Hydronic heat not as efficient as forced air ?

lucky

I have a newly constructed house in Minnesota with very good insulation. The home has 2 complete space heating systems. A Night&Day forced air furnace rated at 92% efficiency and an IBC combi condensing boiler for an in floor hydronic heating system rated at 94% efficiency. I have tested each once for a one week period with similar temperatures of mid thirty temps during the day and around 20 at night and reading the gas meter once each day at the same time and recording the readings. The boiler consistently used almost twice as much gas each day to heat the house as the forced air furnace. I had heard that hydronic heating was more efficient than forced air so was surprised to see that the boiler used so much more fuel than the furnace. I also recorded the electric readings each day and there was a slight reduction in electrical usage during the week that the boiler was running instead of the furnace but not enough reduction to compensate for all the extra gas that the boiler used compared to the furnace.

bob eck

Your radiant system is the pex tubing in concrete? If so that is not a true comparison. When heating up an concrete slab for radiant heat to heat the house it can take two to three days just to heat the concrete mass up so it can heat the house. Also when the radiant slab was installed was there proper insulation put down. If not there is a big heat loss into the ground first before heating the house.
Plus does the gas boiler heat your domestic hot water as an combi boiler or indirect water heater?

GroundUp

What he said^^^. Had the slab already been up to temp and provided there is at least 2" of foam underneath and around the perimeter of the slab, I find it hard to believe the radiant would take a drop more fuel than the scorched air while providing a more comfortable heat. Valid point with the DHW also. Was this boiler installed and tuned by a competent tech?

Gordy

You have to do your comparison at steady state when all mass has reached thermal equilibrium, and then just needs the bump to setpoint.

Also you need to compare one or the other during a week using degree days. Not switch back and forth daily.

Also which one is doing Domestic hot water?

Which one offers more comfort?

Gordy

Another point is the slab insulation detail.

Another point is Return temps to the boiler. If it's not condensing (below 130 return temps) it's only getting upper 80's efficiency.

KC_Jones

I would like to add, the OP is assuming the hydronic system was properly designed and set up.

What are your outdoor reset settings? Have you tuned them? Combi boilers most times are a compromise. To size for DHW load most times they are way oversized for the heating load, which will impact efficiency.

We would need a lot more information to even discuss this topic with respect to your system.

Gordy

I agree there is to little info to make an assumption. Combis are always oversized to do the DHW.

A properly set reset curve is huge, and really can't drive the system properly if it's switched back and forth.

With the type of system the op has it's hard to isolate the gas appliances as just boiler, furnace, or hot water heater running.

If the forced air is going the combi is still making hot water.

This is why I dislike combis, and prefer an indirect with a properly sized boiler for the heat load.

Jamie Hall

Suffice it to say that for a whole lot of reasons the comparison is completely invalid.

However, sadly, it's the kind of thing which gets published every day...

lucky

Yes, both the furnace and boiler were installed by the same professional HVAC company. They did install 2 inches of insulation under the PEX tubing before the concrete slab was poured. The slab was already up to temperature before I started the test comparison as I had been using the boiler to heat the house for several weeks. Then I turned the thermostat down for the boiler and turned the thermostat up for the furnace to make the furnace the heat source for the 2nd week of the test. The house is an 1850 foot rambler type house. The boiler is a modulating boiler from 20 to 125 so it can adjust the heat load as needed. I dont know what the outside reset is set at, the HVAC set that up but the water temp at the supply manifold is 125 and the return manifold temp runs between 100 and 110. The system pressure is 14 at cold and water supply valve for space heating is turned on if that makes any difference. I had previously measured total gas usage this summer when there was no space heating and it is very consistent so I knew how much to subtract from the total gas usage for the other gas appliances and the hot water. The other gas appliances use about .5 of one gas unit each day during the summer months with out supplying any space heat. When the furnace is running the daily gas usage is 2 gas units or in other words a 1.5 gas unit increase over the summer months. When the boiler is running it uses 3.5 gas units each day which is 3 gas units more than the summer months and double the 1.5 gas units used when the furnace is supplying the heat. I am thinking something is not set up correct for the boiler to be using that much more gas than the furnace. Hope that help answers many of your questions.

