Looking at a Combi, What to do?

[DaveD]

Wanting to replace a 27 year old super hot boiler with a combi. Had a few contractors that quoted but all speced out a way too big unit imo after reading a bit on this forum. A few details about house.
3200 sq ft
52' of alum finned baseboard upstairs
Radiant infloor down
I would like to convert to tankless dhw also.

The units that were suggested are
Navien 240
Noritz cb199-dv
I can't see the boiler condensing with the 2 zones upstairs even if looped together with those huge boilers.
Also the radiant is poly b tubing if that matters
Every contractor that came in looked at size of boiler and never asked how much baseboard upstairs. So frustrating.
Any suggestions of what to do?
Thanks

 

[Jadnashua]

There are lots of threads that describe how to properly size a boiler based on past heating fuel use along with heating degree days.

Personally, if you're going to put in a boiler, I'd not go with a tankless...in fact, IMHO, there are few situations where one is the best solution. With a boiler, put in an indirect on a priority zone...properly sized, you'll never run out of hot water and save a lot. A quality indirect may only cool off 1/4-degree per hour, so nearly no standby losses.

 

[Sponsor]

 

[Dana]

With only 52' of fin tube the boiler will short cycle at condensing temperatures if the combi boiler won't modulate below ~10,000 BTU/hr or so. If that 52' is broken up into zones it's a short-cycling nightmare for even the smallest condensing boilers, let alone an oversized combi.

With only 52' of baseboard for the main zones, assuming it has been able to keep up with the design heat load, it's likely the load of the fin tube zones is less than 25,000 BTU/hr. (A tight 2x6 framed ~3200' house in coastal B.C. might have a design heat load of 25,000 BTU /hr @ -8C for the whole house.)

What to do?

First, run these numbers.

Then run these numbers.

Room-by-room, zone-by-zone Manual-J load calculations would be useful too, noting the amount of fin tube in each room, and the load/baseboard ratios. From that we can derive the maximum and average water temperature requirements, and how much of the time the system could run in condensing mode.

Low mass wall-hung combis are most often a good fit for houses with LARGE heat loads and small domestic hot water needs. If your house has more than one bathroom, the description of your house is the exact opposite of that.

Short of adding a lot of radiation, the best bet for a condensing combi system would be something with a self-bufferng thermal mass, making it inherently short-cycle proof. That can either be a system designed around a condensing water heater (with an external heat exchanger isolating the potable water in the tank from the heating system water), or a pre-engineered tank type combi such as HTP's Versa series.

 

[DaveD]

Thanks Dana.
I ran the whole house load calculator and came up with 39262 btu.
then ran the numbers in the first link using the highest monthly bill dec13-jan12 and got 45920 btu.
Dont know if I did it right as a little confusing but do those numbers seem right?

 

[Reach4]

then ran the numbers in the first link using the highest monthly bill dec13-jan12 and got 45920 btu.

Check to see if one of those bills was estimated.

 

[DaveD]

Check to see if one of those bills was estimated.

Good point but no it's actual readings. Seem high?

 

[Dana]

It seems a bit high, but it depends on your location, the shape of the house, and the amount & type of window area. I would have expected a tight 2x6/R19 type 3200' house to come in in the 39-45K range in Kamloops, a bit less Kelowna, and quite a bit less in Victoria. It matters where you are, and what you were using for inside & outside design temperatures.

Fuel use calculations are usually an upper bound, since boiler oversizing reduces efficiency, there may be distribution losses (especially if the boiler i & pipes are outside of conditioned space), and hot water use adds another error. If you have 5 people taking daily showers it could artificially inflate the numbers by 10- 15%, which would be consistent with a 39K Manual-J and a 45K fuel use calc. If the hot water use is minimal and the boiler is in the garage, that too would be consistent.

