Is there a btu difference when heating a radiant slab with propane versus geo?

I have not realized the expected savings from geo and both my installer and manufacturer have tried to figure out why. They postulate that the radiant floor in the basement may the culprit.

They point out two reasons for this:
1. not enough tubing in the slab
2. not enough insulation in the slab (this is obviously conjecture as no one has drilled through the slab.)

My confusion with this theory is that when we were heating with propane, the basement was always the warmest room in the house in winter. With geo it still is.

If there is not enough insulation in the slab, why would that affect the geo savings? By this I mean, it was the same slab that presumably was sucking out energy with propane so wouldn't the same BTUs be required to heat our slab with geo at a much more efficient COP? So basically I would love feedback on this theory and my rationale (or lack of it!)

I think the forum is a fantastic resource and I'd love to join the bandwagon of happy geo customers so any help would be appreciated.

 

A Btu is a Btu... It does not matter HOW they are made, only how efficiently they are made.

Bergy

 

The slab has no idea where the Btus came from or how they were made.

 

thanks. just as I suspected.

 

One begs to ask what water temperature you were delivering to the slab with propane.
Operation of Geo much above 120F for slab heating will drop efficiently dramatically.

 

I'm with TeeTech.
It is likely that your water temp requirements are on the edge of the geo's capabilities creating low COPs or activating auxiliary.
Repair may be as simple as adding a few baseboards, but more info is needed.
J

 

thanks guys. Sorry, I have no idea what temp the propane was delivered before, but are you saying that heating a radiant slab with geo is less effective than with propane?

 

No, the slab doesn't care what the heat source is. The difference is that propane boiler efficiency is relatively insensitive to the water temp - maybe a few percent difference between 90F water and 120F water. Heat pump efficiency depends very strongly on water temp - it may be less then 1/2 as efficient at 120F than it would be at 90F for example. If the slab is not adequately designed for geo temperatures the heat pump may be operating relatively inefficiently (but still hopefully cheaper than propane unless your electricity is really expensive).

If the problem is a lack of insulation under the slab the geo should improve things, not make them worse since it should be running at a lower temperature - therefore less heat lost to the ground.

If you were upgrading an existing propane system there is no excuse for the installer not to have determined the water temp required beforehand and figured that into his performance estimates.

 

Just curious, What temp is the water going to the floor now w/Geo?

Buffer Tank? What size?

Chris

 

Chris?

Does your system have a mixing valve?

I am hearing a lot about non-mixed down radiant floors here of late.

 

No mixing valve, I thought only high temp gas fired systems had mixing valves.
Chris

 

Chris,

EWT 105.5F

80 gallon tank.

thanks.

 

Mark,

I don't know. I'd love to here more about this or how I could find out if I have one.

 

Joe,

Wouldn't adding a few baseboards as a solution throw out the efficiency that I'm trying to get with geo or are you suggesting that the baseboards are more of a top up? After one winter, my installer already switched out a WF Envision and installed a Synergy 3D (at my expense obviously) to better handle the radiant floor needs.

thanks.

 

High op temp requirements blow out efficiency. More radiation/delivery vehicles permit you to operate at lower temps.
j

 

Joe,

I don't understand what "blow-out efficiency" is. Are you suggesting another "delivery vehicle" because of my slab configuration (albeit on the limited information that I provided) or that most radiant floors would benefit from electric heater top-ups?

I only ask because as stated in the original post, I'm not seeing any savings with geo and yet others have weighed in that a BTU is a BTU. Sorry to be so dense about this, I'm just trying to figure out my best course forward.

Many thanks.

 

A mixing valve takes cold water coming back from the heat emitter and mixes it with warm water from the heat source to mix down the temperature of the water going out to the emitter.

The more tubing in the emitter, the lower the water temperature needs to be to provide comfort. I tend to design radiant delivery systems for water in the 90* area.

A mixing valve can be three way, (looks like a T) or four way, (looks like an X). They can be manual or motorized.

 

Emitter= Slab floor??!!?

Maybe I am missing something, but my "mixing valve" would be my buffer tank. Cold water returns to buffer tank from floor(emitter) via cold water inlet on top of buffer tank. The water in tank is being heated to 90*-95*F max, sensor against tank, under 3" insulation. Booster pump circulates water from bottom of tank through exchanger, back in through temp&pressure valve hole. "Hot" water to floor via hot outlet in top of tank.

ManInCold, What Joe is saying is that to get a heat pump to heat water to above 100*F you lose a lot of the efficiency. It can heat water to 90* quite easily and that should be warm enough to heat up the floor. cnygeo2 said it well "1/2 as efficient at making 120* as making 90*.

Chris

 

I think and type for while then POOF

It all goes away

Chris, I design to have the lowest possible temps hitting the heat emitter. If my customer wants 72* air temps then I am sending maybe 85* water to the emitter.

I design for a delta T of 30* on my emitters and usually wind up at between 20 and 10* delta T. I am new at this, and fifteen years of radiant and thirty plus in geo is not enough to have any system turn out as I planned. I continue to live and learn which is why I type here. I always learn more than I give.

A water to water heat pump can make 130* water all day long being feed a diet of 39* EWT. I know this from 5 years of fooling around in the pond in Valley City, OH. So how do we use this 130* water?

I design my radiant panels to use the very lowest EWT possible. I feed the heat pump output into a buffering tank. The heat pump runs until the tank is full of the 130* water. I feed the radiant emitter from the tank, when the room air temp t-stat calls for heat. A pump, on the "mixed" port of my mixing valve, starts when there is a call for heat, because the room air temp stat sees 71*

The room stat wants to see 72* so the pump starts pulling mixed water from the mixing valve at 85* to the floor. To make that 85* water the valve takes 71* water from the floor and mixes it with the 130* water in from the buffering tank.

Timing is every thing so this all goes on until the room stat is happily seeing 72* and shuts off the pump. How long this takes depends on the enviorement.

 

Whoa - that sounds bad for COP!

Designing for emitters needing only 85-90 F water is most excellent - but then why throw that away by heating the buffer to 130? I'm not going to dig through a spec table for this post, but I suspect there is a COP difference of 1.5 to 2.0 for a unit producing warm water at 90 vs hot water at 130 using source water at 39.

Refrigerant COP depends highly on lift - why force temps up to 130 only to mix them down to 90?

A quick glance at a 410a PT chart reveals a 200 psi difference between saturation at 90 vs 130 (I realize that's a simplification - refrigerant has to be hotter than load water for heat transfer to occur, but the principle remains the same) A 200 psi change in compressor discharge pressure translates to a huge difference in both power and refrigerant flow rate.

What am I missing here?