Temperature Change Time

The house is 2800 sq ft sitting on a concrete slab. It has radiant floor heating but it is turned off. This past year I installed a forced air system in the house. I have the radiant floor heating off because I have an inefficient boiler and need to get a new one.

The issue I'm having is that the house takes a really long time to come to temperature. It takes about an hour to change the temperature in the house 2 degrees.

Should I just keep the house at one temperature or have the system let the house cool down when I'm not there? I'm wondering if the cold floors are keeping the house from coming to temp?

Tom

P.S. I had the unit sized to the house and the ducts that I ran were designed by a heating and cooling outfit.

On the same day, how fast does the house cool off?

If you were using the radiant, you'd probably find a fairly constant temperature would work out better, since the lag in getting that large thermal mass heated can be huge. It sounds like the forced air has very little excess capacity (which can be good). Consider one of the Honeywell (or maybe some other brands and types) of setback thermostats where they learn the system. You set the time when you want the house a certain temp, and it adjusts to start soon enough to perform that task. It might be 15-minutes on a mild day, or hours on a cold one. You'll probably find it ends up on most of the time, but at least for a nominal cost, you'll be saving some, and maximizing your comfort.

"Radiant" heat in a concrete slab on the ground is often a losing proposition that can't be repaired after the concrete is poured. Check the report at the link below.
http://www.inspect-ny.com/heat/Radiant-Slab-Heat-Mistakes.htm

When you put heat into a slab on the ground you are always putting some heat into the ground. With fuel oil now at about $3 per gallon, the heat loss becomes unacceptable.

Heat is being lost to the floor when the heat is off. A 4" concrete floor has a heat capacity of about 10 BTU per square foot per degree F. For a 2800 square foot floor that is about 28,000 BTUs to raise the temperature by 1 degree, totally disregarding the heat losses through the floor to the earth.

If you have good insulation under the floor (R=10) with a temperature difference of 15 degrees between house and earth under the floor you would have about 4200 BTU per hour loss from floor to earth. If it is not well insulated (R=4) would have 10,400 BTU per hour through the floor to earth when you are not heating it. The temperature in the slab is much higher if you are heating it and the losses depend much more on the insulation and depth of heating pipe.

The floor can be made warmer by installing carpet over a pad that will provide some insulation if that is feasible.

An hour to warm the house up 2 degrees with a forced air system is just wrong. Are you on a heat pump, or a gas furnace? If it is a heat pump, do you have auxilliary heat?

I have a forced air furnace.

I'm not trying to figure out wether to run the floor heat or not. I'm trying to figure out if what is causing the forced air system to cool the house is the cold floor. If it is. Would it be wise to leave the forced air system on all the time at a constant temperature.

Last night the same thing occurred. The house took 1.5 hourse to move the temp about 4 degrees.

What are your guys thoughts on this?

Tom

I live in a 2000sq ft two story home on a concrete slab foundation that is half carpeted and half wood flooring downstairs. My central heat system raises the temperature from 61 to 68 degrees in thirty to forty minutes in the morning. Thermostat is programmed to kick the heat up about a half hour before we roll out of bed so it's always nice getting up and into the shower.

Sounds like your unit isn't up to the task.

Mort

Most furnaces are somewhat oversized...this is what allows them to recover from a setback. The speed of the recovery depends on how big that oversize component is. Yours appears to be sized to barely hold its own. Note, normally, you only need the design amount of heat on the coldest day it was designed for.

WHen they did the heat load analysis, what numbers did they come up with? How does the output of your furnace compare? Are you sure your filter is clean? Is your return blocked? Is there a decent flow of air out of the vents?

If the analysis said you needed a 100KBTU furnace, and that's what you put in, that means that on the design temperature, it could hold its own if it was running constantly. If it was say 120KBTU, that extra 20K would speed the recovery.

The outside tempurature went up. Actually doubled from the last couple of days. It was 33 this morning. The furnace ran for about an hour and 15 minutes. It was able to bring the temperature up 7 degrees. Much better. I checked the air filter which I had just changed a few weeks ago and it was really clogged. Apparently cutting that travertine indoors is clouding up the air.

I'm going to keep monitoring it.

Thanks

Tom

statjunk said:

. Apparently cutting that travertine indoors is clouding up the air.
Tom

Click to expand...

Ya think!!!

LOL. I was hoping that if you can't see it, it can't kill you.

Tom

Honeywell (and probably others) makes a filter flow warning device. You basically just drill a small hole in the duct near the fan and then screw it in place. If the suction (therefore flow) gets out of range, the suction trips a lever and a red flag shows up. Very simple.

there are also some filters with a whistle device in them - if they start to get clogged, it starts to whistle at you, alerting you to the fact the filter needs to be changed or cleaned.