 

[DaveD]

I'm in surrey bc and its 2x4 walls. I used the highest gas consumption month also. I just had another quote using the navien 240 for heat and dhw and they told me it will automatically condense and modulate on those short baseboard zones and they guarantee it. The owner coming tomorrow to explain and discuss my concerns. Anything else I should ask him?
Thanks guys.

 

[Dana]

The NCB 240eE's minimum modulated input is 18K, minimum output ~17K in condensing mode. That balances with about 85' of baseboard at condensing temps low enough to deliver 95% combustion efficiency. With lesser amounts of baseboard it will cycle, not modulate, based on the thermal mass of that zone, and the programmed temperature differential with which it operates.

If you combined the 53' of baseboard into one zone it would be 17,000 / 53= 320 BTU/hr per running foot, which balances at an average water temp (AWT) of ~140F. The best you can hope for is 150F out, 130F back. The entering water temperature (EWT) at the boiler needs to be below ~127F to condense AT ALL.

At an AWT of about 130-135F it'll be at the beginning of condensing, 88-90% combustion efficeincy, and the 53' of baseboard would be emitting ~13-14,000 BTU/hr, which means there is still 3000 BTU/hr (50 BTU/minute) of excess heat going into the system. There is probably no more than 5lbs of thermal mass in that zone, so it'll be slewing on the order of 10F per minute. It's unlikely that the differential window is wider than 10F, so you're looking at sub-minute burns- it's short-cycling.

So, if what they "guarantee" violates the laws of physics, are you willing to take them up on it?

[edited to add]

In fact, as the temperature rises those 10 degrees out of the condensing zone the output is lower, and the heat rate coming out of the 53' of fin-tube rises, so the burn times would be longer than a minute, but the average combustion efficiency would be about 87%.

If instead of the combi boiler a Navien NHB-55 or NHB-80 boiler were installed (along with an indirect hot water heater) either one of those boilers modulates down to 8K in, 7.6K out, and could modulate at 95% efficiency on a zone with only 38' of baseboard without cycling. Either one would also CLEARLY cover your 39K-45K heat load with margin, and would modulate well into the shoulder seasons, since the minimum output is 17-20% of the load at design condition.

With the combi boiler the minimum output is 37-45% of the design load, and will be forced in to cycling much of the time by the lack of sufficient load, even if there weren't the issue with the zone radiation being too small to operate in condensing mode (on at least the fin-tube zones.)

Regarding the magnitude of the heat load, even 39K is really pretty high for a house that size at Surrey's +25F-ish (~ -4C) 99% outside design temperature, and 45K would be an even bigger outlier for a tight 2x4 framed house, with clear-glass double-panes (or single-panes + storms). Most tight 2x4 framed houses that size would come in at about 30-35K @ -4C outdoors, +20C indoors. Is there a lot of window area (or single-pane window area) or something? Is the foundation insulated? Some amount of blower-door directed air-sealing and spot insulation upgrading might prove very cost effective. If the higher load is due to a lot of single-pane glass, low-E storm windows would cut the window losses by nearly 2/3, and would be very cost effective (far cheaper than replacement windows of similar performance.)

 

[DaveD]

The NCB 240eE's minimum modulated input is 18K, minimum output ~17K in condensing mode. That balances with about 85' of baseboard at condensing temps low enough to deliver 95% combustion efficiency. With lesser amounts of baseboard it will cycle, not modulate, based on the thermal mass of that zone, and the programmed temperature differential with which it operates.

If you combined the 53' of baseboard into one zone it would be 17,000 / 53= 320 BTU/hr per running foot, which balances at an average water temp (AWT) of ~140F. The best you can hope for is 150F out, 130F back. The entering water temperature (EWT) at the boiler needs to be below ~127F to condense AT ALL.

At an AWT of about 130-135F it'll be at the beginning of condensing, 88-90% combustion efficeincy, and the 53' of baseboard would be emitting ~13-14,000 BTU/hr, which means there is still 3000 BTU/hr (50 BTU/minute) of excess heat going into the system. There is probably no more than 5lbs of thermal mass in that zone, so it'll be slewing on the order of 10F per minute. It's unlikely that the differential window is wider than 10F, so you're looking at sub-minute burns- it's short-cycling.

