DHW coil maybe clogged. Can I bypass?

[Big Onion]

We have an old Crane Sunnyday boiler that we use for hydronic baseboard heat. We run it in the winter, off during the rest of the year. (I just put $1000 into getting the combustion chamber replaced, so hopefully no one's suggestion is to replace or upgrade it.) I think this is a "combo" unit, as domestic hot water goes through a coil and feeds into an electric hot water heater. I'm assuming the boiler was the only source of heat for both baseboards and hot water, but the eventually put the electric hot water tank in series. We've had real low flow on the hot water side since we moved in a few years ago -- I had chalked it up to a 20 year old electric water heater which finally failed and was replaced. Still have low flow.

I'm assuming from the inches of sediment that was in the old water heater and that I think the previous occupant ran the well water without the softener for a long time that the coil is probably the culprit here.

1. First, with as old a system as this is, would I have a hard time finding a new coil?
2. Second, is it even worth replacing the coil? During winter, when the furnace runs and the water is "preheated", is there a lot of savings that would offset the coil cost? I understand coil replacements to be around $300 or so.
3. Third, is it possible to straight up bypass the coil and just not worry about it? I would assume capping it off would be a horrible idea, but if I just connected the water supply directly to the hot water heater and left the coil pipes open would it be alright? Or is it possible to pull the coil and put a blank plate in its place?

Pic of the front of the unit below. Water is supplied on the left, feeds into the bottom of that plate, comes out on the right and heads back to the electric hot water heater.

 

[Ron Beck]

You can bypass the DHW coil on the boiler and yes you are correct. Leve the coil pipes open. If you cap them and there will be water left inside it could be a problem with rapid expansion and nowhere for the pressure to go. Also, be aware that when the boiler runs right after you do this it may push out a small amount of water.

 

[Sponsor]

 

[Dana]

It's pretty unlikely that you'll be able to find the compatible tankless coil for that boiler, but maybe you'll get lucky. You may also try de-liming it by pumping undiluted vinegar through it for a few hours. But if it has any pinhole leaks (likely) it may end up overpressurizing your heating system. It's probably best to just plumb around it, and settle for electric-only hot water.

If de-liming the coil improves flow, it's still a good idea to plumb around it, isolating it with ball valves until the heating season. Running the electric tank in series and turning the boiler off was a mistake in the first place. In summer drawing cool water through the tankless coil cools the boiler's internals to a temperature lower than the air's dew point, which causes the fire side of the heat exchanger plates and combustion chamber to "sweat" condensation, aggravating the rust and corrosion issues.

Whether the heating with the tankless coil saves any money depends on the combustion efficiency and duty cycle of the boiler, and your gas & utility rates. That boiler might have had a steady-state efficiency of 80% when it was brand new, but it's unlikely to do better than 70% now. And if it's as oversized for the heating load as it looks (probably 3x+) the low duty cycle drops it to an AFUE of 60% or less. Having to maintain a higher temperature to support the domestic hot water coil also increases the standby losses, you could be looking at a net 50% efficiency, with half the heat going into the boiler room & up the flue rather than into your baseboards.

It's too bad you spent the grand on this beast. It's possible to get a much higher efficiency stainless steel condensing boiler for about 2 grand these days.

 

[Big Onion]

I wish I had come here before agreeing to the repair but it was during this last long winter and I had a 2 year old and a pregnant wife. Also hated not being able to get quotes -- no one around us would work on a residential oil-fired boiler. I'd consider the upgrade if it might mean more efficiency in the future. Any recommendations on what to look for? Is installation going to be as much as the boiler? I have 60amp service to the garage that is unused -- is it possible to use some sort of electric for baseboard hot water and eliminate the boiler and fuel tank altogether?

I'd love for something smaller and more efficient. My first year here the oil bill was around $700/mo, just running the system as we found it. This past winter I reinsulated a lot of the plumbing in the crawlspace (a lot of the insulation and deteriorated) and closed off the crawlspace vents (the autovents had rusted, stuck in the open position) and with the new combustion chamber got it to $200/mo. While down there I saw that a section of baseboard had been removed at one point and the risers just capped -- I think the monoflow tee is still there, which would explain why there's so much variation from room to room (nice and hot in the two rooms before that tee!). We're also having an energy audit next month, so we'll be able to see what else we can do to improve.

