No sure if this post should go under Radiant or Geo forum.   Try here I guess

Have a 3T GeoTherm water to water system   (2 x 260' vertical wells drilled into granite) Works great and have had it for 3+ years.    My total Energy bill is a couple thousand less per year vs my old oil system.   I also pre-heat my DHW (40F well water)

I have a radiant slab controlled by a Invensys wall thermostat.   The system is in a Heating only area of Canada.    The thermostat was installed when the home was built  with the oil fired system.  At that time the water was mixed down to approx 100F.  My geo water is running at about the same temp (100-105).    I am wondering if there is a better way to control the temp in the room.   Right now the thermostat is set for 70F and kicks in at 67 and off course off at 70.  I have a 40 gallon buffer tank in place to prevent cycling on/off.   Is 3F a normal thing for radiant floors?  Is there a better thermostat system or recommendation.  Perhaps a floor sensor?  The lower end of the temp swing sometimes is a little uncomfortable.  (Don't want to go above the 70F cutoff)

To be honest, the floor in the shoulder season (like now) goes from warm to cold because the system is not on as much.  In the middle of winter, the floor is warmer as the system is on more.

Or do i leave alone and enjoy my system.   

Like I said perhaps I should have posted this query in the radiant section of the forum.   I can do that if people feel I may get a better exposure to people who have opinions/experience to share. 

Here's a recommendation:
Get a cheap USB Temperature Logger, or manually log, and capture some additional data from near the floor level in an area of the house that has the most complaints of cold floors.
My amatuer guess is that there is heat stratification caused from heat losses through the floors greater than through the walls and lid.
Is this a slab on grade?
Is there under slab insulation documented in the building's construction?
Is there foundation perimeter insulation? One fix would seem to include longer circ pump run times of more tempered water...

Thanks for the suggestion . Basement and main level have radient. The basement stays warm and we do not turn it on except on the coldest of days. Yes it has insulation under it and there is perimeter insulation. The main level is is buried in a lightweight cement and then tiled.

As for longer run times, I think I would need to have a diff setting on this thermostat. It is not customizable. I was thinking if there was a way to have the system put a min or 2 or warm watter into the floor just to remove the chill. Perhaps there is a controller out there for that

If you really wanted to go all out, I would up the size of the buffer tank to 80gal+ and I would put your floor heating on an outdoor reset so it followed the outdoor temps better. A room air t-stat is full of temp swings because it lags so long behind the floor, or vise versa. If your heat pump buffer tank is not doing any domestic hot water, I might tie the buffer to an outdoor sensor as well.

I'm most certainly NOT a radiant expert. Disclaimer over, 3 deg F is a heckuva swing. That would never fly with my clients, heating or cooling. Given that you have a buffer tank to prevent short cycling, why not find a way (different thermostat?) to clamp the temperature swings?

These systems are, after all, 'comfort systems' designed and installed to provide efficient comfort...there is no point in becoming uncomfortably aware of the system at the valley of every temperature swing.

Buffer tank should be 4 and 8 F temp swing, room thermostats should be no more than 1 F. An outdoor reset for the buffer tank should allow more efficient heat pump operation, longer run time of the zones and thus more even heat in the floors.

No need to increase the buffer tank size, most manufactures recommend 10 gal/ton, so you are good with 40 gal for a 3 ton heat pump.

Excellent. 2 more questions- Is setting up an outdoor reset easy and something I can do or do I need to find a "specialist". Perhaps there is a link to what I need and how? I will cruise this formum and google. 2nd question- any suggestions for a decent stat? Obvioulsy this one with a 3F swing is not suitable

Thx for all the help

The stats should be simple single stage, I like the honeywell single stage vision pro. Outdoor reset is simple, tekmar 260 or similar, wire and outdoor temp sensor, and one at the buffer tank. The rest is reading the instructions for setup.

Posted By docjenser on 13 Nov 2012 06:35 PM
Buffer tank should be 4 and 8 F temp swing, room thermostats should be no more than 1 F. An outdoor reset for the buffer tank should allow more efficient heat pump operation, longer run time of the zones and thus more even heat in the floors.

No need to increase the buffer tank size, most manufactures recommend 10 gal/ton, so you are good with 40 gal for a 3 ton heat pump.

That's interesting. The European manufacturers generally recommend 30-40L/kw buffer which in this case....10kw is roughly 3ton so the storage would be from 300-400L(80-100gal).

