Electric Water Heater as Indirect?

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smyke

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Hello,

Newbie here.

I am in the process of buying a house that we want to convert to NG. It has an old oil boiler for heat and electric WH (about 8 years old). I do not have the specifics on either piece of equipment yet.
Once we close on the house in a few days I will have contractors come in and sort it all out but for now I am just trying to understand what my options are.

My question is: can the existing Water Heater be used as an indirect tank?
In my head it seems pretty simple: hot water from the boiler goes into the tank which will then push it up into the system. Electric element may help in keeping the temp where it needs to be if necessary. Is it this easy? I know I am missing something.

That way I only have to have a new NG boiler installed and utilize whats already in there.

Any and all help will be appreciated.

Mike
 

Jadnashua

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An indirect uses either a water jacket or heating coils to heat the potable water...the boiler's water and the potable water do NOT mix, and a typical WH does not have any way to port that boiler water into the system to heat the potable water. If the tank was designed for say solar, or something similar, it might have a dual mode, but a generic WH cannot be used as an indirect. Essentially, an indirect operates as a heating zone, often a priority zone, so it gets heat first.
 

hj

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Maybe not the simplest, because you CANNOT mix the heating system and hot water system, because the operate at different pressures, and also mixing them would create ENORMOUS problems with corrosion and boiler failures.
 

Dana

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If the electric hot water heater is in good shape there's no financial case for going to an indirect right away.

If you're replacing the boiler, take time to analyze the boiler sizing very carefully for your actual heating load. Almost all oil boilers installed in southern New England homes are 3-5x oversized for the space heating load, and when operating at such a low duty-cycle they never hit their AFUE efficiency numbers.

If history is any guide, 19 out of 20 boiler installation contractors do not run a real a heat load calculation, and will either just size the boiler to whatever your radiation puts out at an average water temp of 180F, or use some idiot's rule-of-thumb of xx BTU per square foot of conditioned space, either of which will usually oversize the equipment by 2x or much more.

The gold standard is to have an energy nerd run an ACCA Manual-J heat load calculation based on the construction details of the house, the electric plug loads, the number of occupants etc to come up with the number. A simpler sub-set of a Manual-J would be to run an I=B=R spreadsheet based on the construction details, with some allowance for air infiltration etc. If done correctly an I=B=R methods calculation will only oversize by about 10-15%, but many on-line or freebie download I=B=R tools overshoot reality by 25- 50% (usually with ridiculous assumptions on air leakage rates.) It's possible to screw up any type of calculation (garbage in= garbage out), and many HVAC contractors doing their own Manual-J's err to the conservative side, oversizing by 2x anyway. To size it correctly you need to be fairly aggressive rather than conservative, and use a bit of judgment.

The ASHRAE and ACCA recommendations for oversizing factor is 1.25-1.4x of the heat load at the 99% outside design temp, which is more than enough to cover you during Polar Vortex coolth. (In Vernon your 99% temperature bin is about 3-5F- definitly in postive single digits. But don't be surprised if an HVAC contractor tries to use -5F or something simply because it gets that cold every once in awhile.) AFUE testing presumes 1.7x oversizing, but that's very close to the knee in the curve beyond which as-used efficiency starts to fall off fast. If you oversize by no more than 1.4x from the calculated number of an aggressive I=B=R or Manual-J you'll probably stay within the 1.7x sizing and it'll still hit it's numbers, but it's safer to shoot for 1.25x. If contractor bias puts even a pinky on the scale it's still very easy to blow it if oversizing from a raw calculation.

With a fuel use history it's possible to verify the calculation, but since you can't vouch for the thermostat settings or auxilliary heat use there's no way to apply those methods without some risk here.

Bottom line, don't let the contractors sort it out. This is a once in every 25 years shot at right-sizing the equipment. There are several threads on this site and others detailing how to go about running your own calculations, but when the time comes start a thread on the Boiler forum here and we can walk you through it.

When installing the new boiler, make sure the system has a zone controller with "priority zone", but leave that zone free for future expansion should you opt to install an indirect.
 

smyke

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Sounds like a plan Dana.

I have a contractor (same one that has been servicing that house for the last 20+ years) coming out a day after closing and we shall go from there.
I am curious to see what they come up with.
Apparently there is a Utica coil in the current boiler that heats the water before it goes to the WH.

Our first order of business will be to blow some more insulation into the walls, but we will have Eversource do the audit first.
 

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With a hot water coil in the beast it's usually at least 3x oversized for the space heating load, and hitting abyssmally below the labeled AFUE efficiency.