After posting and thinking about this more, the boiler is ONLY used for the baseboard heat and it's not wired to come on when there's a call for hot water. Thus, the only way it helps is if the hot water happens to be running at the same time the thermostat calls for heat, and I can't imagine that times up perfectly too often.

I ended up plumbing in some a bypass controlled by some ball valves. There was no change in pressure when bypassing the DHW coil, so I'm going to run with the assumption that coil not the problem and it's somewhere else. Cold water comes out like a rocket, hot water like a propeller plane. At least it comes out and it's not a sad dribble!

Thanks for your response -- thought I had notifications turned on but I must have missed this last month.

 

[Dana]

As a starting put to figure out what makes sense for a replacement heating source, using last winter's exact fill dates and amounts and look up the heating degree-day data (HDD) from a nearby weather station to correlate the fuel used to the temperature. From that it's possible to use the boiler as the measuring instrument for measuring the heat load at any arbitrary outdoor temperature using this methodology. Use ONLY wintertime fuel use, fill-ups intervals starting no earlier than 15 November, and no later than 15 March to have the least amount of error from standby losses, solar gains, etc. If the boiler has been tuned up within recent memory with the raw combustion efficiency scribbled on the tag as the efficiency. Otherwise, we'll have to make a guesstimate based on the age & condition of the boiler.

If you want to share that information on this forum I can run the numbers for you.

Then, measure up all of the radiation, zone by zone. If it's all fin-tube baseboard just give the running linear feet (measured at the sheet metal, not the internal fin-tube).

With the 99% heat load number and the radiation number it's possible to estimate the peak necessary water temperature using the BTU/foot ratio referencing charts like this.

Is the 60A panel 240VAC or 120VAC?

What are your electric rates, all-in delivered? (Take the whole bill, divided by the total kwh.)

A simple electric boiler is fairly inexpensive, but operating cost can be pretty high, and it may take more juice than is available on your panel. In your climate a right-sized ductless (or ducted) modulating mini-split heat pump can have an operating cost of about 1/4 what it costs to run an electric boiler, and even though it's more expensive to install than an electric boiler, the "payback" can be pretty fast. How fast depends on your electric rates (or retail cost of oil.)

Do you have central AC? If yes, are the ducts in the attic, above the insulation or are they fully inside the pressure and insulation boundary of the house?

 

[Dana]

Here's a down & dirty fuel use load calc that could be refined with better/actual numbers, if you have them.

Every year is different, but December & January in nearby Winston-Salem the 25 average for those months is about 800 heating degree-days (HDD) each, base 65F.

The average price of #2 fuel oil was about $3.20 in NC last year:

So $200 that would buy you 63 gallons to cover those 800 HDD for a ratio of 63/800= ~0.0788 gallons/HDD. The source-fuel energy of a gallon of #2 oil is about 138,000 BTU, so that becomes (0.0788 x 138,000=) 10,874 BTU/HDD

This old Sunnyboy came in at 72% raw combustion efficiency after tuning it up- yours is probably similar:

So out of that 10,874 BTU only (0.72 x 10,874 BTU= ) 7829 BTU went into the heating system- the rest went up the flue.

Per-hour 7829 BTU/HDD becomes (7829 BTU/24 hours=) 326 BTU/ per degree-hour. That means for every degree F below the presumptive 65F heating/cooling balance point it takes 326 BTU/hr to keep the house up to 68-70F (if that's where you kept it.)

The 99% outside design temperature for Winston-Salem is +23F, so with a balance point of 65F you'd be looking at (65F - 23F=) 42 heating degrees, and an implied design heat load of 42F x 326 BTU/degree-hour= 13,692 BTU/hr.

Going by ASHRAE's recommendation of 1.4x oversizing for the 99% load the heating plant would need to deliver (1.4 x 13,692 BTU/hr=) 19,169 BTU/hr to be able to cover the higher load during cold snaps, and have reasonable recovery rates from overnight setback.

This is just a boilerplate sketch of the heat load- using REAL fuel use, combustion efficiency and local HDD data and making adjustments to account for your actual thermostat settings it could be twice that, or somewhat less than that.

Converting BTU/hr to units of watts for sizing an electric boiler is 3.412 BTU/hr per watt, so an electric boiler to cover the 1.4x load would be (19,169/3.412=) 5,618 watts, which would be covered by a 60A/120VAC circuit or a 30A /240VAC circuit. There aren't very many 120V electric boilers in that range but the 120V Electro Industries EMB-S-4 (5000 watts) would cover it, since that's the 5,618 watts is at the 1.4x oversize factor.