Rich_49

Something is certainly not set up properly . The radiant slab also gave up heat to the space during your test , at least for a good period of time if the house is well insulated and air sealed . How many zones is this radiant system .T there is alot that can go wrong and it does in probably 75% of the installs , usually starts with design or lack of it , the rest comes from not understanding building science .

Ironman

125* is too hot for a slab if the system is designed and installed correctly.

A lot more info is needed such as tube lengths and spacing, type of floor coverings, the actual calculated heat loss, system configuration, pump and pipe sizes,etc.

Over 40% of a slab's heat loss is through the perimeter; did they insulate the edge of the slab?

How about posting some pics of what you've got?

kcopp

Natural or LP gas?.
I am very familiar w/ that boiler... I have installed many.
LP Adjustments are simple but have to be set up w/ an analyzer. Outdoor reset is the control in the boiler that uses info from a sensor outside that will automatically compensate the boiler temp based on the outside temps. The sensor is light grey 2" in diameter and it does come w/ the boiler. Should be placed on the north side of the building and wired back to the boiler.

Gordy

In reading your last post as @Rich_49 pointed out. When you turned down the radiant thermostat, and turned on the forced air the radiant slab carried part of the load for the forced air.
That heated slab will release energy for a few days.

Not a valid comparison at all.

also the boiler has to see below 130 degree return temps. If it's piped primary secondary it's possible the boiler is seeing higher temps if not pumped right.

Gordy

Another point is solar gains, and wind. Using just outdoor temps is not the whole enchilada. I find it hard to believe the solar gain, and wind were identical over a 2 week period.

Jamie Hall

For starters here. You had the slab up to temperature and measured the gas use for a week. Then you turned the slab off and used the forced air for a week -- and are comparing the energy use. May I respectfully point out that that slab was still contributing heat to the building for at least a portion of the week for which the forced air was working. Probably, in fact, a good portion of the week. Thus you have a very significant undercount on the amount of heat the forced air was being asked to provide. Was it enough to account for the difference in fuel use? Quite possible. In order to get a handle on that, you would need to have measured the heat flow from the slab into the structure. At the very least, you need to have mean mass temperature of the slab at the beginning and end of the measuring period, as well as the mass of the slab. You could then make some assumptions regarding heat loss to the ground, convert the delta T over the week and the mass of the slab into BTU, and convert that into gas and add that to the gas used by the forced air furnace. This is pure thermodynamics, but without the information the apparent results are useless.

Then... we need still more information, which could go either way. What was the integrated total delta T -- not average -- between the inside of the structure and the outside for the two time periods? Integrated total infiltration? Integrated insolation? These factors need to be determined for the two periods and the results corrected for them.

Don't mistake me. I admire your efforts to compare the two heating systems. However, the experiment is nowhere near as simple as it would appear on the face of it to be, and again I will have to say that without the above corrections and information, the flat statement that the radiant floor uses twice as much gas -- while true within the frame which you have used -- is not a measure of anything useful regarding the relative efficiency of the two systems.

lucky

The boiler uses natural gas, I am not sure if the perimiter was insulated. This is my 1st experience with hydronic heat and I love the comfort level of it compared to forced air just surprised that the natural gas usage is so much higher which makes me think something is not set up correctly. I assume having the water supply valve for space heating turned on will not affect the efficiency in any way. Our digital water meter has a flow rate on it and it shows a 0.00 flow rate when the system is on which makes me think the system does not have a leak or it would be a very small leak that the water meter does not register.

Gordy

The radiant gets screwed any way you compare.

If you come off radiant and go to forced air the slab carries over btus as mentioned.

