temp entering manifolds is 125 f.
Main level circulator, Grundfos ups26-99fc. set on high. My question is, can I improve the system by Lowering the water temp or the circulator speed to get a better result? Any help would be greatly appreciated. Thank you.

Jamie Hall

You have two completely different considerations here, which may or may not (probably not) be related.

First, fuel cost. You have paid for 2400 gallons of fuel. That's somewhere around 240 million BTUs of energy Unless the boiler is hideously inefficient -- and the Burnham Alpine is a good, efficient boiler, so that's out -- that's around 200 million BTUs of heat which got into the structure. Assuming that the temperature of the structure stayed the same, that's also about 200 million BTUs of heat which left the structure.

Physics is mean. The only way you are going to use less fuel is to reduce the amount of heat you need, which means reducing the heat loss of the structure -- or possibly making sure that they heat is where you are, and not heating what doesn't need to be heated. Cathedral ceilings are bad news, but you do have overhead fans to make sure the heat is down where you are, I presume?

You might be able to gain perhaps 5 to 10 percent in overall efficiency by tweaking the controls on the boiler to ensure that it is always running as well as it can be. With radiant floors, particularly, properly adjusted outdoor reset can make sure that the boiler is always modulating as far down as it can, and that the floors can run constantly (they should) at whatever temperature is needed to meet the heating requirements. This adjustment may not have been properly made -- it takes some time (as in days or weeks, on and off) and it's much easier for the installer to just turn it on and leave it with the presets.

If either the floor pumps or worse, the boiler, are turning on and off, the short answer to your question is yes, you can gain perhaps as much as 5 to 10 percent by setting the operating parameters of the boiler correctly.

Now the second item is comfort -- is the space comfortable? If so, there isn't much you can do there -- except one thing: do not use temperature setbacks. They are guaranteed to waste energy with a high mass radiant system such as yours. Set the space temperature at what is good, adjust the boiler operating parameters as above -- and leave it alone. If you should happen to have a "learning" thermostat -- such as, perhaps, a Nest or the equivalent, disable all the learning and occupancy features completely and force it to hold a constant temperature.

MrPumpit

Hi and thank you for your response. Last winter I finally got the Burnham tech to come out and properly set the burner. He mentioned that the burner was stopping & staring to many times which was part of my problem. He programmed it and told me that this would make a considerable difference, which I believe it did. I do have ceiling fans but it doesn’t feel like much of a difference. Could the answer to my question be the water temp entering the floor or the circulator speed? My thinking was if I could get that circuit circulator to run 24/7 at a much lower temp governed by an outdoor sensor via Tekmar controller with a variable speed circulator?

MrPumpit

Jamie, one last note, my thermostat is a Tekmar 561

Jamie Hall

Whether you can gain much or not depends on what you have now: with the Burnham, you very much want the return temperature well down -- certainly less than 100. There might be some advantage to a variable speed pump for circulation, with or without a temperature sensitive mixing valve. Keep in mind that what you gain in efficiency comes at a cost of increasing complexity...

hot_rod

How many square feet are you heating? Do you have a load calculation to help determine what the building actually requires?

The home will require X amount of heat to maintained the desired temperature. the building envelop and weather are in charge of that.

Where is the tube in the slab? If it is at the bottom that will take 20° warmer SWT compared to tube near the top 2".

How is the edge of the slab insulated, that can be a huge loss area.

The outdoor reset may help possible and maximize the condensing mode.

MrPumpit

Understood, my current delta T is around 10 deg. I think I would like to try running it at a lower temp and see how that feels. I always have my fan coils if needed. Any thought on circulator speed, the plumber set them all on high?

hot_rod

Slowing down the circulator will probably widen that 10° delta. 10- 15° is common for concrete slabs. I doubt slowing the circulation will change the fuel consumption however.

