Hydronic/Radiant Floor Propane Usage

mjp317

Hey everyone, I’ve been searching through these threads trying to find an answer to my problem for months but haven’t found a solution.

Here’s my predicament.. I have a hydronic radiant system that was installed last year. Since then I have been using an astronomical amount of propane to run the system. It’s warm and seems to be running appropriately but I’m going broke trying to run it.

This winter, I’ve been averaging about about 10 gallons per day or 300 gallons per month of propane. At $2.30 a gallon, I’m close to $700 to heat my house.

Can anyone give me any insight to the potential problem? 

Here are the specs:

- 4000 sq ft of heated concrete slab (including garage)
- Navien NCB-240 Combi Boiler 

- Single zone system with 13 loops
- 1/6 HP B&G Circulator Pump
- 1/2” PEX 250-300’ loops
- each loop running between .3-.5 GPM

Is my Combi undersized to handle the needed flow rate? I’m not able to get any more flow than .5 GPM without my boiler reducing the temp drastically. Delta T has been ~40 difference at a 120 degree supply temp.

Any help is appreciated.. Thanks in advance!

AdamInEvergreen

Getting an idea of your expected heating load may help. To that end;
1) What is your insulation quality in the house / build year?
2) What is the thermostat set at?
3) Approximate location (zip code, town, etc)

Jon_blaney

If you are not cold then it is not a question of boiler size. What is the insulation details of the slab? Do you need to heat your garage like it is your livingroom?

EdTheHeaterMan

Was the concrete slab insulated?

If not, then you are not just heating the home. You are also heating the ground below the floor. If that ground has a high water table, then the water is heated by the ground around it. As the water passes thru the ground, that water is taking the heat away with it.

I know this because I went on a service call to a home in West Wildwood NJ several years ago. The boiler was heating all the radiators quite nicely until flood waters reached the supply mains in the crawlspace. Once the water reached those pipes there was no heat. That's because the freezing cold flood water took all the heat the boiler could make. There was no heat left for the radiators. Once the flood waters receded below the main pipe level, the Radiators magically resumed heating the home.

Do you think , maybe the cold water that was in direct contact with the pipes, was drawing all the heat from the pipes?

hot_rod

just some guessing if the 4000 sq ft is at a coldest, design day you could burn 1 gallon per hour.

An infrared camera looking at the outside would show any big loss areas.

Does the boiler cycle on and off a lot? All the parameters set properly, outdoor reset being used?

mjp317

Wow lots of quick responses.. much appreciated!

The home is a new, built in 2020 and my house and garage are well insulated with 3-5 inches of spray foam in walls/ceilings. 

Under the slab, the installer put 1/2” foam under with 2” around the perimeter edges. Which he said was “code” for my area in middle TN. I am now skeptical of that.

The boiler runs about 2-3 times per day for an average of 2-3 hours per cycle. Single interior thermostat set to 69. Keeps the garage at around 60.

There shouldn’t be any water contact with the pipes. As there was a vapor barrier/insulation installed underneath the piping.

My inclination is that the boiler is always trying to “catch up” to the water flow demand. Causing it to run at a high fire constantly. 13 loops seems like a lot for the Combi boiler to handle at once. But I’m not a pro. 

Attached is a pic my set up. Maybe you guys can see something wrong here?

Hot_water_fan

Seems about average, maybe slightly over average consumption: 300 gallons per month, maybe 20 of which are DHW. That’s 280 in, probably 266 out. Total average heating about 33,000 Btu/hour. If you supply fill dates and amounts and the closest airport (for weather data) we can do a fuel usage heat loss. 

40 degree Delta T is pretty high for a radiant floor. Are you using setbacks? Usually floors are about 10 DT. 

Potential solutions to explore: Increase insulation and air sealing to reduce heat loss, taking a close look at the garage in particular. This probably includes some very easy and very difficult measures. Also, you can explore other fuels: propane is expensive, as you can see. How much is electricity where you are? Zoning would also let you reduce temps in some places. 

 

Are you referring to domestic hot water here?

I’m not able to get any more flow than .5 GPM without my boiler reducing the temp drastically.

mjp317

I’m in Clarksville, TN, so mostly mild winters here. No setback on the thermostat. The house should be sealed very well. Spray foam and new energy star windows/doors throughout.

As far as the flow rate. Whenever I increase the GPM to greater than .5 GPM/loop, the supply temperature cannot reach its set point.

For example, if I try to run them at .75 GPM, the Combi boiler will only pump out supply temps of 100-105 when set at 120.

Jamie Hall

Actually, with that little insulation in the floor, you are probably doing about as well as can be expected. Since the boiler is not running continuously, your fuel usage is determined almost entirely by the heating load.

AdamInEvergreen

I'm a little confused about that pressure release valve (top of the boiler, just to the left of the intake & exhaust vents). Is that going in to a pex line with two other connections just below the boiler then draining off to the left of the image? What are the other two connections (condensate drain?)?

