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Heating Load Calculation

heiner | Posted in General Questions on

Hi All,
I just joined this site, and there seems to be a lot of great information. I want to install a staple-up radiant floor heating system and performed a heating load calculation. Do you think my calculation makes sense? Anything I may have missed? Does it make sense in my case to plan for a 30k BTU system?

We have a 1.5 story house with 1500sq feet base area. The coldest temperature we experience is 40F (California). The house is from 1915 with uninsulated walls and mostly single pane windows. I googled most of the R-values below, would like to know if the calculation makes sense to you.

  area(in^2) area (feet^2) R-value dT (70-40) A/R*dT
net wall 161655 1122.603448 3 30 11226.03448
windo single pane 30015 208.4373666 1 30 6253.120998
window double pane 10346 71.84717624 3 30 718.4717624
roof 236643.2568 1643.354898 26 30 1896.178729
floor 210240 1459.999066 19 30 2305.261683
infliltration loss         3832.282962
SUM         26231.35062

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  1. paul_wiedefeld | | #1

    When are R values are that low, error can be really high so your heat loss could easily be half that. If you’re using a fossil boiler, there’s not much of a point being more precise at this heat loss level since the smallest boiler will probably be 30kbtu anyway.

    What currently heats the space? You can back into the number that way.

  2. heiner | | #2

    We had a gas burner/forced air duct system that broke down. I ripped everything out and will replace it with an electric heat pump + radiant floor system. The gas burner was 30k, which struggled to heat the house, but the burner was very old, and the duct system had leaks etc.

    My next question would be what heat pump to get. I was looking into a Trane XV19 but it doesn't seem to be easily available.

    1. paul_wiedefeld | | #3

      There are shortages of many models currently. In your mild climate, single vs multi speed is the main decision to make, rather than brand.

      What’s going to heat the floor?

      1. heiner | | #5

        pex-staple up for heating the floor. Is multi-speed worth it? Isn't a buffer tank sufficient? Any advice on a multi-speed?
        I was mainly optimizing for low noise, but availability is a good argument as well.

        1. paul_wiedefeld | | #6

          How will the Trane heat the fluid in the pex? That’s an refrigerant-to-air heat pump - do your plans have a refrigerant to water heat exchanger?

          1. heiner | | #7

            you are right. I thought it was air-to-water.
            I am now looking into the Chiltrix CX34. There seem to be some problems about it but mostly due to installation errors.

  3. DC_Contrarian_ | | #4

    If you have a fuel usage history you'll probably get a more accurate number with this method:

  4. paul_wiedefeld | | #8

    Is it a safe assumption that budget isn’t really a concern with this project? Hydronic in-floor heating using air to water in California isn’t going to be a common installation.

    1. heiner | | #9

      I haven't found any other option for in-floor heating that would be more common. The quotes I got were 50k, which is why I am DIY. Hardware costs are indeed a non-issue compared to labor in the bayarea.

      1. paul_wiedefeld | | #10

        $50k sounds about right! Retrofit, in-floor Hydronics with air to water for 1500ft^2 is about the most expensive $/ft installation I can imagine. Lots of expensive hardware.

        1. heiner | | #11

          the hardware is ony 10k actually. The contractors then double the hardware cost. 30k is labour.
          DIY for 10k sounds dead cheap to me.

          1. paul_wiedefeld | | #12

            Hmm a lot of hardware involved in Hydronics. Do you have a finalized design?

  5. heiner | | #13

    Pex tubing and heat plates are actually less expensive than mini splits. I don't think it is an expensive solution at all. The main cost is the heat pump (6K) which you have in any project.

    1. paul_wiedefeld | | #14

      I'm not comparing to ductless - I'm thinking more ducted vs. staple-up. If you can do it for $10k, great! Submit a write-up to GBA afterwards so others can learn.

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