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Levittown ranch slab radiant heat help

slab heating help Goodmorning,

I am looking for some help/advice on heating my 1950 slab house.

We just bought the house this past summer and i have some questions, or concerns with my heating system.

The house is 900sqft slab house with a crude radiant heat.

1/2in copper embeded in the slab pusing 180deg water from a york boiler hanging in the kitchen.

The house is a origonal levitt house.

On the warmer days the thermostat will call for heat one time a day and heat the house to 65deg in about 90 minuets.

the house holds the heat well and has alot of solar gain.

Now that the wether had dipped down a bit the thermostat will call for heat more often, but will always take about

90 minuets to be satisfied.

We keep the thermostat at a steady 65deg with no setback.

I feel that the slab is getting too cold during the times that the heat is not being called for

and taking too long to get back to temp.

Does this seem right? any thoughts i guess what im poking at, is how can i get the most out of my radiant heat


  • Jean-David BeyerJean-David Beyer Member Posts: 2,532
    Does this seem right?

    It depends on what you think "right" is.

    I have a 1950 Cape Cod house of 1150 square feet. The downstairs zone is a concrete slab with 1/2 inch copper tubing in it. Upstairs (that was once the same zone as the slab) was finned tube baseboard that may have been large enough if you put 180F water in it, but the system was made to use lower temperature water, perhaps 140F. Consequently the upstairs was always too cold.  I more than doubled the length of the baseboard in that zone.

    With a slab, you want to run the slab as low a temperature as you possibly can and still heat your house on the design day. Here in New Jersey, this is 14F in most places. It turns out if I heat with 112F water, that will heat the downstairs on the design day, running about 18 hours at a stretch. (I now have a mod-con, and the upstairs zone is now separate, so I can use a different hot water temperature for each zone.)

    When I used my old boiler, I got large temperature swings. I set the thermostat at 70F and the temperature ran from about 66F to about 74 F most of the time, and I figured out it was because of the enormous thermal mass of that slab.

    When I switched to my mod-con, I put water between 75F and 120F depending on the outside temperature. On warm days (above 50F outside), I use 75F water; on medium days (between 50F and 14F, I use water up to 112F; and on cold days (below 14F) I use water between 112F and 120F, I can now hold my indoor temperature within 68F and 70F and most of the time is holds 69F where the thermostat is set.

    I suggest you reduce the temperature of the water you are using to as low as you can get it that will just heat your house. If your house is like mine, and if you are somewhere near me, you could probably set it down to 140F. This will cause it to run longer, but the temperature swings should be a lot less. You might be able to go even lower, but if you do that, you are very likely to get condensation in the boiler and the chimney, and unless you boiler is designed for that, it will rust out and cause other expensive problems.

    Depending on where you are, and the age of your boiler, you might consider replacing it with a mod-con, but you want a capable contractor to do that or you may end up unhappy.
  • dalipaintedmedalipaintedme Member Posts: 17

    but when you say turn down the temp. will that also affect my DHW coil? and it will run longer, meaning that the burn time will be longer? were comfortable with the tems in the house (well maybe my wife would like it warmer) but im looking for fuel economy
  • Jean-David BeyerJean-David Beyer Member Posts: 2,532
    As usual, it depends.

    "but when you say turn down the temp. will that also affect my DHW coil?"

    That depends on the design of your boiler. With my boiler I have an indirect fired hot water heater. This runs off the boiler with a separate circulator. When running the indirect, the boiler operates at a high temperature, so it takes only about 10 minutes to satisfy it. The rest of the time, it runs at a different (lower) temperature to satisfy the heating requirements. I do not like a coil, though my old boiler had one that the former owner did not use. He used a separate electric hot water heater.

    If the volume of your boiler is not too large, the same thing could apply to your setup. The boiler would run high temperature for the coil (turning off the slab), and when the hot water demand was satisfied, the temperature would drop to the heating temperature. It depends on what you have.

    "will run longer, meaning that the burn time will be longer?"

    Here, too, it depends on what you have. It will probably run longer but at a lower overall rate. If you have a new enough boiler, its firing rate will go down when heating the house. If it is the original 1950s era boiler, it will cycle on and off to provide the desired temperature for the house. So if the thermostat is calling for heat for 5 hours and you need  only 20% of the available heat, it will cyle on for on hour and off for four hours, but probably more rapidly than on an hourly basis. You would want it a little faster than that, I expect.
  • GordyGordy Member Posts: 7,122
    Post pics

    If you can of the boiler, and its piping. There has to be a mixing valve or some kind of piping, and valves to mix down with for the radiant. I doubt 180* water is going through the slab. Not impossible

  • MpjMpj Member Posts: 109

    If I remember correctly the boiler water leaving the boiler has a restricter built into the outlet port. I think you have two manifolds under the boiler, check to see if all the circuits (6 or 8) are getting warm

    You may have a bad circulator if it is one of the old B&G's (big and red) or one of the shut-off valves maybe slightly closed.

    I never felt one of those slab go cold. I grew up in a Levit cape lost power for three days in the winter and the slab was just getting cold on third day, they hold the heat well.
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