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Weil Mclain CGt install with CI radiators

R2.0
R2.0 Member Posts: 99
I am converting my house from oil to natural gas.  It's a WWII era brick Cape Cod (solid 12" walls) with cast iron radiators.  I found a good deal on a Weil Mclain CGt boiler that was purchased for a similar job before the owners discovered that natural gas wasn't available.  I've heard good things on this forum about that model, and it's still in the crate with all the accessories.  I have a couple of questions that are related to the boiler specifically (I'll ask other questions about the loop design and control scheme in the Radiant forum.)



1. Tankless DHW: The CGt is set up to provide DHW via a small tank in the boiler.  Currently I have an electric HW heater providing DHW.  But the house originally had a boiler with a tankless coil and the plumbing is all still there.  I could even hook up the boiler and keep the electric tank, as it is located elsewhere in the basement.  The problem is that opens up a can of worms vis a vis needing to get a plumbing permit.  And I don't even really need the fuel savings - it's just me and my son who is only here every other week.  We've never run out of hot water and my electric bills are pretty small.  So the question is, can the boiler be run without having the DHW hooked up?



2. Controls: The boiler controls are pretty rudimentary: contact closes, purge, circ pump, gas valve, ignition, etc.  The only twist is that it will prioritize DHW by recirculating boiler water via a 3-way valve.  The thing is that (as mentioned before) I would like to disable the DHW function.  More importantly I would like to run the system smarter, using outdoor air reset and a PID controller to deal with the huge thermal mass of the house.  The manual is a bit sparse on the controller but it looks like there are no inputs for anything but t-stat contact.  So what are my options here?  The manual simply says "Contact Weil Mclain" but I'd like to get more informed first.



Thanks in advance.



R2.0

Comments

  • SWEI
    SWEI Member Posts: 7,356
    Outdoor reset control

    is simple enough to add.  Look at the Taco SR501, Tekmar 256, or Heat-Timer HWE-SS for that.



    With that boiler, you will be limited to a return temp of 130ºF unless you add another circulator and manage the ODR with a mixing valve or variable speed injection.



    What size is the boiler?  What is the heat loss for the house on a design day?  Those are critical and you do not want to proceed until you know for sure that you have the right size boiler.
  • R2.0
    R2.0 Member Posts: 99
    I used the Taco FloPro software...

    ...to do the load.  The results were ambiguous.  Using the whole house, including the basement, gave me a load of something over 100 MBH.  However, there is no radiation in the basement.  Nor is there in a 200 sf former screen porch - the previous owners cheaped out and did electric baseboard.  Using those numbers the losses are closer to 60MBH.  However however, the previous owners removed some radiators and resized others.  The place is also very loose and drafty.



    The current boiler is an oil fired Burnham V83, which puts out 125 MBH (109 MBH I=B=R net), which is about the same as the Weil Mclain CGt.  I used 1000 gallons of fuel oil last year, which is probably about 20% more than I should have.  So it looks like the boiler is a bit oversized for my application as it stands now.  That means I'm going to lose a bit in standby losses and short cycling, but I'm hoping it's not excessive.
  • SWEI
    SWEI Member Posts: 7,356
    If it were my house

    and I could not afford a mod/con boiler, I would reconfigure a tank-type electric water heater as a buffer tank and install an ODR-controlled mixing valve on the distribution side of that beast.
  • R2.0
    R2.0 Member Posts: 99
    Given that the unit already has a mixing valve for DHW priority...

    ... that shouldn't that hard to implement. But why the buffer tank?
  • SWEI
    SWEI Member Posts: 7,356
    Mixing valves

    You almost certainly have a tempering valve, which supplies a fixed DHW temperature to avoid scalding.



    What you need is a motorized mixing valve controlled by outdoor reset, which will vary the water temperature feeding your radiators based on the outdoor air temperature.



    The buffer tank is to prevent your boiler from short-cycling.  It also provides hydraulic separation of the boiler and distribution loops.
  • R2.0
    R2.0 Member Posts: 99
    It comes with a motorized mixing valve..

    ... which is used to prioritize DHW. It doesn't use a conventional tankless coil; instead there is a small SS tank that sits in a special boiler section. It has its own temperature probe, and when the water inside the tank goes below the DHW setpoint the mixing valve diverts the boiler supply directly to the return, bypassing the heating loop.  This allows the water in the DHW tank to come up to temp more quickly, and when the setpoint is reached the mixing valve reverts to is normal position and all flow goes to the space heating loop.
  • SWEI
    SWEI Member Posts: 7,356
    That's a diverting valve

    which is sort of the same thing, only backwards -- and with no modulation.
  • R2.0
    R2.0 Member Posts: 99
    Missed that about no modulation

    I'll need to look at the valve body to see if it will take a modulating actuator.



    It sounds like you would simply do the temperature reset on the secondary loop and let the boiler water temperature remain constant. Is that correct?
  • SWEI
    SWEI Member Posts: 7,356
    ODR mixing

    You need a new valve, not the one that diverts DHW.  A Taco iSeries-R is probably the least expensive option http://flopro.taco-hvac.com/products/index.html?category=188



    With one of these, you would set the boiler a few degrees higher than what you need for the emitters on the coldest day.



    Tekmar has several controls which will run a motorized valve and also do boiler reset.  If you go that route, install the sensor near the top of the buffer tank instead of on the boiler supply/return.   Unless you need over 150ºF, boiler reset will not do much for you.
  • R2.0
    R2.0 Member Posts: 99
    So I guess the big question is...

    ...do I leave the boiler controls and diverter valve alone and put the mixing valve downstream, or replace the controls and valve?  The former would be cheaper, but the idea of parallel bypasses doing nearly the same thing offends my sensibilities.



    Here's a question - is anyone making a mixing valve that fails closed?  The Taco valve you referenced is a ball valve and I didn't see a spring return in the specs, so that will just stay where it is.  If I found one that defaulted to one position I could keep the boiler controls and use a relay to cut power when the controller sends the signal.
  • SWEI
    SWEI Member Posts: 7,356
    You're overthinking this

    A properly sized mod/con boiler would require no extra parts and probably burn 20-25% less fuel than the buffer tank and ODR valve with the CGt will.  If you elect to skip the buffer tank, probably more like 30% less.



    3-way valves with spring returns are available, but then you need an external control (which will cost at least what the iSeries-R does.)
  • R2.0
    R2.0 Member Posts: 99
    One slight problem:

    I already have the boiler.



    Any ideas on running it without the DHW hooked up?
This discussion has been closed.