Thread: In Floor and Baseboard

Some houses have a mix of in floor radiant on the bottom floor, and baseboard radiators on the second floor. I was taught that the in floor in the concrete slab runs at a lower temperature than baseboard, because of the way the heat is distributed and received.

This is usually done with a tempering valve on the in floor zones to lower the temperature. I have also seen houses that did not use any tempering valve. They just ran the boiler at about 160 degrees or so, and used the same temperature for both. To be honest, it didn't fell all that different, although I didn't walk around barefoot.

So, why would the temperature need to be that much lower in the slab? If the thermostat is in use, the zone would call for heat until the t-stat opened the circuit. Is it because the floor may become too hot in some places to walk on comfortably in bare feet? In short, what is the down side to not installing a tempering valve.

The other aspect is would the boiler not work more efficiently if it was designed to be at 180 instead of 160, or even lower in some cases?

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The depth of the tubing in the concrete has a direct bearing on the design water temp. A 2" depth may requirea 115 degree temp while a 6" depth may require a 180 degree temp. When mixing systems such as baseboard, radiant, and probably domestic hot water the highest design temp determines the boiler setting. Everything else must be mixed or tempered.

It is very unlikely that you will see radiant heat 6in under the floor surface.Not very efficient.
There will be a mixing valve in a modern radiant heat floor system.The temp coming out of the boiler will be same as the baseboard heat but the mixing valve allows for seasonal changes in the radiant heat.The mixing valve controls the amount of hot water that circulates through the tubing, just like a faucet.

Originally Posted by Michael Garrity

It is very unlikely that you will see radiant heat 6in under the floor surface.Not very efficient.
There will be a mixing valve in a modern radiant heat floor system.The temp coming out of the boiler will be same as the baseboard heat but the mixing valve allows for seasonal changes in the radiant heat.The mixing valve controls the amount of hot water that circulates through the tubing, just like a faucet.

The 6" depth I gave was just as an example. Obviously it would be less efficient at that depth.

Well, that's pretty much what I said. The question is what is the down side of not having a tempering valve for the in slab zones, as well as what effect operating the boiler at a lower than normal temperature has on the efficiency of the boiler.

Location,location.If you live in a mild climate and you keep the boiler temp lower and have no mixing valve on the radiant heat then there is probably no downside because your house does not require much heat.If you live in the Northeast you want your heat hot and fast in the winter.In cold climates use a mixing valve for radiant heat{seasonal use]Boiler efficiently again is about location and need.How long will it take to heat the house when the boiler is set at 180 as opposed to 160 or less?Depends on your boiler,rooms size,amount of zones.Is it being used for domestic hot water?Having more than one zone is the most efficient way of heating your home.

There is NEVER a need to run 180 deg water in the slab unless you are trying to dry meat, regardless of the depth. Your floor temp should not be seeing water temps above 105° F. There is a very real potential for the concrete floor to reach and hold the same temperature as the water. In this case that could be very dangerous not to mention the overheating concerns.

Baseboard radiation requires a minimum temp of 140° F to do any real work.

NEVER. If I ran my radiant heat at 105 my house would be freezing.My boiler runs at 180 and it is just nice and warm.My 460sft kitchen stays at 68-70deg.Now in the spring and fall I use the mixing valve to adjust the amount of hot water that flows through the pipes for the radiant heat.105 deg, I don't think so.

Originally Posted by Rod Butler

There is NEVER a need to run 180 deg water in the slab unless you are trying to dry meat, regardless of the depth. Your floor temp should not be seeing water temps above 105° F. There is a very real potential for the concrete floor to reach and hold the same temperature as the water. In this case that could be very dangerous not to mention the overheating concerns.

Baseboard radiation requires a minimum temp of 140° F to do any real work.

I have learned to never say never. Every installation has its own requirments, the amount of tubing in the slab, whether or not there is good insulation and vapor barrier, design temps. There are also slab sensors that can be installed to prevent overheating of the concrete. But to just proclaim a system shall never need 180 degree water is close minded.

Originally Posted by David Bell

I have learned to never say never. Every installation has its own requirments, the amount of tubing in the slab, whether or not there is good insulation and vapor barrier, design temps. There are also slab sensors that can be installed to prevent overheating of the concrete. But to just proclaim a system shall never need 180 degree water is close minded.

