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Conversion from gas hydronic to heat pump?

Treebeard
Treebeard Member, Email Confirmation Posts: 11
What would it take choose a heat exchanger, and heat pump to provide enough energy to heat a house and domestic hot water that now uses a 94,500 BTU high efficiency boiler?

Comments

  • Hot_water_fan
    Hot_water_fan Member Posts: 899
    First, I'd start with what the heat loss is as usually boilers are extremely oversized. How much gas was used last year?
    Treebeard
  • pedmec
    pedmec Member Posts: 664
    Can you rephrase your question so we understand
  • Jamie Hall
    Jamie Hall Member Posts: 19,960
    I'll give it a shot, @pedmec ... our OP has a gas fired boiler running at 94,500 BTUh -- neglects to say whether that's gross or output -- powering a hot water heating system, and he or she wants to get rid of that and pick a heat exchanger and heat pump to do the job.

    Well now. First, you don't need a heat exchanger -- you need either a ground source or air source to water heat pump. Next, you need to know what temperature your existing radiation requires to provide the heat your space requires. Which means you need to know what that hear requirement is. Then, if you are using air source, you need to know what the design condition outside air temperature is.

    Oh -- and it's not energy you are dealing with, it's power.

    Then you can begin to think about trying to find a heat pump to do the job...

    So, @Treebeard , we need some more information.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    GGrossTreebeard
  • pedmec
    pedmec Member Posts: 664
    And the answer is.......
  • hot_rod
    hot_rod Member Posts: 17,537
    What type of heat emitters? Baseboard, radiators, radiant?

    A heat load is a must have first step. Does the current boiler keep the entire home warm on the coldest days?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Treebeard
  • Treebeard
    Treebeard Member, Email Confirmation Posts: 11
    Hot_water_fan said:
    First, I'd start with what the heat loss is as usually boilers are extremely oversized. How much gas was used last year?
    Gas used includes cooking, and clothes-drying. Will gas usage data still help?

    I'll give it a shot, @pedmec ... our OP has a gas fired boiler running at 94,500 BTUh -- neglects to say whether that's gross or output -{OP That BTUH is from the installation manual "Btuh input Natural Gas"} - powering a hot water heating system, and he or she wants to get rid of that and pick a heat exchanger and heat pump to do the job. Well now. First, you don't need a heat exchanger -{OP I thought that the heat exchanger will be costly and necessary to get the heat from a heat pump into the hydronic system}- you need either a ground source {OP no space for ground source} or air source to water heat pump. {OP I did not know that there was such a device} Next, you need to know what temperature your existing radiation requires to provide the heat your space requires. {OP I think the hydronic water temperature is around 120F but I don't have an easy way to measure it}  Which means you need to know what that hear requirement is. Then, if you are using air source, you need to know what the design condition outside air temperature is. {OP FWIW, I think that minimum outdoor temperatures over the last ~40 years have been -15F for one week (once)}  Oh -- and it's not energy you are dealing with, it's power. {OP I got it, the rate of transfer not the amount thanks!} Then you can begin to think about trying to find a heat pump to do the job... So, @Treebeard , we need some more information.
    hot_rod said:
    What type of heat emitters? Baseboard, radiators, radiant? A heat load is a must have first step. Does the current boiler keep the entire home warm on the coldest days?
    Heat is hydronic through convectors (baseboard fin-tubes in tall boxes like radiators). The boiler runs often when the OAT< 0F. The boiler runs continuously when the outside temperature gets to -15. The intelligent (learning) thermostat is set to 65F daytime and 60F nights.

    pedmec said:
    Can you rephrase your question so we understand

    I just wanted to see if it was practical to replace the boiler with a heat pump system and use a heat exchanger to transfer the heat from the heat pump into the existing hydronic system. Come to think of it, it might be a good idea to just leave the boiler in place (it is compact), and have dual sources of heat Gas and Electric.

    Because of real estate limitations it is not possible to use ground source (as much as I would like to be able to). There is no way to get enough horizontal space, and no way to squeeze a drill rig (unless it is an ATV that can climb a 50 degree grade) in to go vertical. Perhaps we could crane an ATV in...nah too expensive.
  • hot_rod
    hot_rod Member Posts: 17,537
    You would want heat emitters that could work with 120f water or lower to have a good match for any heat pump. The type of emitters you have, and the load size doesn’t seem like a good heat pump match. Right now anyways.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Treebeard
  • hot_rod
    hot_rod Member Posts: 17,537
    go here and download these free journals. https://idronics.caleffi.com/

    It will help you determine what is possible or not for HP conversions. The one issue gives you ideas on how to modify existing high temperature systems so they could maybe be a match for heat pumps.

    Two things happen with HPs are temperature drops. Their output drops, and so does the COP. So at some point switching to another fuel source make the best $$ sense.

    I'd look at covering part of the season with HPO, switching to the boiler when the HP just doesn't cut it.

    I think the NYSERDA simulation program is now up and running, give it a try with your actual data.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Treebeard
  • Treebeard
    Treebeard Member, Email Confirmation Posts: 11
    Thank you for the links and guidance. I have thought that using the "emergency heat" thermostat wire to start the boiler was a good idea. I saw a contractor use reclaimed heating oil (free) as their main heat and Heat Pumps for backup "emergency" heat for when the oil was used up. Three zones and two AHUs, complicated... 
  • Treebeard
    Treebeard Member, Email Confirmation Posts: 11
    hot_rod said:
    go here and download these free journals. https://idronics.caleffi.com/ It will help you determine what is possible or not for HP conversions.
    The graphs indicate that the lower the temperature output of the HP, the more efficient it is. So, I have to measure the temperature of the water in the hydronic loop. I wonder if preheating the water to the boiler with the HP would be worthwhile.