So, if what they "guarantee" violates the laws of physics, are you willing to take them up on it?

[edited to add]

In fact, as the temperature rises those 10 degrees out of the condensing zone the output is lower, and the heat rate coming out of the 53' of fin-tube rises, so the burn times would be longer than a minute, but the average combustion efficiency would be about 87%.

If instead of the combi boiler a Navien NHB-55 or NHB-80 boiler were installed (along with an indirect hot water heater) either one of those boilers modulates down to 8K in, 7.6K out, and could modulate at 95% efficiency on a zone with only 38' of baseboard without cycling. Either one would also CLEARLY cover your 39K-45K heat load with margin, and would modulate well into the shoulder seasons, since the minimum output is 17-20% of the load at design condition.

With the combi boiler the minimum output is 37-45% of the design load, and will be forced in to cycling much of the time by the lack of sufficient load, even if there weren't the issue with the zone radiation being too small to operate in condensing mode (on at least the fin-tube zones.)

Regarding the magnitude of the heat load, even 39K is really pretty high for a house that size at Surrey's +25F-ish (~ -4C) 99% outside design temperature, and 45K would be an even bigger outlier for a tight 2x4 framed house, with clear-glass double-panes (or single-panes + storms). Most tight 2x4 framed houses that size would come in at about 30-35K @ -4C outdoors, +20C indoors. Is there a lot of window area (or single-pane window area) or something? Is the foundation insulated? Some amount of blower-door directed air-sealing and spot insulation upgrading might prove very cost effective. If the higher load is due to a lot of single-pane glass, low-E storm windows would cut the window losses by nearly 2/3, and would be very cost effective (far cheaper than replacement windows of similar performance.)

I totally agree with you on the sizing.
I may have messed up the load calculations. I'm a newbie and only done this once. House has average amount of windows which are low e. Slab not insulated only on sides of foundation. R40 attic and r12 2x4 walls.

 

[Dana]

Precision on the heat load isn't necessary- you have two very clear stakes in the ground, the Manual-J, and the fuel use calc (which defines an upper bound, and includes any distribution losses.) It's clear that any ~50KBTU/hr or bigger condensing boiler will cover your load. There aren't very many boiler models smaller than that in the north American market, so you're looking at the smallest boilers in almost any manufacturer's lineup.

The important number to focus on then becomes minimum modulated output relative to your zone radiation, which is what makes wall-hung combis something of a solution-problem. In order to have reasonable domestic hot water performance the biggest of the series is usually necessary in locations with incoming water temps as low as yours, but the bigger combis also have substantially higher minimum output. eg:

The smallest of the line NCB-120E would be able to edge into the 90s for combustion efficiency on a ~50' fin tube zone without cycling, but it's really marginal on hot water performance at your incoming water temperatures. It will support one 2.5 gpm shower but not two, and if somebody ran a load of laundry or washed their hands with decent flow or ran the hot water to the kitchen sink at high flow while someone was in the shower there might be hell to pay later.

 

[DaveD]

Ok the owner came by today and said they put a primary loop in but I can't see how that helps. An indirect tank will really boost cost like 3k by the time you put extra pump and zone valve etc. Or I could go an Olsen with b vent and keep existing hwt but only about 85% efficient and still have dinosaur hwt.
Guess I'll talk to some other contractors

 

[Dana]

That's really sportin' of 'em that they would install a primary loop...

...since Navien would VOID THE WARRANTY if it were installed without it.

Seriously?? Do the even read the installation manuals?

The internal pump on the NCB series is sized only as a primary pump, designed to work with a hydraulic separator such as their pre-fabricated closely spaced tees manifold, not as the heating system's radiation pump. If somebody does all of the hydronic design math some systems can be pumped direct, SOMETIMES using the internal pump that it's shipped with. But I doubt very much that your multizoned system with radiant + fin tube can be reasonably pumped with just the internal pump using a NCB as the boiler.