MrPumpit

Hi Bob, thanks for your input, The area of the main floor which is on one thermostat is 1800 sq ft. I do not have a load calculation, however the average cathedral ceiling height is 16’.
The slab edge is covered with 1-1/2” insulation (open cell). Most of my loss is in the sun room on the north end of the house. Three walls of glass windows approx. 10’x10’ each. That room only gets to 66 deg. In mid winter while the rest of the area maintains 70 deg. I think I am going to drop the water temp 5 deg. To 120 and put the circulator on med. and give that a try. Right now it’s 39 outside and I have the fan coil on and it’s comfy. I’m not a fan of hot air heat. Thank you, Chris

Zman

Your boiler is capable of outdoor reset control and would have been shipped with an outside air sensor.
If your plumber tossed yours out, I would highly recommend installing a new one in the shade on the north side of the house.
once the sensor is installed, you should be able to run your boiler around 125 degrees on the coldest day and as low as 80 degrees on the warmest. You will not only save energy due to the increased condensing efficiency but you will also save energy and improve comfort by not overheating that massive slab.

hot_rod

Probably if you have 3 walls of glass you have some south facing? That can pose yet another challange for radiant, any HVAC system really.
I've been in Colorado trophy homes where 1/2 the house is heating, south facing have the AC running.

I love the view but large glass poses an R-value and solar gain concern.

Tall ceilings are not usually a big concern for radiant floors as there is very little stratification, but, again, big glass can be and often the floor radiant alone may not meet the load in those "glass" rooms.

Run a load calc on just that room see how it pencils out. I've used panel rads and even trench convectors, recessed into the floor under the glass, to make up the radiant shortfall a few times.

No harm in trying some things, I doubt reducing flow and dropping SWT will get you to where you want to be? You may get to a near constant circulation condition, but you still need to add heat at the same rate that the room is losing it.

Zman

The floor of your house weighs around 135,000 pounds.
For every degree you overheat the slab beyond what is required to offset your heatloss, you are storing about 28,350 btus that you don't need or want. It may not fix all of your problems but outdoor reset as a first step will absolutely improve comfort and save propane.

Robert_25

MrPumpit said:

Hi Bob, thanks for your input, The area of the main floor which is on one thermostat is 1800 sq ft. I do not have a load calculation, however the average cathedral ceiling height is 16’. The slab edge is covered with 1-1/2” insulation (open cell). Most of my loss is in the sun room on the north end of the house. Three walls of glass windows approx. 10’x10’ each. That room only gets to 66 deg. In mid winter while the rest of the area maintains 70 deg. I think I am going to drop the water temp 5 deg. To 120 and put the circulator on med. and give that a try. Right now it’s 39 outside and I have the fan coil on and it’s comfy. I’m not a fan of hot air heat. Thank you, Chris

How big is the house in total?  If I read your post correctly the area heated via radiant is 1800 sq ft, but if that is only half the house that makes a big difference in how your fuel consumption looks.

MrPumpit

Hi guys, thanks for your input, I do have an outdoor sensor for the boiler. One of my problems I was having certainly played a part. For nearly 3 to 4 months I was experiencing a soft lock out accompanied by ignition failure and the service tech finally figured it out. Behind the gas valve is a round plastic air/fuel mixer that had shattered losing ability to mix the fuel & air properly. Now the boiler is working fine, low temps, but I am heating with the fan coil. I will start up the radiant in Nov. my goal is to try and stop the boiler from going on & off so frequently. Any thoughts on that?

MrPumpit

The entire house is all radiant w backup fan coils. The lower level mostly below grade w/ west side walk out. (1840) sq ft.
Two radiant zones. Lower level heat rarely goes on. The upper floor total area is (2400) sq ft. Main zone.1 (1800) sq ft. Includes Living, dining, kitchen & sunroom. Zone 2 (600) sq ft includes master bedroom & bathroom. (4 loop run w/zone valves.

Zman

If the damage was to the inner venturi, that can sometimes be caused by the combustion air vent being too close to the exhaust or in an area where the exhaust cannot escape. Can you post a picture of the vent on the outside of the house?

Your boiler is short cycling because it is 2 to 3 times larger that required (like driving a 1 ton truck to get groceries). There may be some settings that can be adjusted to help somewhat. Beyond that, a buffer tanks or a smaller boiler is the next step.

hot_rod

Read through the control setting section of your manual, it has a number of settings to prevent or mostly eliminate short cycling. The boiler can pretty much match any load down to its lowest turndown.

The Central Heat Maximum basically allows you to set the highest firing rate. If you knew the heat load you set the boiler to fire up to that. You could trial and error it, set it at 50% and give it a try, Factory default is 100% so it tries to go full throttle on every heat call now.

