HW heater feeding existing tankless coil

[bc3510]

Good evening,

I'm a degreed mechanical engineer and have an 1800 ft. sq. colonial built in 1921 with typical 2x4 construction. It currently has an American Standard Arcoliner oil fired boiler firing at .85 with an efficiency of 67%. The location is Long Island, NY with the coldest day of a typical year approaching 0F.

I have modified the supply system extensively and it currently has two supply and two return loops.............each to opposite sides of the dwelling. The loops are all 1" copper. From the loops are 3/4" feeds to each of the nine cast iron radiators............each equipped with a Honeywell zone valve.

The system is controlled by two Taco zone control units. One of them has an input directly from the tankless coil and monitors its temperature.............closing the zone valves if the boiler drops below 150F. at the coil. The coil is massive..........12 gallon rating and it provides hot water at 130F even with 150F boiler water.

150F. boiler water is just barely sufficient to maintain heat in the dwelling to 70F. on a 0F. day. It is not sufficient to climb if the thermostats are lowered overnight.

The nine radiators provide 572 square feet of EDR.


I'm a bit fond of the American Standard as I can easily maintain and clean it myself. Of course, the expense of oil is getting a bit onerous. The dwelling does have natural gas available (currently utilized only for cooking).

My original thought was to simply replace it with a gas boiler and separate hot water heater (basic unit without electricity required).

HOWEVER, after mulling it over a bit...........why can't I do the following:


Install a new higher capacity hot water heater such as the AO Smith GPVX-75L. With 72K BTU it could easily heat the dwelling.

I would run the heater at 160F and use the side connections to feed the tankless coil on the existing boiler. I would estimate that it could bring the boiler water to 150F. given sufficient flow. This would be sufficient to heat the dwelling on all but the coldest of days. At that point, I could always start the oil burner.

Of course, I would utilize a tempering valve for the DHW.

It's a simple and elegant solution and very cost effective (other than the venting...........that's currently a problem).

Thoughts?

[BadgerBoiler MN]

Too many moving parts. How about a new condensing boiler with outdoor reset and an indirect water heater--no moving parts.

Tankless coils have been obsolete for some time now. With the notable exception of the BoilerMate, all indirect-fired water heaters are very well insulated and produce standby losses below 1/2°F/hour. This in contrast with your tank-less coil and poor boiler insulation combined with an open draft hood and energy-sucking chimney.

I would shut down the flue and save 5% on my fuel bill. Then run a 2" PVC vent to the rim-joist. All condensing boilers feature outdoor reset; save another 5-10% on your fuel bill while improving comfort. Finally, the indirect with virtually zero standby losses and a 30 year life-span sized to any load you likely could produce and you have the perfect integrated heating system.

[bc3510]

Too many moving parts. How about a new condensing boiler with outdoor reset and an indirect water heater--no moving parts.

Of course, if I was capable of supporting such a system for the next 10 years, without assistance, that would be the direction I would take.

However, maintenance and repair on the condensing units are your bread and butter and one call per year would easily consume any fuel savings to be derived from them. From my perusings online, one call per year would be a truly exemplary performance from them.

My suggestion has no moving parts other than one additional circulator.

Sure, the existing boiler loses a decent amount of heat to the basement, and, I might consider replacing it with a .82 gas fired boiler in the future, if my proposed system doesn't pan out.

But, understand the the current price for 100K of oil is $2.53 and the current price for 100K of gas is $1.25. This assumes equal efficiency between the American Standard and the new AO Smith HW heater. I'm fairly sure the water heater would beat the American Standard somewhat.

Also, the cost of my proposal is less than $.

[pecmsg]

With 150*f supply water on a 10*f day does the boiler cycle on and off?

[BadgerBoiler MN]

Hard to argue with the price. I use a lot of water heaters for space heating--mostly condensing like Polaris. Maintenance on condensing boilers is highly dependent on the initial installation and reasonable maintenance after the fact--about a dollar-a-day for hard-working systems. I just worked on a 10 year old system that had never been serviced; a couple of grand later, just like new. And another similar boiler, same run time and fifteen hundred; same result.

As far as "consuming fuel savings; this is a highly subjective issue. The bigger the fuel bill the bigger the savings. We often pay for new condensing boilers on commercial buildings in two seasons or less, here in Minneapolis and all of our heated driveways have great returns when ease of installation and total fuel savings are accounted for.

I would prefer a condensing water heater with a plate HX to separate potable from space heating, still, a few thousand to get started.

The new water heater will likely combustion test at 80%. A new atmospheric boiler should be 85% and I would be happy if you lose the tank-ess coil.

Your plan will work.

[bc3510]

With 150*f supply water on a 10*f day does the boiler cycle on and off?

I typically keep the entire house at 60F. with the exception of my office. So, in answer to your question, the boiler does cycle (estimated duty cycle of 50%) to maintain a temperature delta of 50F. in the house.

The boiler used approx. 6 gallons of oil per day in January and February firing at a rate of .85 gallons per hour. Granted, the average temperature in those months was probably 25F.

[pecmsg]

I typically keep the entire house at 60F. with the exception of my office. So, in answer to your question, the boiler does cycle (estimated duty cycle of 50%) to maintain a temperature delta of 50F. in the house.

The boiler used approx. 6 gallons of oil per day in January and February firing at a rate of .85 gallons per hour. Granted, the average temperature in those months was probably 25F.

Like the rest of LI its probably oversized.

[bc3510]

Like the rest of LI its probably oversized.

I believe it has the capability to fire at 250K and the requirements of the house are not more than 60K.