They may have put the electric tank in series with the boiler so that they could just turn off the boiler for the summer to save oil, or it's possible that the tankless coil in the boiler wasn't delivering adequate hot water performance due to the boiler being too small to serve both loads.

Since you're making thermal envelope upgrades on the house, the load calculations should be on the "after" picture, to avoid oversizing.

A heads-up: The true heat load of a typical retrofit-air-sealed 2x4 framed 2500' house with an insulated basement with R13 in the walls and a sub-code R25-30 in the attic and clear-glass double panes or clear storms over wood sash single panes is on the order of 35,000-40,000 BTU/hr. If it has no foundation insulation it might be 45-50,000 BTU/hr. Most old-school boiler contractors in our region are going to tell you it needs something like 60-90,000 BTU/hr (so you'd be good down to -50F or -75F or something). There are many houses in central CT with heat loads under 30K being served by boilers with over 100K of output.

If the oil boiler is in good shape the cheapest legal thing to do might be to install a retrofit gas burner on the thing, but that's not necessarily the right thing to do if it's 3x oversized for the post-insulation heat load.
 

smyke

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The house is 1600 sq ft ranch. Currently no insulation in the basement (2 garages) and unknown insulation in the walls. Attic is insulated with some old loose stuff and covered with plywood.
Lady that currently lives there used 1000 gallons of oil over the last year but she kept it very warm in there according to the oil company. I forgot to ask for the specs on the boiler, but will find out in a week once I get the keys. The coil is a Utica POBT-5 and the WH is AO Smith ECT-52.

I only scheduled the contractor to come out now so I could possibly have enough time for Eversource to hook me up to the gas line by winter.
I may have to wait with the changeover to gas and insulate the house first and go from there. Still not sure what to do.
 

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Oil is cheap right now relative to the recent 5-year average ( though still quite a bit more expensive than natural gas per delivered-BTU), so it's not insane to focus on tightening up the place first.

If you're going to be heating with oil for a season it's worth downloading the NORA FSA calculator tool, which uses boiler models based on testing done at the Brookhaven National Lab to estimate the heat load from fuel use based on a "K-factor", and the nameplate efficiency of the boiler. (The paper is fairly illustrative on just how badly efficiency suffers with gross oversizing. Take a look a the regression curves of the various tested systems in the appendices.)

At tightened up garage-under 2x4 rancher that size should end up with a heat load under 30,000 BTU/hr @ 0F, and quite possibly under 25,000 BTU/hr, which puts you potentially in mini-split heat pump territory, which is cost-comparable to 85% efficiency gas. So it's probably worth spending the initial money on tightening up, then deciding the HVAC solution based on the actual heat load, the existing radiation, etc.

Is the boiler in the garage, or is there a half-house basement with a boiler room as well?
 

smyke

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There are 2 single car garages with the basement (will be eventually partially finished) between them. Boiler is in the basement and its not closed off in a room. There are some plywood partitions put up separating garage areas from the rest so its not all wide open.

Funny you mention mini-splits as that's what I was thinking about for AC. I figured the heat from that could be supplemental at least.
We are at 8 cents per kW right now so maybe not a bad idea.
 
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Dana

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Be sure to insulate the basement (even before you finish it!), even if Eversource won't spring for any subsidy on it. They would probably subsidized insulating and air-sealing the foundation sill & band joist with a couple inches of closed cell spray foam- be sure to ask about that during the audit.

If the full-in retail cost of electricity (including grid costs and other fees) is only 8 cents, that's less than half what I'm paying a few miles north of you in MA. With electricity that cheap a cold climate mini-split would heat for under half what it costs heat with $2 oil with a best-in-class boiler.

An un-insulated non air-sealed partition wall separating the garage from rest of the basement is a huge heat leak that needs to be dealt with too. It's impossible to make a garage air-tight to the outdoors, so it's a major air leak at the bottom of the house, which maximizes the total stack effect pressure driving conditioned air out the top of the house.
 

smyke

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No, its not all-in price. Fees are more than the actual cost of power used.

I will see what the Eversource audit will get me and go from there. They haven't even called me back yet. Hopefully they get around to it before winter comes.
 

Jadnashua

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Dense packed cellulose is better at stopping air movement. Fiberglass works well as an air filter, and can work well if there are no air leaks to start with.
 

Dana

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Both have their optimal uses, depends on where.

For the partition wall between a garage & conditioned basement, R15 rock wool is probably a better bet, with an inch of fire rated polyiso (Dow Thermax) on the interior side, which makes it substantially more fireproof than either low density fiberglass (worst) or 3lb cellulose (better).