David, I agree partially, particularly about the never say never part. I can hear my dad saying that in the back of my mind.


I guess I should add that a properly designed system should never need to see temps above 105° in the floor slab, but I stick by my comment that an in floor system should NEVER need to run at a 180°.

Agreed that there are slab sensors that are designed to prevent overheating of the concrete but if you have those sensors installed are you not limiting the water temp anyway? In reality walking on a floor over 90° is very uncomfortable. The amount of heat released at that temperature will easily heat a home and is more effective at limiting over heating cycles.

Back to Jim's question about a boiler operating more efficiently at (higher temps). That depends on the boiler, modern high efficiency boilers LIKE to see low inlet temperatures, the lower the better for higher efficiency. The higher the water delta T the lower the flow.

Bottom line is that if you are looking at a system that is delivering that high of a temperature to the floor slab, it is worth noting to the home owner the potential risks and in my opinion the short comings of the system.

Table of heating water temperatures needed with radiant tubing at different depths in the concrete slab

Table of Insulation Material PropertiesAverage water temperatures needed for heat output of 15 and 30 Btuh/sq ft.Upward Heat Flux Requirement (Btuh/[ft.sup.2])Tubing Depth 2" Below Slab Surface, Average Water Temp. Required degF.Tubing Depth at Bottom of 4" Slab, Average Water Temp. Required degF.15 Btuh95 degF102 degF30 Btuh120 degF134 degF (1)
NOTE: (1) regarding the "134 degF" in the bottom right of the above table: This is moving down just 2" deeper. We estimate maybe 168 degrees water temperature would be needed at 4" down and well over 200 deg heating water would be needed in tubing 6" down. In the slab in our construction project, the critical tubing, leaving the heating boiler, was placed more than 12" deep in poured concrete. Heating energy costs will increase consistent with the increase in heating water operating temperature requirements.

The table didn't paste right, sorry.

Originally Posted by David Bell

The table didn't paste right, sorry.

No worries, I think I can understand what you are saying. Can you tell us know where you got that information?

Concrete is such a poor insulator that in my experience the depth is really not a major concern unless you are over 8 inches. The problem being that in an uninsulated installation the heat will radiate to the earth at the same rate it radiates up (+_).

InspectAPedia.com Building & Environmental Problem Diagnostic & Repair Encyclopedia

I think we started talking about boiler temp and mixing valves.Start at the first post and continue reading.In a cold climate it is all about air and floor temp.If radiant heat is the heat source for a home then a tube temp of 105deg will require 2 extra sweaters and smartwool socks.In a cold climate it is the air temp that matters in a room.At a height of 5ft I want the air temp to be around 68deg.As I said before radiant heat temp is about loction and need.

Originally Posted by Michael Garrity

I think we started talking about boiler temp and mixing valves.Start at the first post and continue reading.In a cold climate it is all about air and floor temp.If radiant heat is the heat source for a home then a tube temp of 105deg will require 2 extra sweaters and smartwool socks.In a cold climate it is the air temp that matters in a room.At a height of 5ft I want the air temp to be around 68deg.As I said before radiant heat temp is about loction and need.

Sorry Michael but we will have to agree to disagree. I have been involved with over 500,000 SF of radiant flooring in Park City, Salt Lake City and Deer Valley, Utah, a pretty harsh climate. The maximum temperature delivered to any part of the flooring never gets above 105 deg F. The radiant panel association concurs with this design practice.

I was addressing Jim's original questions regarding slab temperature and boiler efficiency. If 180 degree water is working for you, go for it, I am just saying it is poor design.

Was there ever a mention of the tube temp being 180deg?I don't think so.We were talking about boiler temp and mixing valves in a combined system with baseboard and radiant heat and in my case domestic hot water.In your system if the tube temp is 105deg,what is the boiler temp?My system works great and is efficient.

To high of a infloor heat can effect the floor covering such as wood flooring Also you can heat a whole house with low temperature water, but requires more baseboard Typical residential baseboard puts out 550 to 600 BTUH per foot with 180 degF water Lower water temp. just requires more baseboard. Have seen homes (small) heated with a75 gallon water heater.