If you use forced air long enough for the slab to cool. Then go to radiant the slab has to work harder to get warmed back up.

A real comparison in comfort, and efficiency. Do a complete season comparison. One year with forced air, and one with radiant. I'm assuming the forced air is for ac. Or is it supplemental to the radiant.

Then I would use the degree day per sf formula as a base line to compare.

Jamie Hall

I think you will find, @lucky , if you read my commentary closely you may find out why the gas usage appears to be so much higher: to put it more simply, your slab was still heating the structure while you were using the forced air. The forced air was freeloading off the nice warm slab.

To put it into another perspective: I know of one house which is passive solar, and has a truly massive slab as the heat store (which is really what your slab is). That house will go a week with no solar input, in the winter, and only a 4 degree F drop in temperature. Wouldn't take much gas to bring that four degrees back...

You may also find that if you ever let the slab temperature drop for some reason that it takes a long time -- days -- for it to recover in temperature, and will use a lot of fuel to get there -- and then use very little to stay there.

None of this has anything at all to do with the relative efficiency of the two systems. Rather, it has to do with the different behavior of the two systems.

lucky

We have central AC so already had the ductwork, the builder added the forced air furnace to use in early fall and late spring when we do not need to heat up the slab for heat and also to supplement the heat in the coldest part of winter when temps can get down to 30 below in Minnesota.

hot_rod

I think the only realistic comparison study comparing two systems was done in Canada years ago. Two identical homes were built and compared over a period of time. But daily owner usage could skew numbers even then.

Gordy

I'm still,going to go back to a properly set up outdoor reset strategy. When the curve is properly dialed in the thermostat is nothing more than a high limit device. The reset is in the drivers seat. This could take a whole season to dial in the correctly.

Another is how the boiler is piped. Is it primary secondary, or direct piped?

If primary secondary, or using a hydraulic separator. If the system side is flow rate is not more than the boiler side the boiler may not be seeing condensing return temps as much.

Gordy

hot rod_7 said:

I think the only realistic comparison study comparing two systems was done in Canada years ago. Two identical homes were built and compared over a period of time. But daily owner usage could skew numbers even then.

Yes two identical houses next to each other can have dramatic differences from exterior variables also. Solar gain one could shade the other, and wind one may block the wind from another.

SuperJ

I have both in my house. I can't accurately determine the difference in cost. But I'm sure I spend a little on the boiler due to maintaining all my zones precisely at setpoint. With my furnace running off a single thermostat in my living room, many of my other areas of my house suffer especially in the shoulder season when the furnace doesn't cycle much.

I kinda see a furnace like the Toyota Corolla of heating. It's simple and efficient but doesn't bring much joy or pride of ownership. Sitting in my home office next to my micro zoned panel rad on a cool gloomy fall day (when the furnace would barely cycle) I'm thrilled with with my hydronic panel rads. It makes my house way more comfortable for 2/3rds of the year.

I love my wood stove too. Between my rads and wood stove I'm never thought I would love a heating system so much.

Zman

With a high mass slab like that, the slab will give several days of heat to the space while you are running forced air.

lucky

I understand that, but the boiler was still using all that gas each day when the slab was already warmed up for weeks before so the carryover affect of the slab would have been the same during both the radiant heat and furnace part of the test so I don't agree that is a factor in why the furnace used less gas than the boiler. There was carryover affect from the slab being warmed up for both the radiant heat and furnace part of the test.

Gordy

There's no way that a properly tuned condensing boiler, and radiant system should use double the gas of a forced air system solely for space heating.

Something is a miss in the radiant system design /setup, meter readings, or your calculations.

I would review heating degree days for the periods both appliances were in use also the winds, and solar influence each day. As I said before "similar" day, and night temps are not accurate enough.

