Monoflow system upgrade to combi

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George Day

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First off let me thank you for opening and reading this post. Any advice will be appreciated.

I currently have a 1948 3500sqft (3700sqft with walk in attic storage-currently unheated) house heated by 19 wall convectors and a Burnham series II boiler. Last winter 2-3 of the radiators towards the end of the loop were weak, this year 5 are weak or give off no heat. I am pretty confident I have bled all of the air out of them, by going around the house a couple of times with solid stream outcome. I also replaced the circulator pump. A plumber friend of mine believes the diverter tees in the loop are worn out at those units. In my area it will cost an arm and a leg to get the tees replaced only to be putting them in 70 year old pipes, so that is a not a preferred option.

Additionally, I am about to rehab the utility/laundry room and would like to relocate the boiler from there to the garage where the water heater is. With that in mind, I am looking at replacing both the boiler and water heater in favor of a Navien NCB 240 combi unit. I am looking at changing all the boiler pipes under the house from steel to PEX. Running the system as a home run design with 2 zones. Zone 1-13 convectors on first floor (568" of convectors) and zone 2 will have 6 convectors up stairs (280" of convectors). I could split these out differently if needed.

Another option is to keep the boiler I have and pipe the loop using pex, and tees as designed. That is to fix what is here and see what needs upgraded at a later date.

In my research I have performed a heat loss calculation through the SLANT/FIN app which provided me a 84,487 BTU/hr, I have also double checked this with utility bills using Dana's blog "Out With The Old, In With The New" found at http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new#ixzz4boKywBNi That calculation put me at 84,935 BTU/HR. My water heater is also gas and I have gas logs, so my estimations could be high.

The Navien NCB 240 comes in at 120K BTU/HR max with a min at 18K.

More details:
Downstairs heat loss- 54,763 BTU/Hr
Upstairs-29,724 BTU/Hr (may want to add 2 more convectors for walk in attics adding nearly 19BTU/hr)
Boiler-164,000BTU/hr input 136,000 output and 118,000 net (82% eff)
Watched the boiler cycle today and measured the temp at 163F out and 136F in, while checking the 8th or 9th convector in the loop at 155F in and 140F out.
Need the ability to run 1 shower, 1 dishwasher, and 1 clothes washer at time.

Goal is to design a system that will last 10-20 years and pay for itself in energy effieciency.

Concerns: navien/combi reviews online, using convectors at a low temp, house has hard water (which may increase maintenance on any boiler), also concerned with have the additional cost of removing convectors and putting in wall rad panels if convectors are not up to the new boiler at lower temps. Domestic HW not enough flow when my kids get a little older an start the Hollywood showers of teenagers.

All comments welcome. George
 

Dana

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If the bypass plumbing between the tees is accessible and easy to work on (eg: 3/4"-1" copper, not 2" iron) you may be able to restore performance to a low-output convector by installing a ball valve on the bypass (main loop) between the tees to force flow through the convector, and tweak it until the room temperatures balance well. That should work fine for one or two convectors, but it's probably impossible for higher numbers.

The NCB 240 has enough burner to deliver 100% of the design day heat and a 1gpm shower simultaneously, but not a 2gpm shower. But IIRC it automagically gives priority to the domestic hot water side (might be a programmable option default, or a DIP switch setting- read the manual.) Unless you're planning on taking a shower during the coldest hours of the coldest days (usually just before dawn during a January/February cold snap) even without priority it should be OK.

A heat load of 85K for a 3500' house in VA implies that the house either leaks a ton of air, has no insulation, or has only single-pane windows, or some combination thereof. Most 2x4 framed houses that size with fluff in the wall framing cavities, clear glass double-panes (or clear storm windows), and 6" of blown fluff in the attic would come in under 70,000 BTU/hr if it's reasonably air tight. If it has a full basement (not counted in the 3500') that's reasonably insulated and air tight it could even come in under 50,000 BTU/hr.

If you're moving the boiler outside of conditioned space on the other side of the insulation (such as the garage) you'll be increasing the distribution losses, and you need to derate the output by 10-15%, since some of that heat is no longer going into the house. A wall hung combi boiler doesn't take up anywhere near as much space as a 6 plate cast iron beast like your P206. Keep it indoors if you can!

If you have more than 5' of vertical drain downstream of the shower, a 4" x 48" or 3" x 60" or larger drainwater heat recovery heat exchanger can cut the ~55-70,000 BTU/hr burner capacity needed for the shower by half. The largest one that fits is the "right" one, since the marginal performance improvement is more than the marginal cost of installation. It has to feed both the water heater and the cold side of the shower to get the full benefit:

power-pipe-dana.jpg
 
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George Day

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Dana,
Thanks for your reply. My house is fairly drafty and the perimeter walls are not insulated. As I tackle remodel projects I am fixing that. I only have 1 bath (on the second floor) that would have a 5' vertical drop but its drain dumps into a main cast iron drain stack before the vertical run. I have the desire to put the unit in the garage for aesthetics, but at 10-15% loss I will have to really consider that. With my work, I am up and showering before dawn so your data on the DHW gives me concern. Do you know of any other unit that would be better for my heating and DHW situation? I believe the main loop is 2" iron or steel. I know it is not copper. Would the home run layout be feasible with 1" PEX?
 