Have you ever noticed that PRV activating? Should make some noise and maybe shake around a little bit. It is abnormal for them to go off, and if they go off with any regularity it is indicative of a problem with the system. For that reason the manufacturer (and perhaps code) instructs they not be tied directly in to a drain line do instead terminate like 4" above the floor so the homeowner can notice when it goes off. I'm getting way beyond myself here but I think it is theoretically possible when you increase the flow rate the system is dumping through the PRV, ejecting the heated water and refilling with make up water which is why it seems like you never get up to temp with the higher flow rate.

You also might want to install a condensate neutralizer.

Hot_water_fan

For example, if I try to run them at .75 GPM, the Combi boiler will only pump out supply temps of 100-105 when set at 120.

It would be a tragedy if a two year old house in your climate had a heat loss > 30btu/sqft, so hopefully it's not that. That sub-slab insulation is lacking however. That being said, radiant floors should have low supply temps as it is, so that's actually preferred.

The boiler runs about 2-3 times per day for an average of 2-3 hours per cycle.

Can you supply a propane fill date (and when the last fillup was) and amount? Getting the Gallons/Heating Degree Day is key. It sounds like the boiler is running max 9 hours a day, but we don't know if that's on the average day or coldest day. It should instead be running almost nonstop at a lower fire.

Nonetheless, you don't have much room for improvement unfortunately with regards to the boiler. You'd ideally have it running slightly lower temps for longer run times. In terms of the sub-slab insulation, that's a bit of a lost cause unfortunately. R-10 is usually what you'd want, so you're losing 3x more heat than that. Lowering supply temp will cut down on the sub-slab heat loss too. That leaves fuel switching, which might work for your situation, might not.

mjp317

Latest fill up was 236 gallons with fill up 3 weeks prior. 9 hours max would be close, but only on the rarest sub 15 degree temp days. Average is about 5-6 hours of run time with a supply temp of 130.

maybe I should try running at 100-110 to introduce longer run times and low fire?

yes it’s a shame on insulation.. knowing what I know now, it’s too late. 

hot_rod

So 236 gallons will go 21 days, or about 11 gallons a day? Assume the cold nights could have the boiler running non stop to heat the home.

Your best shot at increasing fuel efficiency would be to run the lowest possible supply that will get the job done.

Some boilers have features like ramp delay which keeps the boiler at low firing for a set period of time.
Also anti cycling function that prevents the boiler from firing after shut down for the period of time you set., preventing inefficient short cycling on milder days.

Boost function is another nice feature that allows the boiler to ramp up if not meeting the load for a time you set.
See what functions the model has and try to use as many as possible to squeeze as much out of the fuel as possible.

EternalNoob

Regarding sub-slab insulation, there is something of a long-standing debate here. Traditional and logically-intuitive view is to always insulate at least 2" foam below the entire slab so you don't loose heat to the earth below. Somewhat more progressive (and non-intuitive) view is to only insulate the perimeter foundation and below the perimeter 2 feet of slab, allowing the earth below the center of the slab to build up a heat gradient. This gradient will take time to "fill" with heat, but once reaching a steady-state, ultimately the gradient itself provides an R-value the same as what the insulation would achieve.

Practically infinite thermal mass makes the system very unresponsive, but extra thermal mass can be advantageous in moderate climates, taking advantage of passive solar gain, or using solar thermal. I think it was the either the NREL, or maybe the lawrence berkeley labs' building-science department that did a case study on the subject and that is what determined the the building code requirement for perimeter foam only. I think the physics may break down or make less sense in much colder climates, and also with excessive ground-water flow, where the heat gradient becomes disrupted, but i know that 's what the code calls for here in California (zone 3) and we have a pretty stringent energy efficiency protocol with the Title-24 program. I have seen the seen the perimeter-only slab insulation work quite well here, particularly on a project with a massive solar thermal system where the extra ST energy could be stored in the slab mass.

In any case, just sayin i wouldn't jump to conclusion blaming the problem on lack of slab foam. Not doing any math but judging from instinct, i think if your 2020-built house in Tennessee is chugging 30kbtu/hr then it probably isn't the building envelope but something going on with your boiler. I don't know what your building department situation is like over there but insulation is always inspected in new construction here, and there's no way a builder can really get away with skimping on it.

Hot_water_fan

Latest fill up was 236 gallons with fill up 3 weeks prior. 

Do you have exact dates? Basically we want to see gallons/ Heating degree days, so we have numerator (236) but not the denominator exactly. 

If you’re reasonably sure about the 9 hours at 15 degrees, that means the boiler is about 2.5-3x too big for heating (but right sized for DHW) which is usually the case. The ideal is that the boiler spends the most time at the lowest supply temperatures and the lowest modulation, so I wouldn’t be surprised if it ran for weeks at a time once it’s dialed in. There’s a lot of boiler settings that work with you and against you when trying to reach the ideal. One easy option is lowering the temperature. Next would be throttling the boiler back (heating side only) - you’d be in effect electronically turning it from a 120kbtu to 60kbtu boiler. You might have to disable/dial back a boost function if it has one of those. Likewise, if you have outdoor reset, you can use that to further match supply temperatures to outdoor temperatures. Most boilers come with a preset high mass radiation curve, which would fit your situation perfectly if the outdoor reset is enabled (sometimes installers don’t install it). There’s a great chance that with your square footage and insulation (in the walls at least) that the slab temperature will end up being 80 on the coldest days and cooler the rest of the time. That’ll about max out the boiler and system efficiency, so after that you can zone and/or look into fuel switching (assuming you have central air, a heat pump would probably be the cheapest heat source when used to supplement the floor). 