With better numbers on the fuel use gallons & weather along with the raw combustion efficiency numbers, much higher accuracy is possible.

 

[Big Onion]

Thank you for that very detailed response! Here is the fill information from Berico's website, which I wish would show the dates in some kind of order but still:

Excluding the amount filled on 11/19, that's 322.9 gallons were consumed from 11/19 through 1/14. That comes to 3.07 gallons per day, on average (counting 105 days from 11/19/18 to 3/4/19). Compared to the graph you provided, I think we got a good deal on our fuel pricing. (I may be getting a slight discount for being on "autofill" ... regardless, still good!)

They scribbled nothing when they serviced the boiler, but my estimate is that it's original with the house, which was built in 1956. I can try and find the model/serial number on there if that would help. I use a Nest thermostat and set up a data collection script in December 2018, so it has some very detailed date from 12/2018 through to 4/2019 when we stopped using it. Included is outside temp, thermostat setting, and how long the heat was on for that day. You're welcome to access it here.

https://docs.google.com/spreadsheets/d/1OTPUTohteNHzTCy4OStyLAIHf-pD0H0zH6Qw5wZkFDc/edit?usp=sharing

I didn't start this script until December (and simply cannot process the json data that Google sent me for the other time period), so have no usable data for 11/19 through 12/6. I'll use that data from 12/16/18 on, since that's what I have that matches up my oil fill information. We kept it kind of chilly in the house, using space heaters to make up for the 55 thermostat setting. So this might skew some of the data, or just show how inefficient the heater is if the average 3.07 gallons per day. So with the data I have, this is 284.56 hours total usage that burned 245.6 gallons of oil, averaging 0.86 gallons per hour of run time, to keep the house an average thermostat setting during that time period at 61.4 degrees, although the thermostat temperature sensor read an average of 64 degrees, while the average outside temperature was 45.9 degrees.

I used a site to get HDD data for that time period (12/16/18 - 3/4/19) and it added up to 1700. So that 245.6 gallons we used represents (245.6/1700) 0.1445 gal/HDD. If that oil burns at 138,000 BTU then we have 19,941 BTU/HDD. If my boiler is similar efficiency, then we're looking at (0.72 x 19,941=) 14,357.5 BTU to the heating system. Represented per hour is 598.23 BTU to keep the house at that average temperature of 64 degrees (or do I go by thermostat setting of 61.4?).

I'm actually in Greensboro, so the 99% outside design temp is +19F, so the 65F balance point difference is 46F. So 46F x 598.23 BTU/degree = 27,518.6 BTU/hr. Using 1.4x oversizing, 38,526 BTU/hr. If I'm looking at an electric system that typically handles 3.412 BTU/hr per watt, then (38,526 / 3.412 =) 11,291 watts. I was wrong -- it's 30A 240V service, which would not be able to handle this.

Cost per kwh is $0.1036. I would assume some 12,000w electric system, used for 284.56 hrs of heat would be 3,414.72 kwh total, or an estimated cost of $353.76. Is that math right? I spent $678.87 on those 245.6 gallons of oil, so it sounds like I'd be cutting my costs in almost half (not factoring the cost of equipment).

Note - I did not measure up and include the length of heating elements. I can do that in the next day or so if it is meaningful.

FYI, we have central air only on the first floor. Second floor is window units only. I've been eyeballing some mini-splits since the window units are a pain (it's a .5 second floor and two window units occupy 40% of the windows in the two bedrooms) but I've got a small budget to work with and have gotten some outrageous quotes. Considering some DIY (Mr. Cool) systems, but would like to see if a ducted concealed mini-split might be doable given the access to the "attic" space along the one edge of the house, that's less DIY.

Let me know if I can provide more information. I really appreciate your time.

 

[Dana]

"...at that average temperature of 64 degrees..."

If you're keeping the house at 64 degrees use base 59 or base 60HDD not base 65F, assuming it's a 2x4/R11-R13 type house. That will result in higher BTU/HDD slope constant due to the fewer heating-degrees.

Then add 4 degrees to the total your heating degrees to figure out what it takes to keep it at a code-minimum 68F indoors when working a fuel-use load calc. For NEST data when it was kept at 61F in the house use base 57F. Degreedays.net has very good weather station coverage for most of the US, and allows you to adjust the base temperature over a wide range.

The fact that the boiler is in the garage (?), outside the fully conditioned space means you're distribution losses are something like 15% of the total use.