I have a combi boiler, 120K on my home with a 30K load, so I set it to fire at 30%. It stills goes to high fire, high temperature for DHW calls.
Use ODR and anti cycling function also.

MrPumpit

Hi guys, Zeman, you are right about the intake/exhaust being to close, they were at the same height in a corner, 2 weeks ago I raised the exhaust 3’ higher, hopefully that will solve that issue.
The Burnham tech recently made some adjustments & it seems to be much better, much quieter, running on lower temps. I am going to calculate the heat load today to see what needed. I’m thinking of running the radiant governed by the floor sensor and not the air temp, set to run at 70* min to 80* max, leaving the fan coil on as a backup set at 68* any thoughts on that?

Zman

I don't think adjusting the boiler parameters will help much. That Honeywell MCBA controller does a nice job of turning down under low load situations. It may have a anticycling delay feature but you would have to go deep into the service manual or tech support to enable.
As for the rest, if you do not improve overshoots with outdoor reset, slab sensors and indoor feedback would be the next step. A Tekmar TN2 or TN4 system would do a nice job of managing all that. Given that you have air as a backup, using it to take over after the slab gets a certain temp would be a good strategy. Control logic is the tricky part of all this.

MrPumpit

I worked up the heat loss calculations and came up with
36,364, based on bat insulation, not spray foam.
I’m certain I can get close to where I would like to be. Just a matter of finding the sweet spot. Thank you guys for all your help.

GroundUp

36k is likely pretty light for that space, but it's definitely not anywhere near enough for that size boiler. As far as the pump goes, the 26-99 is grossly oversized for a 9 loop system even on low speed, so all you're doing on high is wasting electricity. Turn it to low speed for sure. Then, I would suspect that your water temp is far too high and you're flywheeling with the thermal mass of the 6" slab. I have never once encountered a properly insulated building that requires more than 110* SWT even with the -40F temps we see here in MN every winter, so I'm betting you'd have quite a bit to gain by backing down the SWT to 110 or possibly even less on a design day (lowest temp/worst case) and using the ODR curve to back it down on warmer days. Also limit the firing rate to 50% as has been mentioned. Once that's done, I think you'd benefit from balancing the flow in your loops to increase flow to the sun room while decreasing flow to the rest of the house to try to balance the air temp instead of having a cold sun room. Maybe even add a new thermostat and zoning actuators if it's a big deal to you

Leon82

If you have the manual it will spec out the height difference for the intake and exhaust pipes. Mine was 12" from the centerline's if I recall. The intake was a downturn 90. The exhaust went up and out with 2 90s

MrPumpit

Thanks guys, My plumber originally installed taco series 0015 on all four manifolds, but I thought that it wasn’t enough for the large manifold having 2200’ of tubing,, 10’ of head, etc. After the change, there was a noticeable difference, most of the tiled areas were warm to the touch. I am taking your advice dropping the temp to 110* and putting the circulator on slow.
I followed the advice from the Burnham rep. by raising the exhaust by 3 ft., hopefully that with work.
I can’t thank you guys enough.

MrPumpit

Incidentally, the 36K heat loss is only for the upper level only (2400)Sq ft. The lower level is fine. Two zone radiant, below grade on 3 sides, 10’ concrete walls, R Rubber spray waterproofing, 2” rigid insulation.

GroundUp

The Taco 0015 was perfect. Your "10 ft of head" does not exist, as that lift is canceled out when the water returns back down. If the longest loop in your system is 250ft, your total head loss is only about 3ft at .5 GPM per loop.

archibald tuttle

so, if i'm reading this correctly you've got 2400 feet on oneish living level and additional 1800 feet partly below grade with a walkout? so presumable 600 sq. ft. is either over crawlspace or on grade?

I assume the heat that seldom goes on downstairs is also in a slab but on grade? and how much of what kind of insulation under? and that is similar on edges as to your indication of 1.5" open cell. open cell is funny choice for that setting but i digress or maybe that is your spray foam, is the 1.5" in addition to what depth in walls?

hmmm, 6" is a lot for a suspended slab. not that its that hard to support the dead weight but I'm happy with 3.5 is my typical where i use 2x4s on edge for screeds and often use 1.5" setting them the other way in troughs between more regularly placed screeds to which i attach the floor. which also is an issue for response or if there is some detail of wicking heat to outdoors that got missed and you essentially have better or equivalently insulated yourself from the floor with carpet and pad or . . . toward those ends you might take some exterior infrared shots to identify any problems. that technology is much more affordable either on a do it yourself basis or engaging a professional to help you assess.

