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Sanity check: small elec boiler, indirect DHW and plate heat exchanger

Ahead Member Posts: 2
[my first posting here!]

I'm spec'ing out for a new house build: near passive house design as far as efficiency goes. Manual J output on my new house design is coming up with a heating load of 16.7btu (cooling is not a concern). I am trying to figure out how to provide DHW.

DHW loads aren't expected to vary from our current use, which is being served via a simple, electric 50 gallon heater: pretty sure that it's a 4,500 watt unit. Our current TOTAL average daily electrical use is 17kw/hr/day. DHW is the top draw (we currently have wood for heat and propane for cooking and for the clothes dryer). Maybe the DHW is 80% of that?

Researching on boilers for the new build, I ran across a boiler that is close to matching the heating load requirements (of 16.7btu). A 20.5kbtu (6kw) unit, electric: electrical rates are low here. By just the numbers this is "over-sized" by about 22%, which led me to think that there's room to pick up the DHW. The unit is supposed to be able to also serve DHW (though I don't know if it has a priority setting or not- need to inquire). So, it seems like there's a shot here...

I figure that I'd want to be going with a "larger" indirect tank (larger than small) to help offset the fact that the boiler wouldn't provide for very quick recovery times. Thinking 50 gallons minimum. Leaning towards making the tank totally indirect, using a plate heat exchanger (w/circ pump) and mixing valve on the output side: pretty much eliminate concerns over bacteria; because we're using a contact tank for treatment (mostly for a non-health concern reason [iron reducing bacteria]) our water throughput is a bit limited to about 2gpm sustained (40 gallon contact tank; requires a contact time of 20 minutes). I need to do a temperature check on our incoming water.

I want to right-size the boiler. The Manual J output is for the maximum load. I know that at max it is going to be a stretch for a smaller boiler, though when it's that cold the total DHW consumption tends to be reduced anyway; further, coldest parts of the day occur when we're sleeping (DHW loads about zero). Going larger starts to push electrical capacity (we're kind of stuck with 100 amp service due to the primary feeders being sized at this- they're run under a roadway and it would cost something like $10k to upgrade). This is one reason why I'm looking to leverage the boiler rather than going with a separate device for DHW: with just the boiler there's no concerns over concurrent power draws for the two heating requirements. Also, our backup power is only 7.5kw: while one could say that we could get a bigger one, well, let's just NOT say that! (anyone familiar with this generator would certainly understand)

My fingers are crossed that I can get all this, along with potable water distribution bits (manifolds and such), an ERV and an electrical panel, stuffed into an 8' x 7' conditioned mech room (which is under the magic number of 700 cubic feet- will need to ventilate). It's like I'm designing the house around all this!

And... ALL subject to change once I commission an architect and an engineer.


  • mattmia2
    mattmia2 Member Posts: 8,857
    Your difference in cost of materials and labor between a large indirect and a good either plastic or stainless superinsulated electric water heater may be similar to the cost of upgrading the service.
  • bburd
    bburd Member Posts: 779
    In that situation I would probably use propane for heat and DHW, unless you are planning a solar panel electric system.

    Using an electric boiler and pumped indirect DHW tank will involve some losses in the piping and pumping energy compared with a super insulated electric tank water heater, in addition to much higher construction cost and more maintenance (pump). 

  • GroundUp
    GroundUp Member Posts: 1,745
    Why aren't we considering either using a solar tank (with electric backup) in reverse (use the coil for space heating and the tank for DHW) or just a regular old tank water heater connected as normal for DHW with a pumped plate HX as a sidearm for space heating? With the boiler, you'd be spending a lot of unnecessary money on parts and complexity, not to mention the extra space taken up by the extra appliance. A single 5500W element in any tank=18,766 BTU. As much as I love to condone the use of electric boilers, this is not an application where I feel it'd make any sense at all if your local code doesn't mandate an H stamp. If it does, then nevermind
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,139
    Hi, I'll add that if you can put in a shower drain heat recovery unit, it can cut your energy demand for showering roughly in half. That might help. Do you have adequate sun for solar water heating?

    Yours, Larry
  • JakeCK
    JakeCK Member Posts: 1,356
    edited March 2021
    Ahead said:

    [my first posting here!]

    I'm spec'ing out for a new house build: near passive house design as far as efficiency goes. Manual J output on my new house design is coming up with a heating load of 16.7btu (cooling is not a concern). I am trying to figure out how to provide DHW.

    Do you mean 16.7k btus? 17btu's is less than what I release after hearty bowl of baked beans.