It is absolutely amazing how just about every boiler sold in this area is ridiculously oversized.

If I do replace this boiler, I'm getting a 70K unit from either Bryant or Smith. It'll definitely be an induced vent model at .82.

[bc3510]

I would prefer a condensing water heater with a plate HX to separate potable from space heating, still, a few thousand to get started.
The new water heater will likely combustion test at 80%. A new atmospheric b
oiler should be 85% and I would be happy if you lose the tank-ess coil.

Your plan will work.

I am convinced that you are correct. The standby losses from the existing American Standard are huge. Why not park it (holding it in reserve for any brutally cold day) and use the plate HX as you suggest:


http://www.zorotools.com/g/00052685/k-G0761643?utm_source=google_shopping&utm_medium=cpc &utm_campaign=Google_Shopping_Feed&kpid=G0761643&k w={keyword}&gclid=CN3Zz4rltb0CFYyhOgodNHQA9Q


Would seriously reduce the volume of water that needs to be maintained at 150F.


Is there a preferred mounting configuration for these things? Where do they live?

[R Mannino]

I'm with the Badger on this one, get rid of that dinosaur and upgrade to anything with natural gas.



This one went to the scrap yard on Thursday.

[bc3510]

I'm with the Badger on this one, get rid of that dinosaur and upgrade to anything with natural gas.

There is no question about switching to NG. The only question is which way to go.

Conventional wisdom uses a new gas boiler with an indirect HW heater.

If I chose a gas HW heater that is sufficiently large (70K), I could provide HW and domestic heat for about 1/2 the cost of the conventional wisdom solution.

I agree that attempting to backfeed the existing tankless coil is probably foolish due to the standby losses in the existing boiler.

However, feeding a plate HX to provide domestic heat appears to be a decent solution.

Of course, it's not conventional wisdom, and cannot generate any revenue for you fellows...........but, strictly from a technical and economic standpoint, it sure does look attractive.

After further research, there is no economic way to utilize atmospheric vent. I'd need to spend well over $ to attempt to open the second flue in the chimney and line it. So, I'm leaning toward the Polaris, despite my misgivings about the technical complexity of the unit.

Any thoughts about the Polaris...........???

[beenthere]

Sorry, we don't talk prices here.

[bc3510]

Sorry, we don't talk prices here.

Sorry about that.

It's a bit difficult when the discussion of which direction to take is heavily dependent on component costs.

[beenthere]

Sorry about that.

It's a bit difficult when the discussion of which direction to take is heavily dependent on component costs.

I just go by the rules. From the site rules.

3. No pricing questions please

Only people who actually see your job can price it accordingly. There will be no pricing allowed in the public access areas of this site. Please do not ask for or give pricing comparative information.
Posts of this type will be deleted.

[BadgerBoiler MN]

I design and install combi systems daily using Polaris and Taco Xblocks. The cost is similar to a condensing boiler with an indirect but will fit in a 36" circle on the floor with a 2" PVC vent. I used to worry about "the complexity of unit", but that was nearly 20 years ago when I sold them at wholesale. We used two of them as primary heat for our 100,00 sq.ft. warehouse for 15 years, recently replace with Viessmann, a new line for my old employer.

They are stainless steel commercial water heaters and properly sized and installed will serve brilliantly. I particularly like them for small loads and micro-zones with design water temperature below 160°F, 140 is better. Just as most boilers are grossly over-sized, most radiation falls into the same category; perfect for us.

[bc3510]

I design and install combi systems daily using Polaris and Taco Xblocks. The cost is similar to a condensing boiler with an indirect but will fit in a 36" circle on the floor with a 2" PVC vent. I used to worry about "the complexity of unit", but that was nearly 20 years ago when I sold them at wholesale. We used two of them as primary heat for our 100,00 sq.ft. warehouse for 15 years, recently replace with Viessmann, a new line for my old employer.

They are stainless steel commercial water heaters and properly sized and installed will serve brilliantly. I particularly like them for small loads and micro-zones with design water temperature below 160°F, 140 is better. Just as most boilers are grossly over-sized, most radiation falls into the same category; perfect for us.

Thank you for that confirmation.

I made a bit of a mistake regarding the model. The one I was looking at was "Vertex" and not "Polaris". Do you see any significant difference between them?

My concern over "the complexity of the unit" might be overblown. They do seem to be quite reliable with minimal maintenance necessary.

I will have to be a bit careful of the flow to and from the unit through the HX and onto the cast iron heating. The very high mass of the radiators could easily kill the temperature in the tank. I like the idea of the variable speed circulator with suitable temperature sensors.

[BadgerBoiler MN]

The Vertex, though thermally similar, is a dressed up glass-lined water heater, six year warranty. The Polaris is 316SS, ten year for residential. Cast iron is perfect for condensing equipment and flow rates were often in the gravity system range originally. The XPB will do it all once you program it correctly...not necessarily easy, but I feel you have the right stuff ;-).

http://www.badgerboilerservice.com/hotwater.html

[R Mannino]

There is no question about switching to NG. The only question is which way to go.

Conventional wisdom uses a new gas boiler with an indirect HW heater.



However, feeding a plate HX to provide domestic heat appears to be a decent solution.

Of course, it's not conventional wisdom, and cannot generate any revenue for you fellows...........but, strictly from a technical and economic standpoint, it sure does look attractive.

The flat plate works as long as you have storage and the additional circulator as well.
Not one of my better jobs, but it works.

[BadgerBoiler MN]

This instead of indirect water heater...oh my...

[R Mannino]

Storage tank was existing and customer wouldn't go for indirect.