For insulating the foundation walls, an inch or more of rigid foam (any type, but seams taped & edges sealed) trapped to the concrete with a non-structural 2x4 batt-insulated studwall is fine. To meet full code-min performance you'd need R15s if going with only 1" EPS, but with 1" polyiso or 1.5" EPS you'd get there with cheap contractor-roll kraft faced R13s. Put an inch of foam (EPS or XPS, but not polyiso) under the bottom plate of the studwall as a capillary & thermal break. EPS i& polyiso are both cheaper per-R and greener than XPS, due to the far lower impact blowing agents used.

Don't put wood in contact with the foundation wall or floor in a zone 5 climate.
 

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Dense packed cellulose is better at stopping air movement. Fiberglass works well as an air filter, and can work well if there are no air leaks to start with.
 

Dana

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Dense packed cellulose is better at stopping air movement. Fiberglass works well as an air filter, and can work well if there are no air leaks to start with.

High density blown fiberglass (1.8lbs per cubic foot or higher) is every bit as air-retardent as 3-3.5lbs density cellulose. So are R15 fiberglass or rock wool batts. But batts don't offer the same air-sealing aspects of blown fiber. Blown fiber gets pushed into the air leaks of a wall cavity during installation, whereas with batts it's far more important to air seal that cavity first. It also takes near-obsessive attention to detail to install batts perfectly, whereas blown fiber finds the voids and fills them, with no compressions or gaps.

For the above grade walls there's some advantage to using cellulose than fiberglass, despite the slightly lower center-cavity R. Unlike fiberglass, cellulose is hygroscopic, and will safely wick & redistribute and buffer wintertime moisture accumulations away from the structural wood, lowering the mold risk. For below grade walls there's some risk of cellulose accumulating ground water coming through the foundation. If there's even a remote chance of minor flooding it's better to stick with non-hygroscopic foam + fiberglass or rock wool, since the mineral or glass fiber can often dry in place and won't wick moisture all the way to the top of the cavity with only 4" of water on the floor the way cellulose will.
 

smyke

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I should have specified. I want to blow cellulose or fiberglass into the walls and for the basement walls I was thinking rigid board.
 

Dana

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I should have specified. I want to blow cellulose or fiberglass into the walls and for the basement walls I was thinking rigid board.

How the wall is built may make a difference on what's the safest/best way to retrofit insulate the walls.

What is the stackup of materials on the above grade walls, from the exterior paint to the interior finish, eg:

3 layers oil paint | asbestos shingles | #15 felt/tarpaper | 1x8 ship-lap plank sheathing | 2x4 framing | wood lath | hard plaster | foil wallpaper

In the basement it takes at least 3" of rigid foam to hit code-min with an all-foam solution which is fine, but expensive to do with virgin-stock foam (much cheaper with reclaimed roofing foam), and makes it difficult to route the electrical. (If the rigid board is EPS it takes 4".) It's usually easier to do the combination of 1-2" of rigid foam trapped to a poured foundation wall with a non-structural fiberglass or rock-wool insulated studwall. In a combi stackup it's good to have at least R5 of foam on the above grade section for dew point control on a 2x4 fiber insulated studwall. Putting foam between the concrete and studs/fiber keeps ground water from wicking in or entering the susceptible wall cavity creating a mold problem. Putting a minimum of R5 foam allows you to use unfaced batts with no interior vapor retarders other than latex paint, so any moisture (whether vapor , or incidental liquids) can dry toward the interior easily, and the fiber/foam interface on the above grade section will have an average wintertime temp above the ~40F dew point of the basement air. That takes at least 1.25" if EPS, but only 1" if polyisocynaurate. More foam is of course better, and there is a lot of 2"-3" polyiso and 3"-4" EPS roofing foam available through the recyclers for dirt-cheap.

In southern New England there are multiple vendors of reclaimed roofing foam, which runs 25-35% of the cost of virgin-stock, which can make it cheaper than high-density batts. (I did my basement with 3" reclaimed roofing polyiso.) If you search the local craigslist you'll find some of the smaller vendors. The larger reclaimed foam vendors are Green Insulation Group in Worcester, MA and Nationwide Foam Recycling in Framingham, MA, but there are others.

If you use polyisocynaurate rigid board against the foundation wall it's best to keep the bottom edge of the foam off the slab, since it can slowly wick ground moisture. If there's a flooding history stop the polyiso at the high-tide mark, and use only polystyrene (EPS or XPS) below that. It's also important to put an inch or more of polystyrene between the bottom plate of the framing and the slab as a thermal & capillary break, so that the bottom plate doesn't wick ground moisture or adsorb air moisture.
 
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