Cost per kwh is $0.1036. I would assume some 12,000w electric system, used for 284.56 hrs of heat would be 3,414.72 kwh total, or an estimated cost of $353.76. Is that math right? I spent $678.87 on those 245.6 gallons of oil, so it sounds like I'd be cutting my costs in almost half (less the cost of equipment).

The cost calculation looks right.

Mind you, even a cheap 1 ton Mr. Cool DIY has an HSPF of 8.8 (= 8800 BTU/kwh as opposed to 3412 BTU/kwh with an electric boiler), and if right-sized for the zone load would actually beat that average efficiency, and per it's AHRI rating can deliver at least 12000 BTU/hr @ +17F outdoors 70F indoors. It will have somewhat more capacity than that at 64F indoors.

The 3/4 ton ducted Fujitsu 9RLFCD uses a more efficient cold climate vapor injection type compressor, and can deliver 16,000 BTU/hr @ +17F, with an average HSPF efficiency of 12.2 ( 12,200 BTU/kwh). You can beat that efficiency in practice in your location if right-sized, and you're keeping the house at 64F.

The AHRI heating-hours zone maps put Greensboro in zone III, whereas the HSPF efficiency models it for Zone IV, sometimes also Zone Vwhich is somewhat cooler for a higher heating duty cycle, but also a lower COP efficiency:

Of the ducted mini-splits Fujitsu units have the most flexible installation configurations, since they can be mounted vertically or horizonally and the cooling season condensation can still drain correctly. Most are horizontal-only. They also have a large turn-down ratio (all sizes can modulate down to 3100BTU/hr), partly due to the more sophisticated compressor design.

Midea has some attractively priced ducted mini-splits that use Toshiba's compressor technology, which might be a reasonable to do as a "DIY-mostly". I don't know too much about them (I'm usually looking at equipment for heating design temperatures 10-25F colder than yours.) Carrier mini-splits are all rebadged Midea units these days- it's a first-tier Chinese manufacturer, after buying into Toshiba's compressor factory over a decade ago. The ones listed on the NEEP (New England) site are only the larger sizes, but they make as small as 3/4 ton and 1-ton, at HSPFs in the 10s.

When designing duct systems for ducted mini-splits pay careful attention to the static pressure specs. The higher efficiency Fujitsus can handle 0.4" w.c., many of the competitors on the smaller sizes are only good for 0.2", as compared to the ~1.0" w.c. most bigger air handlers are specified at. Some mini-split vendors now offer "mid-static" version that can handle up to 0.8" w.c., but they're usually slightly lower efficiency and more expensive. If you design the duct velocities to stay <400 feet per minute at the maximum blower speed and use hard-piped duct (or tightly stretched flex), only radiused and use only large pleated filters (eg Aprilaire 2400) the static pressures stay low enough to use the wimpier duct cassettes. Using smaller registers and tapered boots is sometimes useful to get good mixing in the rooms at the lower cfm.

This is a 1.5 ton Fujistu mounted vertically in a 7-8 square foot "utility closet". The big grill is the common return & filter- the cassette is mid height, tied to a short fat plenum with flex running to four separate rooms (some of those ducts ended up in soffits below the ceiling level.)

A horizontal mount can sometimes fit under the ceiling in a closet, keeping it fully inside the thermal and pressure boundary of the house to avoid distribution losses and air-handler driving outdoor air infiltration, which becomes important at the high temperature differences in heating mode (50F peak delta compared to 25F in the cooling season)

If your first floor AC ducts are reasonably routed mostly inside of conditioned space it may be worth swapping in a right-sized heat pump (or a right-sized mini-split using the existing ducts.) Even a pretty-good HSPF 8.5 two-stage Goodman can be dramatically cheaper to heat with than an electric boiler. Most existing central air is grossly oversized for the actual loads- it's worth running a Manual-J, even a freebie/cheapie version such as CoolCalc.com or Loadcalc.net is better than a WAG. If you also have NEST data on the AC cycling you can probably infer the cooling load from the duty cycle, which would be even better yet. LoadCalc usually oversizes by 25-35% even when selecting "tight" construction type, so be aggressive rather than conservative on the R-value assumptions and duct tightness, etc.