I'm not defending radiant because of your statement. I'm about using accurate data, and base lines in the comparison. If your absolutely sure your radiant heating system is using double the energy something is deffinetly wrong, and it's not because it's a radiant heat system.

lucky

I agree that water is a much better transfer of heat vs air and agree that theoretically the boiler should be using less gas than the furnace, but my numbers are accurate. I am a professional cost accountant and very good at meausuring costs and usage etc, I have no training as an HVAC and so depend on your comments to lead me in the right direction in this matter. I also agree that I did not have the capabilities to meausure solar gain and wind speed each week etc so that may be part of it. I am not trying to say radiant heat is bad or worse than forced air, I am just trying to figure out why the furnace appears to be using significantly less fuel than the boiler to keep the house at the same temperature. Based on the comments here, I believe that my radiant heat system is not set up properly.

kcopp

post a picture of your set up please...

Jamie Hall

Whatever...

Solid_Fuel_Man

Hot air does have some advantages over hydronic systems. One is that there is no real setup, just set a unit in place and duct it relatively well and it's done. Cheap and easy. Condensing furnaces are well proven and efficient, unless the ductwork is really botched.

Hydronics have some advantages over hot air too. Long run times, superior comfort, small diameter pipes carry heat rather that large ducts etc.

Above are just two points and not intended to be at all conclusive.

That said hydronics have really two main disadvantages over air: water can freeze, setup and design is more involved (read more expensive upfront).

A few pictures and more information about the systems is needed. Also as has been said, a much longer term comparison is needed for all the reasons stated.

hot_rod

I believe it is possible for the radiant slab to use more energy, compared to forced air.

My bet would be a massive edge loss. Get an infrared camera for a few hours and you may find where all that heat energy is going.

I see slab shops with snow melted back several feet from the building due to poor or no edge insulation. That edge should be at least 2" and down a few feet, depending on your climate, and tight against the slab and sealed so rain or run off doesn't wash away the heat.

Covering the raw edge of the foam is another detail as it needs to be to the top of the slab, no exposed concrete at all. As Bob mentioned this could account for 40% of the load.

The entire slab needs to be encased in a foam block to be an efficient radiant slab. I've made this mistake myself and have witnessed the result and payback

Run time on the boiler would show some results also, many of the new mod cons have that data logging ability. If it firing, the heat is going somewhere?

Ironman

Though it's been said numerous times, I don't think you're grasping the significance of what we call the "flywheel effect" with a radiant slab. The boiler will pump heat into the gigantic mass of concrete for days until it reaches its thermal equilibrium. Conversely, it will continue to give off that heat for days. If you run the radiant for a week and turn the FA on on Monday, the slab could still be contributing heat that the boiler made on Thursday.

Forced air has no mass because it only heats the air. It's contributing virtually nothing when the boiler is turned on.

You may be good with crunching numbers, but you're missing the necessary parameters for comparing FA to radiant in your test.

As has been stated, the only accurate method would be to run each separately for a season and compare both to the actual heating degree days for each year.

And as mentioned, there could be numerous issues with the design and installation of your radiant system. We're glad to help determine that if you'll provide some pics and info.

lucky

Picture of boiler set up, 6 zones controlled by one thermostat.

I agree that during the test the furnace had the benefit of left over radiant heat, but so did the week that the radiant heat was on as the radiant heat had been on 3 weeks prior to the test so any carry over affect would have also benefitted the week I was measuring the gas usage of the boiler. They both had the same benefit of left over radiant heat from prior days so that negates that as a reason why the furnace used less gas than the boiler.

TheKeymaster

That makes no sense. Yes they both had the same radiant heat added because of the slab from the week before. What your missing is that the gas that paid to heat up that slab each week was paid for by the hydronic system week. In your forced air test week some of the heat output was paid for before. You can't say the hydronic system had the same benefit because that is how it works. The forced air system works differently. You can't count the extra slab heat in the forced air as the same because that system doesn't heat the slab at all so it would never get that benefit. They work differently so you can't mix parts.

