Cut out tankless coil or keep with HPWH?

[Nsherman2006]

Hi all,

I've been searching for answers, and have found lots of great info on this site, but I'm still scratching my head about this one. I don't think there's an "answer" I'm looking for so much as advice. I bought a 1960s single-story ranch with an oil-fired boiler for both baseboard heating and DHW. Boiler appears to operate fine.

I'm in the process of installing solar panels that will likely outproduce my energy usage, so I'm trying to transfer as much of the heating load to electric as possible.

My current plan is to install 2 mini-split units (1 on the main floor and 1 in the basement) to provide both heating and air conditioning, as well as help manage moisture in the basement. I'm also planning a hybrid heat pump water heater (again for both heating and moisture management).

If this was new construction, I wouldn't bother with a boiler at all. However, since I already have the boiler here and working, I'm planning on leaving it installed. This will be particularly useful in the case of a power outage as I can run the furnace off a generator. (I'm in northeast CT so extended winter outages are not unheard of)

So, my options:

1) Leave the boiler in place, but decomission the tankless coil. I'm assuming this would require a modification to the aquastat so that it doesn't call for heat for DHW anymore. This would also leave me with no hot water during a power outage, but that's not a huge deal

2) Remove the boiler entirely. This would be pretty simple, but I don't see any reason to go through the work to tear out a functional system

3) Somehow plumb the HPWH to be the primary source of hot water, but leave the tankless coil operational for supplemental heating or to provide DHW in the case of an outage.

I'm leaning toward option #3. Is it possible to just connect the outlet of the HPWH to the inlet of the tankless coil, so that the HPWH is the primary water heater and the tankless coil supplements it in the case of water temperature dropping? Are there any reasons this is a bad idea?

The thread that seemed the most relevant to this one is:
https://terrylove.com/forums/index....r-heater-alongside-ancient-boiler-coil.69098/
But I think my situation is slightly different with the solar installation making electricity nearly free.

Thanks!

-Neal

 

[Dana]

However, since I already have the boiler here and working, I'm planning on leaving it installed. This will be particularly useful in the case of a power outage as I can run the furnace off a generator. (I'm in northeast CT so extended winter outages are not unheard of)

What IS (almost) unheard of is an oil boiler that runs without electricity. Without electricity the burner doesn't spin or ignite, the aquatat controls don't work, etc. Since the oil boiler doesn't run in the event of a power outage either, and the HPWH buffers a LOT more heat, so those aren't grounds for keeping the tankless coil hooked up.

Until you're sure that the mini-splits are really going to work well for your house, keep the oil boiler. A couple of ductless heads may leave some rooms cold, but if one of the mini-splits is ducted that issue may go away. The main risk of keeping an oil boiler that isn't really used is the cost of clean up when the oil tank eventually leaks. If you still want to use the existing radiation as backup, a right-sized electric boiler is pretty cheap to install (and 3x as expensive to run as a mini-split.) To size the electric boiler, use the oil bills from last winter to calculate the design heat load based on fuel use using this methodology. With two mini-splits you don't need to up-size the electric boiler to cover the higher loads of cold snaps, since it's unlikely BOTH mini-splits will be out of commission at the same time.

It's not hard to plumb the HPWH & tankless coil so that the either the coil OR the HPWH can be cut in/out using isolating ball valves. It's not a good idea to try use them both at the same time. You actively don't want to plumb them in series, using the coil as "pre-heat-maybe-backup" for the HPWH. With cold water running through the coil of a boiler that isn't being maintained at temperature there will be considerable water condensation & corrosion on the heat exchanger plates over the summer, reducing the life and efficiency of the boiler. If the HPWH is feeding the tankless coil in series the boiler will suck heat out of the flowing hot water unless you're keeping the boiler temp above the HPWH temp, suffering ALL of the standby loss, for no benefit in efficiency. If you turn off the boiler (or it fails), the HPWH would be heating up the boiler as the water flows, cooling the water before it hits your distribution plumbing. The only rational & efficient way to do it is as an either/or backup, plumbed in parallel, not in series.

It's your call, but I personally wouldn't bother plumbing in the boiler as backup.

 

[Sponsor]

 

[Nsherman2006]

Dana,

Thanks for the comprehensive reply!

What I was saying is that in case of a power outage, I already have a generator wired to provide power to the furnace, fridge, and some other circuits. A hot water heater would require removing 2 circuits from the existing transfer switch to provide 220V, and I would prefer not to do that.

The oil tank is only 3 years old, so it's not reason to abandon the system, and as much as I would love to have the appropriately sized boiler...I don't see the point in putting money into a system that is basically backup just to achieve a slight efficiency gain. So I think that I will likely go ahead with leaving the current system as is.

I didn't think of the boiler acting as a heat sink if the coil is plumbed in series with the HPWH, so thank you for that insight! It looks like isolating each system with ball valves and keeping both seems to make the most sense. I'm thinking that the cold water inlet would go to a tee that splits to go to either the coil or HPWH, and the outlet of each would join together at a tee to the domestic hot water supply, with ball valves before and after each of the coil and HPWH to isolate each (as you said, plumbing them in parallel).

With this solution, should I be concerned about leaving stagnant water in the coil? I'm assuming that I would still have to modify the aquastat to prevent the stagnant water in the coil for calling for heat?

Thanks again!

Off-topic: Where is that bump run in your profile pic? Looks like a blast!

 

[Dana]

It's worth purging the stagnant water in the coil by running several gallons when first switching over to the coil, but it's unlikely to grow pathogens running at basement room temperatures with the boiler off.

Typically the tankless coil isn't controlled by a separate aquastat- is yours? Most just rely on setting the idling/low-limit aquatstat on the boiler to a temp that still provides adequate hot water performance.

As for the profile pic, Terry came up with that- I'm not really sure where that is, and doesn't quite characterize my typical skiing venues. A google images search came up with this. It implies that it's somewhere in NC, but it really could be anywhere.

When in fixed-heel bindings in winter I'm usually running gates (or preparing to), or in the spring simply goofing off, (< that's at Mt. Wachusett more than a month after the lifts stopped running). I'm not often running bumps these days- my back can't really take a full day of it anymore. I tend to favor telemark mode both at resorts and in places like this (<that's on the Muir snowfield on Mt. Rainier, WA) for recreational skiing, winter, summer, or in-between.

This is a shot of me ducking branches skiing down the Metcomet Menadnock hiking trail on Mt. Grace MA (a local little known out of the way snow-pocket when the wind is right) where there is still the remnants of a tiny abandoned ski area:

^^^Maybe I should swap that in as my profile pic, even though it's over a decade old and out of focus (?). (That was a blast!)

^^Here is one is me dropping into a steep-ish gully on the north slope of the Sourdough Ridge (Burroughs Mt., inside Mt. Rainier Nat'l Park). The entrance fee that day was a 2-ski-length wide shot through the rocks on a ~50 degrees pitch (similar in pitch to the Left Gully at Tucks or the Airplane Gully in the Great Gulf in the Whites, only narrower.) That shot is more than 15 years old (I'm surprised it's still on line!), my first run on a flexy pair of ash-core prototype Scotty Bob's back when they were a start-up. (Perhaps not the best ski for hard summer snow and steep pitches, but what the hell- I had fun anyway! I still have a pair of the stiffer maple-core ScottyBob's in the quiver.)