Heat Exchanger and pump sizing

Discussion in 'HVAC Heating & Cooling' started by mikeyvon, May 30, 2008.

  1. mikeyvon

    mikeyvon New Member

    Nov 14, 2007
    I am using flat panel radiators to heat my new home. I am trying to size my heat exchanger and pumps to run off of my water heater (Smith Vertex 76K BTU). My total heating load is 36K BTU per hour.

    I am using Veha radiators. There are 9 radiators on 8 zones (2 will be in series in the basement). I will be using a homerun system with a manifold. The average length run is around 70' using 1/2 O2 barrier pex. The longest run is 120' the shortest being 40'. I will be using the radiator valves to balance the flow (i hope). I will be using trv's to control zone temps.

    I will be purchasing my supplies from pexsupply or fergusons.

    I am looking at Heat Exchangers, but I am uncertain what size I need. They have 14, 20, & 30 plate in the 3"x8" size and they have many different models in the 5"x10" size. Is there a general rule on BTU transfer for heat exchangers?

    For pumps, I have been thinking about using taco's OOR 3 speed on the radiator side

    Does this size seem correct? I need a bronze pump on the water heater side correct? What size would I need there?

    I am also planning on an expansion tank, bypass valve, air eliminator, various temp and pressure gauges.
  2. mikeyvon

    mikeyvon New Member

    Nov 14, 2007
    ^^^bumpity bump bump
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  4. cattledog

    cattledog New Member

    Jan 29, 2006
    Portland, Oregon

    You have some more calculations to do. There is no simple rule of thumb, and the heat exchanger manufacturers or suppliers typically provide some analytical support.

    You know that you want to deliver 36kbtuh across the heat exchanger, and the way this happens is with two streams of moving water, one loosing temperature and one gaining. The key factors are the flow rate and temperature change on each side. There will be a balance for heat loss and gain. Most importantly, there is the difference in temperature (driving force for the heat transfer) between the two streams of water on either side of the heat exchanger.

    I would start by trying to understand the room radiation or secondary side of the heat exchange. Based on your radiators in each loop, what incoming water temperature and flow rates do you require, and what is the return water temperature.

    Then, you will need to balance this on the primary side, and come up with a designed flow rate and delta T to supply your max load. Knowledge of the flow rates on either side of the HX helps you size the inlets/outlets and the size and number of plates to keep the pressure drop and pump requirements down. The HX manufacturer/supplier can provide the pressure drop table for different flow rates. You can use the simple rule of thumb that a flow of 1 gpm with a delta T of 20F provides 10K btuh.

    Finally, the heat exchanger supplier should be able to provide the analytical program to size and select the number of plates for the mean temperature difference seen between the primary and secondary side. As the driving force goes down, the heat exchage surface area must go up to get the heat transfer. You should also oversize slightly to allow for some fouling and loss of efficiency with time in service.

    The selection of the size and number of plates (i.e. the heat exchange area which determines the transfer between the two streams and the pressure drop across the unit) and the pump requirements is a trade off of costs.

    Looking beyond the selection of a heat exchanger, one problem you may have is that your water heater (for domestic water purposes) is going to be running at about 140F max, and given your baseboards/radiators this may not provide the water temperature on the secondary side you need to provide the 36kbtuh room radiation. Most domestic water heaters used for hydronic heat, provide heat to in floor loops which run at significantly lower temperatures than panel radiators or baseboard units.

    Another potential issue is with the on/off control band of the water heater. For example, some water heaters may drop 15F (140 to 125) before firing back up. You will then have a different heat delivery across the HX.

    You are wise to isolate the domestic water from the heating side with a heat exchanger, and some jurisdictions actually require a double walled HX for applications with potable water on one side.
  5. mikeyvon

    mikeyvon New Member

    Nov 14, 2007
    cattledog - thanks for the response. It does seem that I still have some homework to do. I am trying to do my heating as simple and economical as possible. We heat primarily with wood heat. We designed our house plan to be open to make wood stove most effective. That coupled with our relatively mild climate, low degree heating days, and new efficient construction. Code requires us to have another heat source. I wish (and hope) it was as easy as someone just telling me which pumps and heat exchanger to use.

    I did take a gamble on the water heater. Marketing says it is effective for hydronic heating and it comes with side hook-ups designed just for that. It was only a slight ($200) up-charge from the powervent water heater I am required to use anyways. It is thousands less expensive than a mod-con boiler. The radiant expert I used to do my heat calcs and bounce ideas off of even suggested using a water heater in my situation. Unfortunately, he wants a lot more money to help design my "boiler room controls". I am hoping it will work out.

    I am covered on the 140 degree water temp. I am using flat panel radiators and sized them correctly for 130 degree water with a 20 delta t.

    I am not a heating expert and do not plan on doing this for a living. I am a public school teacher and building this house (our first) as owner builder. I simply cannot afford to spend too much on a heating system that essentially will act as a back-up and background heat source. I have everything figured out (i think) in the controls but the pump sizes and heat exchanger. I did do my homework on the heating system and feel once I get the pumps and heat exchanger correct, it will work out nice.

    Thanks again and keep the comments coming. :)
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