Dana

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With 2" iron it sounds like your system has fairly high flow requirements.

Like the NCB-240, most low mass wall-hung combi boilers stop at 199,000 BTU/hr in (at 200K it's a different boiler class), and that is your ultimate limiting factor. A single 2 gpm shower at a 65F rise (40F in, 105F at the shower head) takes about 65,000 BTU/hr of boiler output, which at 95% combustion efficiency takes about (65,000/0.95=) 68,500 BTU/hr of boiler input. Unless you have a LOT of radiation, on at outside design conditions you won't be able to run the boiler at condensing temperatures and would be getting about 86% efficiency, so the 85K of heat load takes (85,000/0.86= ) 99,000 BTU/hr of boiler input. (If you put it in the garage, add at least 5% to cover the additional distribution losses that weren't accounted for in the fuel-use heat load calc. ) Combined that's 167,500 BTU/hr of input BTU, which leaves you only 31-32K of "extra" burner, which is only good for another ~1 gpm of hot water.

A tank type combi such as an HTP Versa would get you there. Even the smallest (PHE 130-55) has enough burner (130,000 BTU/hr total, 100, 000 BTU/hr of space heating output) to cover your design day heat load with a bit of margin, and 55 gallons of domestic hot water capacity, with enough burner capacity beyond the heat load to have reasonable recovery times (about as fast as a 50 gallon electric tank, even when running 85K of space heating), and the domestic hot water flow rates would not be burner-limited, the way a low-mass wall hung combi is. In non-condensing mode it's only barely covering your space heating if you put it in the garage, and you might consider bumping up to the 199K burner version, which delivers 135,000 BTU/hr of space heating, more than covering the additional distribution losses. (If it were my house I'd go with the smaller burner.)

If the walls are framed wood with plank or plywood sheathing it's possible to blow the cavities full of cellulose without gutting the walls. If you're insulating it only one room at a time the room to room temperature balance is going to be changing with every upgrade.

In most houses insulating the basement walls to the current IRC code minimum (R10 of continuous insulation, in US climate zone 4, which is all of VA) it will take something like 10% off the whole-house heat load without changing the temperature balance. If it's a poured concrete or CMU (concrete block) wall this is pretty cheap & easy to do using 2" reclaimed roofing polyiso (typically about $10-15 per 4x8 sheet in pretty good condition), held in place with 1x4 furring through screwed to the foundation 16-24" o.c. with 4.5" TapCons. Tape the seams of the foam with housewrap tape, and seal the top & bottom to the foundation with can-foam to prevent convective thermal bypassing and moisture transfer. (The cut bottom edge needs to stay off the slab, since it can wick ground moisture from slabs that don't have vapor barriers under them.) To meet fire code the foam needs an ignition barrier- half inch wallboard mounted to the furring works. Air sealing and insulating the basement is well worth doing, even if you never plan to finish it into living space.

These folks have a series of short videos on how to do it, with perhaps finer detailing than you'd use for a basement you'd never finish, using glue to hang the foam (which would require hanging the gypsum on a studwall in some areas, depending on local code interpretations, which is more expensive than doing it with furring & masonry screws.)
 

George Day

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Thanks Dana, You gave me a lot to think about/ research, thus a few days to reply. I do not have a basement, I am on a 30" high vented crawlspace. The floor joists for the first floor have batted insulation but that does leave a very large volume of vented air under the house at any given time. The position I find myself is a catch 22.

On one side, I have a 12 year old boiler that has given me no real problems. A 8 year old water heater also with no problems and all I NEED are a few piping repairs under the house to get my heating system back in spec.

On the other hand, I am already committed to a laundry room remodel and figured while I was doing upgrades I might as well completely reconfigure/replace the boiler piping, relocate the boiler to the garage therefore buying a lot of extra usable space. Sound advise is telling me not to relocate the boiler. The costs for this laundry room additional upgrade is starting to really spiral out of control. I was figuring on $2.5K for the boiler and $1.5K for the piping (with me doing all the work and shopping for deals). That is $4K extra for project that would fall under the WANT category, and I was wrestling with that choice.

The HTP VERSA looks to be a $6-7K investment for the boiler alone, which when added to the overall budget is just unfeasible at this time.

