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Help with system design.

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I currently have a gas boiler with indirect dhw and cast iron radiators. I would like to replace my system with a heat pump. I have some basic knowledge when it comes to boilers and design, but I would like some help with the heat pump. I was wondering if some of you could assist me in that respect. Since the heat pump doesn't have the same temperature output as my boiler, I might gets things wrong on my own. I'm also concerned about flow rates and such. So if anyone is game, I can provide many more details.
Cokomo

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  • hot_rod
    hot_rod Member Posts: 22,254
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    Suggested steps:
    Do a room by room heat load calculation
    Determine what SWT are required for you radiators to meet that room by room heat load
    See if any of the current HPs can supply adequate SWT to heat under design conditions.

    At best expect around 130 SWT from current low ambient A2WHPs, efficiency drops of course at those conditions.

    Your radiators will probably need to be oversized a bit for this to work. But they may well be, especially if the home has energy upgrades.

    A few free load calculators

    https://www.usboiler.net/heat-loss-calculator.html

    www.slantfin.com load ap
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • drooplug
    drooplug Member Posts: 26
    edited February 2019
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    I did a room by room heat loss quite awhile ago. I can tell you the results were higher than actual. I've made some insulation upgrades since then.

    I measured my total heat loss to be about 35,000 btu/h at 10F outside and 70F inside. I was able to heat my home with 120F water when it was 3F outside. I didn't time it, but the boiler pump was not running constantly. My design temp is 10F

    I have been looking at the infamous Chiltrix. I believe maximum water temp is 120F. At that temperature, maximum btu output is just about 13k at 5F. I would need two units to cover both my heating and cooling loads. So at design, I'll have 25,000 btu from the heat pump and their electric back up unit offers and additional 15k btu.

    My concern comes from the flow rates. Each chiller has a design flow of just under 5 gpm with a max of 6 gpm. Now, with two chillers piped in parallel, won't this deliver a very high flow in my heating loops if only one zone is calling and my pipe size is the same or smaller than the chiller?
  • hot_rod
    hot_rod Member Posts: 22,254
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    The Solstice SE SCM 036 has two refrigeration circuits so 18,000, 1.5 ton each, it stages to the load. A buffer tank is pretty much a must with any HPs. You disconnect the HP flow rate from the distribution as the buffer becomes the hydraulic separator.

    Solstice uses heat from the buffer to defrost so you don't need to use resistance heat and drive operating cost up. Generate the defrost heat at the higher COP.
    .
    Solstice shows about 25,000 120F SWT at 3°F. At 10°right around 30,000.

    i know of one installation in Upstate NY on it's second heating season.

    SpacePak has a great design manual available.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • drooplug
    drooplug Member Posts: 26
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    Spacepak wasn't very responsive to my requests. I read in another thread that the are introducing a modulating heat pump. Maybe I will try again when that comes out.

    The strange thing with Chiltrix is their dhw heating. You can only use one chiller per coil in the dhw tank. If I wanted to use both, I would need a tank with two coils. They said it was to control flow rates. I thought a hydraulic separator or buffer tank would solve that issue, but they say that it won't work. Any thoughts on that issue?
  • drooplug
    drooplug Member Posts: 26
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    Is that spacepak design manual available online?
  • hot_rod
    hot_rod Member Posts: 22,254
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    drooplug said:

    Is that spacepak design manual available online?

    I think you need to go to their site and register

    http://spacepak.com/water-works
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • drooplug
    drooplug Member Posts: 26
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    I got my hands on the Spacepak Design manual. I also bought John Siegenthaler's Heating With Renewable Energy. I looked over what he wrote for Spacepak and checked out the heat pump section of that book. It seems like my thinking on design wasn't too bad.

    I don't know what Chiltrix's deal is. Maybe I'll submit a design to them and see how they respond. Otherwise, I'm kind of likeing the Solstice Extreme again. Since I want to do a bunch of repiping, it will be some time before I make a final decision. Hopefully Spacepak's new heat pump will be out and I can have that as an option.
  • mikeg2015
    mikeg2015 Member Posts: 1,194
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    Spacepack has a new inverter compressor (minisplit) reverse chiller coming out. You might look into that.

    But as mentioned, you need ot look at your EDR, then look at the heat up put of the radiators at the supply water temp proposed at design. Then see if that matches your heat loss. If it doesn’t you’ll need ot add more radiation or increase water temp.

    You could add a small, cheap cast iron atmospheric boiler to supplement. If your on propane, you can perpurchase it at a lower rate, and with dual fuel arrangement, you can ration it accordingly.

    also can add a hydronic AHU in series-parallel after the radiators and pull more heat out and increase delta T, dropping RWT/EWT which increases heat pump capacity and economy.

    Or use a combi boiler for DHW and supplemental space heating demand.


    NOTE - heat pump specs only show GROSS steady state capacity and COP. They do not include defrost, which below 20F rapidly degrades NET output depending on relative humidity.
  • drooplug
    drooplug Member Posts: 26
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    My EDR estimates understate the capacity of my system. As I said above, I am able to maintain 70F indoors with 3F outside using 120F supply water. My design temperature is 10F. It rarely gets that cold let alone down to 3F.

    My original thoughts were to use the air handler as a second stage for those very low temps, but it seems I might not need to do that.

    I have an existing gas boiler and since I started reading Steigenthaler's book, I've gained some valuable insight on how to use that in the system as an aux heat supply for space heating or DHW. I was originally figuring I would probably need to keep it in service to do DHW to start with. It's less than 10 years old and in good condition.

    Our longer term plan is to put an addition on our house which would provide a good roof space for solar PV and that will help offset any additional costs associated with electric heating of hot water.