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Running a steam boiler and heat pump together as temperature hits 3F tonight?

llsrvd | Posted in General Questions on

We have 2 heat pumps and a steam heat boiler.  The overnight low tonight will be 3F.  We have NOT been using the boiler so far this winter, but since the basement has been about 48-50F recently, we are concerned about pipes freezing with the cold tonight and tomorrow night.  Should we have the heat pumps and boiler both on?  How should we set each one? Or should we just run the boiler and turn the heat pumps off (or would that be bad for the heat pumps). This is our first winter with the heat pumps so we’re not sure what to do.


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  1. charlie_sullivan | | #1

    It won't hurt the heat pumps to turn them off. But you can also have both on. You might need to experiment with the settings to get them all working together effectively, but I don't see any problem with either turning off the heat pumps and just using the boiler, or turning both on and adjusting the thermostats until you get what you want.

    1. llsrvd | | #2

      Thank you for your quick reply. We ended up turning both heat pumps off and will keep them off today (high 7F) and tonight (low -2F). The basement did warm up to about 56F (from 48F before the outside temp went down to the single digits) which gives us peace of mind about the water pipes down there.

      1. Expert Member
        BILL WICHERS | | #3

        You need to get down to 32 or below and stay there for a little while before you’ll have a freeze problem. 48 leaves some room. Now if you have pipes in exterior walls you might need to keep things a little warmer to be safe.

        I agree with the other posters. Your boiler will probably be the better option in these really cold temperatures we’re currently seeing.


  2. llsrvd | | #4

    Thanks Zephyr7, we do have some pipes close to the basement windows, but fortunately none actually running up inside an exterior wall. We're probably being over cautious because we did have to deal with a burst pipe in a previous house.

  3. WPmichael | | #5

    When it gets this cold I will run the boiler for an hour a few times a day. When the temperatures get so low the heat pump get less efficient because there is less energy to extract from the outside air. I live in an area where electrical costs are high. So this helps keep the utility bill down.

    1. Expert Member
      Dana Dorsett | | #6

      The net system efficiency of steam systems with the boiler located in the basement is pretty low. Using standby & distribution losses to heat the basement isn't usually the cheapest or best way to get there.

      The raw combustion efficiency of a steam boiler might be 80%, but something like 25% percent of the fuel use is lost to standby & distribution, even with insulated steam pipes in the basement. That's a net-efficiency of about 60%.

      So, assuming buck-a-therm natural gas you get 60,000 BTU/ therm of heat into the fully conditioned part of the house for $1. Normalizing to MMBTU (millions of BTU) it takes 16.67 therms/MMBTU, which at a buck a therm costs $16.67/MMBTU. If your natural gas costs $1.50/therm (like mine) it would cost $25/MMBTU

      Most cold climate mini-splits will deliver a COP of about 2 at full speed at 0F - +5F. At a COP of 2 you get 2 x 3412= 6828 BTU/kwh of heat into the house. Normalizing to MMBTU it takes 146.5 kwh/MMBTU. If your electricity costs 20 cents/kwh (like mine does) the cost is about $29/MMBTU a that temp, about 16% higher than heating with a steam boiler, not a huge difference compared to buck-fifty gas when it's less than 2% of the heating season.

      Setting up a space heater on an inexpensive (<$20) freeze protection line voltage thermostat plug (eg. Dayton 48GP69 ) that turns on at 35F, off at 45F would probably be worth it in the short term even if you continue to use the steam boiler in that inefficient manner. Placing the plug on a heavy duty extension cord near the most susceptible plumbing can work out well.

      In the longer term it will be worth air sealing insulating the basement walls to current code minimum, at which point the basement temperature will only rarely (if ever) drop be low the deep subsoil temperature in your area.

      1. Expert Member
        BILL WICHERS | | #7

        Using a heat tape and/or pipe insulation would probably be even more efficient than a space heater and a freeze plug. If using an extension cord, try to use one of the flat “air conditioner” type cords. The flat cords allow better cooling for the conductors and thus are a bit safer when running heavy loads in near continuous operation.

        I would argue that distribution losses in lower levels do help beat the rest of the building, and thus aren’t entirely “lost”. If the losses are enough to keep the basement at something high like 80 degrees while the rest of the house is more reasonable, then I’d consider those losses more of a real “loss”.


        1. Expert Member
          Dana Dorsett | | #8

          Keeping an inactive steam boiler from freezing & cracking with heat tape isn't a sure thing.

          While it's the standby & distribution losses to the basement aren't totally lost, if the foundation is not insulated they are lost-mostly- at least 75% of it is going out the foundation walls and into the slab/floor. A typical CMU foundation is no more than about R2, so when it's 3F outdoors and 53F in the basement (a 50F delta) every square foot of exposed above-grade foundation is losing 50F/R2= 25 BTU/hr, with lower but steadier losses per square foot below grade. With 2' of exposure and a 150' perimeter (say a 30' x 50' footprint, 1500 square feet per story) that's 300 square feet x 25 BTU/hr = 7500 BTU/hr for JUST the above grade portion of the wall. With poured concrete or stone foundations it's usually a bit worse.

          A heat loss of 7500 BTU/hr is WELL into double-digits percentages of the heat load of most insulated houses with 1500' foundation footprints. It could be as much as 1/4 of the total heat loss, even at a relatively cool 53F indoors. With more exposure than that it's usually more than 25%.

          Insulating the walls all the way down to the slab to IRC code min cuts the heat losses from the basement by over 75% (sometimes by over 90%) despite the now higher ~55-60F basement temperature even without actively heating it.

          1. Expert Member
            BILL WICHERS | | #9

            Good point about the potential losses for uninsulated basement walls, I had not considered that. I should have thought of it too since I’m in the process of insulating my own basement...

            I had thought the OP was concerned with regular water pipes freezing and not the steam pipes themselves. I’d think the steam pipes should have drained back down to the boiler if they’ve been inactive for a while. Even if they have some water in them, a freeze won’t cause any problems since the water would have room to expand. If there are seperate return lines, those might be a problem if there are parts that are completely filled with water.

            I agree the safest option for the boiler and associated piping is to use a space heater over a heat tape. I’d be concerned with using a heat tape on a steam line anyway since the extended periods of high temperature might degrade the heat tape over time.


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