  • hot_rod
    hot_rod Member Posts: 20,762
    What are your electric rates? Make sure you include all the fees and access charges. My meter access fees are more than the power cost at my well house. Compare fuel cost carefully.
     You want an electric boiler that modulates as you at at design a small part of the year. A 6 kw boiler with indirect should perform close to what a 4.5 kw tank would. With a minimal DHW use it would not be on DHW priority for excessive periods of time. I’d size an indirect close to what you have now for capacity. Any preheat for DHW would help, a recovery tube as @Larry Weingarten mentioned or thermal, if you could install it affordable
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Ahead
    Ahead Member Posts: 2
    Really appreciate the feedback!

    Jake, yeah, typo ;) Missing the "K."

    Electricity rates (and fees [one of the largest not-for-profit public utility organizations in the country, so no gouging going on here]) are, compared to most parts of the US, cheap: we're talking PNW - hydro power. That said, there's no guarantee that they will stay that way. Pretty certain, though, that fossil fuel sources are going to see larger price increases: I don't relish this as I depend on diesel fuel quite a bit.

    GroundUp, yes, I'd looked at solar and electric. I've considered a lot of things. I dismissed this solution because I am concerned about having two large electrical loads that are self-running (unlike switching on a cook stove or such) operating concurrently. With mains power no concern. On backup power, there most definitely is a concern. Add in the well pump, though only 1/2hp, and there can be instances of big spikes: to-date our generator has managed to handle fairly normal activities when the mains power is down; of course, having wood heat and a propane cook stove makes this a lot easier! Things might be a bit different if this wasn't all in the context of a complete house build from the ground up. Regarding the solar aspect, I comment on this in my response to Larry below...

    Larry, I'm not aware whether I'd be able to do any waste water heat recovery with a single story slab-on-grade house: I am always interested in learning :) Plumbing, in general, and especially waste water plumbing (slab-on-grade), has me a bit nervous: one reason why I'll be looking to run all water supply lines within the building's envelope (and a reason I decided to not do radiant hydronic floor heating [John Siegenthaler's writings on radiant baseboard pulled me in that direction]). Tough call on the solar. When the sun shines it shines. It's the PNW; and, at our place we clocked 101" of rain last year, so clouds are more of the norm outside of the summer season. I do know that it doesn't take a lot of solar to heat water: years ago I attended a training course on solar water heating, in which case I am a bit nostalgic. I figure that my design (indirect tank and plate heat exchanger) would make it fairly easy to plug a solar system in to. Solar during the "cooling" season makes sense.

    Bob, your response kind of matches my thinking of "it could work, but it'll be a squeaker." Your reply means a lot to me (no offense to others!).

    My wife and I are pretty good with being able to adjust. As noted, we already have water constraints via water treatment throughput. DHW is really already baked in.

    Oh! I checked with a vendor of the boiler and they confirmed that it has priority DHW (though the literature doesn't present it as a bullet point- there's the circuitry for it). With an energy efficient envelope I figure that the area heating can readily incur the likely longer interrupts (which wouldn't be all that much).

  • GroundUp
    GroundUp Member Posts: 1,745
    The solar tank would take less power and have more storage than the 6kw boiler with the indirect, was my point. I did not mean to utilize solar thermal collectors, I meant to simply utilize a solar tank in reverse and use the solar input coil as your output for the radiant while the element in the tank does all the heating. Perhaps I am misunderstanding your thought
  • Ahead
    Ahead Member Posts: 2
    GroundUp, you're saying to utilize this setup for both DHW and space heating, no boiler? 6kW is 6kW: the numbers are there.

    I suppose it would then come down to setting up controls/logic for DHW priority, which, I'm thinking, should be fairly simple (though it'll be working my brain cells a bit hard to figure).

    I'd likely have to forgo the plate heat exchanger on the DHW side. Positive would be (I believe you noted it) less energy loss. Bacteria issues, however, would reenter the equation; nothing, however, that couldn't be handled by having the temperature set up high. Mixing valve would adjust temps down for DHW use: not sure on the space heating side- mixing with returning perhaps.

    I'd stashed away info on the Chiltrix units. Just took another look: I'd plucked docs for their 80 gallon tank; saving money on a boiler would mean more money available for a larger tank. Their electric heating element is 6kW, which meets and matches that of the target boiler (and likely less overall losses, so better).

    I appreciate your having patience with me. And I appreciate your input! (I'm now excited to see where this can lead.)