I have thought of encapsulating/sealing my vented crawlspace and putting a dehumidifier down there, but have heard mixed comments on doing so. Can I assume that you would be in the pro-category for sealing a vented crawlspace?

My boiler output piping is 1.25" copper that runs approximately 25-30' before reaching the main loop (180-200') of 2" iron. It then transitions back to 1.25" copper for the final 25-30'. I am assuming the copper was added when the boiler was installed about 12 years ago, part of the problem is this copper piping is all out in the open making the laundry room look very haphazard. It is this 1.25" copper piping that makes me go down the path of a PEX home run piping system with the manifold being centrally located under the house. I just don't know if I would be restricting the flow too much to get adequate transfer out of my convectors. I plan to run 2 zones (upstairs and down) of 1" pex suppling to a manifold and then 1/2" to the convectors (http://www.supplyhouse.com/Caleffi-6686O5S1A-1-TwistFlow-Manifold-13-Outlets) Using this style manifold would allow me to balance the flows for each heater. Would this work?
 

Dana

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Thanks Dana, You gave me a lot to think about/ research, thus a few days to reply. I do not have a basement, I am on a 30" high vented crawlspace. The floor joists for the first floor have batted insulation but that does leave a very large volume of vented air under the house at any given time. The position I find myself is a catch 22.

On one side, I have a 12 year old boiler that has given me no real problems. A 8 year old water heater also with no problems and all I NEED are a few piping repairs under the house to get my heating system back in spec.

On the other hand, I am already committed to a laundry room remodel and figured while I was doing upgrades I might as well completely reconfigure/replace the boiler piping, relocate the boiler to the garage therefore buying a lot of extra usable space. Sound advise is telling me not to relocate the boiler. The costs for this laundry room additional upgrade is starting to really spiral out of control. I was figuring on $2.5K for the boiler and $1.5K for the piping (with me doing all the work and shopping for deals). That is $4K extra for project that would fall under the WANT category, and I was wrestling with that choice.

The HTP VERSA looks to be a $6-7K investment for the boiler alone, which when added to the overall budget is just unfeasible at this time.

I have thought of encapsulating/sealing my vented crawlspace and putting a dehumidifier down there, but have heard mixed comments on doing so. Can I assume that you would be in the pro-category for sealing a vented crawlspace?

My boiler output piping is 1.25" copper that runs approximately 25-30' before reaching the main loop (180-200') of 2" iron. It then transitions back to 1.25" copper for the final 25-30'. I am assuming the copper was added when the boiler was installed about 12 years ago, part of the problem is this copper piping is all out in the open making the laundry room look very haphazard. It is this 1.25" copper piping that makes me go down the path of a PEX home run piping system with the manifold being centrally located under the house. I just don't know if I would be restricting the flow too much to get adequate transfer out of my convectors. I plan to run 2 zones (upstairs and down) of 1" pex suppling to a manifold and then 1/2" to the convectors (http://www.supplyhouse.com/Caleffi-6686O5S1A-1-TwistFlow-Manifold-13-Outlets) Using this style manifold would allow me to balance the flows for each heater. Would this work?

A vented crawlspace in a place with summertime humidity as high as it is in VA is bringing in more moisture than it is purging. It's also a huge air leak at the bottom of the house, which maximizes the height of the stack-effect stack, and could easily be as much as 20-25%of your total heat load. Spending the boiler money on converting the crawlspace into an insulated & conditioned crawl space is probably the better value, even if it means keeping the beastie-boiler for another decade or more, assuming the existing boiler is fully inside the insulation & pressure boundary of your house.

Again, putting the boiler in the garage will reduce the overall system efficiency, since all of the standby losses and some of the distribution losses are then truly lost. You can only beat that if you build an insulated boiler room/closet in the garage and remove any insulation in the partition wall between the new boiler room and the fully conditioned house.

Using manifolds with home-runs to the convectors and valves for tweaking the flows can work, but it's pretty messy if you don't do the full math first.

IRC 2015 code min for crawlspace walls is R10 continuous insulation, just as with basements. It's fine to leave the existing joist insulation in place, but you definitely want to put down an EPDM (membrane roofing) or 10 mil plastic vapor barrier on the floor (does it have a rat-slab, or is it dirt?) , lapped up 8-12" onto the foundation walls and sealed in place with duct-mastic. Then lap the wall-foam over the vapor retarder, and use a horizontal 1x4 furring through-screwed to the foundation to clamp the foam & vapor barrier to the foundation. In termite zones you may be required to have a 3" inspection gap in the wall foam, which is fine- you can cut some strips of R15 rock wool batt to stuff in the inspection gap. Be sure to caulk the band joist to the foundation sill and subfloor, and insulate the band joist & foundation sill, and any ledge at the top of the foundation sill, sealing it in place with can-foam:

jZTXU.jpg
 
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