Jamie Hall

On under slab insulation. The concept of heating up the ground underneath has its points. However, if there is any ground water at all (something California doesn't have much of, but anywhere east of the Mississippi surely does) it doesn't apply at all. Consider. We around here use water to move heat from our boilers to our buildings. Mother Nature does exactly the same thing, and she will quite happily move heat from under your house to somewhere else

Ground water moves, folks. That's why wells work. That's why septic systems work. And it will hold the bottom of the insulation under the house at groundwater temperature indefinitely -- and that is typically no more than 40 to 50 F. The R value of that foam is around 3. You easily be losing upwards of 20,000 BTUh through that slab -- all the time, year round -- if the ground water is anywhere near (within two to three feet) the slab.

hot_rod

EternalNoob said:

Regarding sub-slab insulation, there is something of a long-standing debate here. Traditional and logically-intuitive view is to always insulate at least 2" foam below the entire slab so you don't loose heat to the earth below. Somewhat more progressive (and non-intuitive) view is to only insulate the perimeter foundation and below the perimeter 2 feet of slab, allowing the earth below the center of the slab to build up a heat gradient. This gradient will take time to "fill" with heat, but once reaching a steady-state, ultimately the gradient itself provides an R-value the same as what the insulation would achieve. Practically infinite thermal mass makes the system very unresponsive, but extra thermal mass can be advantageous in moderate climates, taking advantage of passive solar gain, or using solar thermal. I think it was the either the NREL, or maybe the lawrence berkeley labs' building-science department that did a case study on the subject and that is what determined the the building code requirement for perimeter foam only. I think the physics may break down or make less sense in much colder climates, and also with excessive ground-water flow, where the heat gradient becomes disrupted, but i know that 's what the code calls for here in California (zone 3) and we have a pretty stringent energy efficiency protocol with the Title-24 program. I have seen the seen the perimeter-only slab insulation work quite well here, particularly on a project with a massive solar thermal system where the extra ST energy could be stored in the slab mass. In any case, just sayin i wouldn't jump to conclusion blaming the problem on lack of slab foam. Not doing any math but judging from instinct, i think if your 2020-built house in Tennessee is chugging 30kbtu/hr then it probably isn't the building envelope but something going on with your boiler. I don't know what your building department situation is like over there but insulation is always inspected in new construction here, and there's no way a builder can really get away with skimping on it.

Heat will continue to travel into the ground until the soil temperature at some depth below the heated slab is the same temperature as the slab,  At what depth would the ground be, say 80F or more?

No amount of insulation can stop the movement from a warmer surface to a colder, basic thermodynamics. It could be argued a large diameter gravel bed below the slab would have less conduction transfer, I suppose.

There we’re a number of tube manufacturers promoting perimeter only insulation back in the 90’s. That doesn’t seem to be the case anymore,

Maybe they had to pay the fuel costs on those uninsulated slabs😗

Even moving the tube from the bottom of the slab to mid point reduces downward losses.

All this heat transfer in heated slabs has been modeled with FEA, and sensored in actual operating conditions, no need to guess or imagine heat transfer

I think the healthy heating.com site has some of those models.

The code in areas of western Canada required 4” under slab for a short while. The $$ was scaring folks away from radiant slabs so they backed off to 2”

mjp317

Thank you everyone for the insight. Many well educated responses that help a lot. 

I was off on my fill up dates. It was 4 weeks. Dec 14th - Jan 15 used 236 gallons.

We had 1-2 weeks in that time frame of highs in the 60-70s and plenty of weeks with lows in the 20s

Zman

More insulation would have been nice, but you need to optimize what you have.
I would recommend increasing the flow rates so you get a minimum delta of 20 degrees, then set the boiler up with outdoor reset so the boiler target temp will vary and will generally be lower.

When you lower the flow to the floor your heat gets less even. It seems like you are helping the boiler satisfy, but you are really just tricking the sensors.

Hot_water_fan

I would recommend increasing the flow rates so you get a minimum delta of 20 degrees

I was confused by this at first but I think the OP meant the initial delta T was 40, I’d expect it closes over the course of the call.  If the 120kbtu boiler is only running 1/3 ish of the time, the steady state delta T can’t be 40 at .5 GPM per loop.  

Dec 14th - Jan 15 used 236 gallons.

That implies a heat loss of 64k btu or so. Not too bad for a house that size! 

TAG

I use that about the same new 300g for my 4000+sf in PA ....+ 1500sf SIP studio. Propane is expensive. NG has gone up .... but my place in NJ is about the same SF and it costs 1/4 the amount using natural gas and the boiler is CI. PA house is newer with all foam. It eats through about 1500g for the 6 months that captures all the heating season.