Upsizing a modulating mini-split by 1.3x for either heating or cooling is fine of that's what it takes to cover both, 1.5x only if you must, but beyond that it starts to inch down in as-used efficiency ( with a few notable exceptions for "hyper heating" or those with cold climate compressor types, like the Fujitsus.) Look up the capacities from the extended temperature capacity tables in the engineering or installation manuals- the AHRI nominal capacities are based on temperatures that can differ from your actual 1% & 99% outside temperatures and humidity conditions. If using a fuel-use based load calculation you'll have to guesstimate the distribution losses of the boiler relative to those of the ducts & air handler. Just the standby losses of an old way-oversized boiler in the garage can be pretty bad, so it's unlikely that your ducts & air handler for the first floor would be worse (TBD.)

You might want to read up on this (get a free trial subscription) and the related articles on that site before finalizing any heat pump or mini-split decisions.

 

[Big Onion]

That's a lot to take in so I'll need to take some time to read through it and study up! Thank you for taking the time to respond.

I think basically what you are saying is that adding a heat pump to the first floor central AC system (I'll have to look into how that all works, our current unit is very, very old and probably has a short lifespan anyway) and then adding mini-splits upstairs would ultimately be more efficient than using the boiler at all. I hate not utilizing a system that is installed and in place (hydronic baseboards), but if there's no real way of making the means by which it works (heating up the water and circulating through the house) happen more cheaply than it does not, then I suppose looking to a bigger change makes sense.

I love the idea of the ducted mini-split on the second floor, but that might be more of a job for a pro. I can't imagine that installing that unit and running ducts (I can handle the work to hide it, such as was done in the pic you included) would be a low cost job, though. I have a friend who flips houses and has a "guy", so I may have him come out for a consult. In the winters it's wickedly cold up there and in the summer it's extremely hot. Something needs to be done, certainly.

I have a small budget I can work with for this year ($2-3k max) so I might need to stick with just trying to make what we have work a little better and next year really look into a larger plan on what to do. I'll be getting under the house to remove that leftover monoflow tee from when they cut out a section of baseboard, but that might be it for this year if everything else would be major work.

Again, thank you for your input.

 

[Dana]

If you have enough baseboard to deliver the heat at low temperature you could heat the water with a reversible chiller such as the 3 ton Chiltrix CX34, but that's a more serious design project.

If the ducts are in good shape, yarding out the old AC unit and installing a right sized heat pump sized for the first floor's loads would be a good start.

If the upstairs is "...winters it's wickedly cold up there and in the summer it's extremely hot..." the first $2-3K is almost certainly better spent on air sealing & insulating the place, with an emphasis on air sealing prior to insulating. Many states have programs subsidizing this sort of thing, but I'm not sure what's available in NC. (The means-tested NC low income weatherization program reads like a sick parody of what really needs to be done to make a place comfortable.) The most important leaks to fix first are those at the top of the house (stopping the air leaking out of the attic, which gets complicated in kneewall attic spaces) and those at the bottom of the house, such as the foundation walls, foundation sills, and band joists. This is because the stack-effect infiltration drives are function of the vertical distance between the leaks. A square inch of air leak at the mid point moves very little air compared to those at the bottom & top of the house.

Not all air leaks are obvious, but some are, but going at it DIY can be dirt cheap and quite effective once you get past some knee in the curve. Start with the obvious stuff, bigger leaks first and work it down until you're stumped. (Don't be afraid that you'll make the place "too tight"- that literally never happens on the first pass, and only rarely when pros with the experience and tools are going at it.) When done with everything you can think of, it's possible to find most of the rest using a big reversible window fan and a pistol-grip infrared thermometer once the indoor to outdoor temp is more than 20F. Depressurising the house or room with the fan blowing out will suck cold air in through leak paths, making cold patches around where it's leaking in. ( It's also possible to find gaps in the insulation with the IR thermometer even without the fan.) If you're a gadget-geek, a $200 Flir One infra-red camera that uses your phone or tablet computer makes it even easier to spot leaks and defects in the thermal envelope of the house.

With a better description of the house construction more pointed suggestions might come to mind.

While thinking about this stuff it's worth reading Nate Adams' freebie Home Comfort Guides, including the videos. Nate was a frustrated insulation contractor when working under his state's weatherization programs because they were too narrowly defined, and didn't address the whole house a system. Insulating a house with a 5x oversized furnace may save energy (or not) without making the occupants more comfortable, etc. He has since made a business out of rehabbing older homes for comfort with the right balance of weatherization/insulation and the right sizing of the equipment. Many of his projects are described in detail on his blog page (scroll down to Case Studies for links.)