In any event, i'm a little skeptical of depth of pipe issues, esp. in a suspended slab or really other than poorly insulated slab on ground because the physics is that eventually you get that heat unless failure of insulation design is wicking it away or 'storing' it under the house. if the thermostat had setback functions then the slower response might mean you are overrunning. i don't know the tekmar 561 and it might have learning functions to try to overcome that so maybe not so important; but the first thing i learned about radiant was throw away the thermostat.

In that vein I think the best thing you could do is throw away the thermostat and work over time to get the temperature response curve on the ODR to take care of the thermostat function. or employ the best brainiac features of an advanced thermostat that can sense rise rather than setpoint and maybe takes other inputs as well.

As far as loss goes, sounds like the glass is on the wrong end of the house for best sunny day gain. Do you have thermal blinds (or really anything is better than nothing, don't gotta spend through the nose) for nighttime or non-use during day on that space? All in all that doesn't sound like a really bad number for the cubic footage but you are honing in on a number of small improvements. (i may have missed it but you don't indicate your location/climate zone)

Short cycling definitely could be helped by buffer tank but smaller boiler with lower turndown can also help. Assume since this sounds recent that it is all barrier pex because otherwise you can use a DHW with heat exchanger to intermediate oxygen issue as well. That would encourage you to use lower temps in floor because you would have two deltas to worry about and keep the return temps in full condensing range.

Are you monitoring return temp. to the boiler. Depending on near boiler piping, if it is primary secondary or there is a bypass you want to insure that the boiler itself is not seeing returns that go much higher than 125 and the alogrithm for longer cycles for instance could lead to periods of higher return temp.

Toward that end, I never go above 6" spacing and often use 4" or less in my lower mass approaches. Pipe is the cheapest component in the system and more runs is good both to keep length down but also in case you every hole one by accident. Gonna be horse and barn door re that but just doing the math, 9 runs of 240 feet in your main 1800 sq. ft. zone is barely above 1 ft./sq. ft. whereas my rule of thumb is 2 or more. not to mention your average reinforcing mesh is 6" spacing so its easy frequency.

At the risk of getting buried under comments and then stoned to death, i'll say I have never seen better condensing than in a scorched air furnace where the return temp is low end room temp., e.g. 68 or 70. Still, according to theory, your setup should allow you to take full advantage of condensing boiler if you iron it out . . . bon chance.

Zman

That sure clears it up....

MrPumpit

Well said Archibald, Having been in the concrete since 1973, it’s second nature to me. Yes , 600sq ft of the 2400sq ft is slab on grade, with perimeter insulation 2” rigid both vertical and horizontal Rienforced w/ 6x6-4/4 sheeted mesh. Same scenario for the lower level slab. As for the structural arch upper floor, that slab has close to 10 tons of rein. steel in it. The pex is in the middle of all the slabs. As for the open cell 1-1/2” insulation, I was referring to the exterior sheets used for “Drivit” synthetic stucco. As far as the 6” slab being a lot for a suspended slab, unless it’s placed on Q deck, 6” is a minimum, depending on loads. Anyway, I think I’m going to lower the manifold water temp to 110* & keep an eye on the delta T and hopefully I will get a better result. Thanks guys.

TAG

I have a large room off the back of my house --- three side with a lot of glass -- cathedral ceiling. The floor is thick limestone over a thick wet-bed -- have it on continuous circulation linked to the boilers ODR. But -- it does require some tweaking. I use a floor thermostat so it's does not shut off. I can also supplement the floor with other heat .... I try to keep the floor at a steady output .... it will over heat on occasion when it's an odd day -- lots of sun.

I can also vary the water temp to that zone with a manual bypass -- it's normally dropping the ODR water temp a bit. But when it;s really cold I know to bump it a bit. If I stop the flow the slab cools and it takes too long to heat.