Jamie Hall

It occurs to me that there is another way to make a test. Won't be really accurate, but will be much better than what has been described so far (which, accurate cost accounting or not, is -- bluntly -- garbage). Let the slab cool down completely. That is to say, close the house for a couple of weeks (it will take that long), running neither heating system. Then fire up the forced air system and measure its usage. Correct for the items which we have mentioned previously, and compare that to a period when only the slab was running but was already up to temperature -- that is, had been running for at least two weeks previously to starting the measurements.

kcopp

Piped wrong... The boiler loop needs to be 1" minimum. Boiler is prob short cycling.

Gordy

kcopp said:

Piped wrong... The boiler loop needs to be 1" minimum. Boiler is prob short cycling.

This is @kcopp go to boiler.

Gordy

Well I hate to beat a dead horse, but there is a diffence on how forced air heats a structure, and how radiant does.

Forced air heats the air in the structure which in turn residually heats the (objects) mass of the structure.

Radiant heat heats the objects (mass) in the structure first, and the air is residually heated from the mass.

Forced air has benefits as to quick fake comfort recovery over radiant coming out of setback. I use fake because while the air temp is warmer the objects are not which in turn pulls heat off the human body. Creating discomfort.

I’m not going to debate efficiency anymore until a better testing platform is used, and the boiler piped properly.

I can tell you if forced air was solely used at the start of the season, and there is no perimeter insulation your concrete floors would be miserably cool.

If you want to check my statement run the forced air solely for a month in say January. Take a IR thermometer, and shoot the slab at the wall floor detail around the inside perimeter. I wouldn’t be shocked if it were mid 50’s. That cold will conduct into the room through the slab.

I will also state with similar efficiency appliances (forced air, and boiler) there is no way that the gas usage could be double for one over the other. You are dealing with a combi so it must be tracked for dhw usage, and that separated.. I really don’t know how you can accurately do that other than your summer usage pattern.

One other thing. Besides the piping for the boiler there are other settings that need to be properly setup to get the most efficiency out of that boiler @kcopp can direct you on that.

Is the boiler producing ample amounts of condensate? Running radiant floor temps it should be condensing a lot with the return temps a radiant floor can produce.

Brewbeer

I converted my forced air house to hot water baseboard in 2015. Both the furnace and the boiler were/are high-efficiency units. With several years of data before and after, there isn't an appreciable difference in the amount of natural gas consumed, whether forced air or HWBB.

The BIG difference is in the comfort level.

And I'm happy to pay more for a heating system that is noiseless, consistent, even, and draft-free. There is nothing more annoying than having to turn up the TV when the heat comes on.

GroundUp

Boiler is piped wrong and water is too hot, both leading to poor efficiency. If there is no edge insulation, a good portion of your heat is ending up outside. Guaranteed the slab and objects in the house were warmer with the boiler running versus the forced air week, moreso toward the end of it when the slab was cooled. This is another case of a "contractor" who doesn't read the damn manual, and looks eerily similar to another one I repaired earlier this season from a big name "contractor" out of the North Branch/ Cambridge area. Same guy by any chance?

Ironman

At 1850 sq. ft. in MN, your tubing should have been space at 8" O.C. maximum. That would mean you should have had 2775 feet of tubing in the slab. The maximum loop length should not have exceeded 300' ( 250' is a better design number). 2775 / 6 loops = 462.5' each. Either your loops are way too long or your tube spacing is too wide.

That error is gonna make the slab take longer to heat up and also have less output. That's probably why they cranked the SWT up to 125* when a properly designed system would only require 100*, maybe 105*.

You haven't stated what type of floor coverings you have, but if it's carpet and pad, that will also lessen the output.

So, you have a boiler that's not piped sufficiently, either too little or too long loops, the perimeter possibly not insulated, maybe no ODR, maybe other unknown issues, and a slab that's emitting heat for days after the boiler's turned off and you think your test was accurate??