All our radiant slabs we use slab sensors.  We used to use Tekmar but have switched all controls and stats for radiant to HBX. 

Using a floor sensors, allows you to maintain a minimum temperature in the slab so there is no long recovery times.  You can set it to keep a minimum and control the air for those cold days the slab minimum is not enough.

 Even folks keep their floors just a tad warm in the summer to help keep things dry. 

Posted By MikeSolar on 16 Nov 2012 08:22 AM

Posted By docjenser on 13 Nov 2012 06:35 PM
Buffer tank should be 4 and 8 F temp swing, room thermostats should be no more than 1 F. An outdoor reset for the buffer tank should allow more efficient heat pump operation, longer run time of the zones and thus more even heat in the floors.

No need to increase the buffer tank size, most manufactures recommend 10 gal/ton, so you are good with 40 gal for a 3 ton heat pump.

That's interesting. The European manufacturers generally recommend 30-40L/kw buffer which in this case....10kw is roughly 3ton so the storage would be from 300-400L(80-100gal).

"The size of the buffer tank should be determined based upon the predominant use of the water-to-water equipment (heating or cooling). For heating, buffer tanks should be sized at one U.S. gallon per 1,000 Btuh [13 Liters per kW] of heating capacity at the maximum entering source water temperature (EST) and the minimum entering load water temperature (ELT), the point at which the water-to-water unit has the highest heating capacity, usually 50-70°F [10-21°C] EST and 80-90°F [26-32°C] ELT. For cooling, buffer tanks should be sized at one U.S. gallon per 1,000 Btuh [13 Liters per kW] of cooling capacity at the minimum EST and the maximum ELT, the point at which the water-to-water unit has the highest cooling capacity, usually 50-70°F [10-21°C] EST and 50-60°F [10-16°C] ELT. Select the size of the tank based upon the larger of the calculations (heating or cooling). The minimum buffer tank size is 40 U.S. gallons [150 Liters] for any system."

Climatemaster water-to water system design guide, Page 7 Europe is different, high mass buildings in radiant in concrete floors require larger thermal mass buffer tanks for quicker response time.

Posted By docjenser on 17 Nov 2012 01:15 AM

Posted By MikeSolar on 16 Nov 2012 08:22 AM

Posted By docjenser on 13 Nov 2012 06:35 PM
Buffer tank should be 4 and 8 F temp swing, room thermostats should be no more than 1 F. An outdoor reset for the buffer tank should allow more efficient heat pump operation, longer run time of the zones and thus more even heat in the floors.

No need to increase the buffer tank size, most manufactures recommend 10 gal/ton, so you are good with 40 gal for a 3 ton heat pump.

That's interesting. The European manufacturers generally recommend 30-40L/kw buffer which in this case....10kw is roughly 3ton so the storage would be from 300-400L(80-100gal).

"The size of the buffer tank should be determined based upon the predominant use of the water-to-water equipment (heating or cooling). For heating, buffer tanks should be sized at one U.S. gallon per 1,000 Btuh [13 Liters per kW] of heating capacity at the maximum entering source water temperature (EST) and the minimum entering load water temperature (ELT), the point at which the water-to-water unit has the highest heating capacity, usually 50-70°F [10-21°C] EST and 80-90°F [26-32°C] ELT. For cooling, buffer tanks should be sized at one U.S. gallon per 1,000 Btuh [13 Liters per kW] of cooling capacity at the minimum EST and the maximum ELT, the point at which the water-to-water unit has the highest cooling capacity, usually 50-70°F [10-21°C] EST and 50-60°F [10-16°C] ELT. Select the size of the tank based upon the larger of the calculations (heating or cooling). The minimum buffer tank size is 40 U.S. gallons [150 Liters] for any system."

Climatemaster water-to water system design guide, Page 7 Europe is different, high mass buildings in radiant in concrete floors require larger thermal mass buffer tanks for quicker response time.

The 30L is also recommended for the high temp Altherma type units that heat rads and is probably more important for ASHPs as it can also be used as the source of heat for defrost so heat is not taken from the space. Like expansion tanks, there is very little drawback to a larger tank (other than space and cost) as it can reduce the cycle time of the HP and that is the biggest issue. Allowing multiple zones is a benefit, but any size tank can do that.