Low head mid-mass fire tube boilers with a big turn-down ratio such as HTP's UFT-080W can often be pumped direct, and would be a reasonable fit here. That boiler has pre-plumbed ports and controls for an indirect, which makes it pretty easy to install too.

 

[DaveD]

They always install the loop and it would have 2 pumps 1 for radiant 1 for fin tube. What is the reason for that primary loop?

 

[DR-DEATH]

That's really sportin' of 'em that they would install a primary loop...

...since Navien would VOID THE WARRANTY if it were installed without it.

Seriously?? Do the even read the installation manuals?

The internal pump on the NCB series is sized only as a primary pump, designed to work with a hydraulic separator such as their pre-fabricated closely spaced tees manifold, not as the heating system's radiation pump. If somebody does all of the hydronic design math some systems can be pumped direct, SOMETIMES using the internal pump that it's shipped with. But I doubt very much that your multizoned system with radiant + fin tube can be reasonably pumped with just the internal pump using a NCB as the boiler.

Low head mid-mass fire tube boilers with a big turn-down ratio such as HTP's UFT-080W can often be pumped direct, and would be a reasonable fit here. That boiler has pre-plumbed ports and controls for an indirect, which makes it pretty easy to install too.

I second this. I was looking at boiler options last year and all companies were only wanting to install combi units like the navien - which were highly oversized. I have 3 zones and my basement zone is only 14ft. Dana did exactly what he is doing for you and I found someone to install the HTP UFT 80. Been installed since November and worked great all winter in NH. I have been able to tweak the curve and it def doesn't short cycle on the shortest zone when it's the only zone calling. I have been super happy with this boiler and so far no issues. The other plus is that it can be piped directly per the manual which saves on install labor and the boiler itself is pretty cheap. I had all the same experiences that you are having while shopping right now and it sure is frustrating. Dana knows his stuff and I would definitely go with whatever he suggests.

 

[Jadnashua]

A primary loop on a boiler heat distribution is a fairly short loop from the outlet of the boiler back to its inlet. The secondary loop taps off of the primary loop and returns to the primary loop, so there's a mix of primary water and secondary water that makes it back to the inlet of the boiler. There's normally some mixing between the primary and secondary (well, if there wasn't, you wouldn't be heating anything useful!). This helps moderate the temperature of the water going back into the boiler and probably other reasons.

 

[DaveD]

Thanks for the explanation. I'm trying to get pricing on doing the indirect tank but think it'll be too much. Would the return water temp on those short radiator runs increase or decrease if flow of pump increased? Thinking of a programmable variable speed pump may help and also combine zones.

 

[Jadnashua]

The faster you try to pump water through a loop, the less time it has to release its heat. Generally, faster isn't better. The faster you pump it, up to limits that increase the friction and limit the volume (this also increases wear, uses more energy, and can become noisy), the higher the return temperature is, and you lose more possibilities of condensing, and the efficiency that comes with it. Increasing the speed might help if you had more radiation than needed, and the water had cooled off too much to do much by the time it got to the end. More volume/speed, more available heat, but it won't help if you're radiation limited. ANd, there are also limits in the heat exchanger in the boiler, too.

 

[Dana]

Faster is definitely not better, since that increases the temperature of the returning water, which decreases the raw combustion efficiency.

There are limits to how slow the flow can be on the primary loop- too slow and the boiler becomes more susceptible to flash boil, and increased the delta-T at the boiler, which for most heat exchanger designs shouldn't go over a few 10s of degrees Fahrenheit. (I'm not sure what the max delta-T is for the heat exchanger on the NCB-240. It could be pretty high, since it's a glorified tankless water heater.) Pumping the primary loop too fast will erode water-tube heat exchangers (such as the NCB-240) , creating pin-hole leaks. So when flow rate requirements for the radiation is higher than what is appropriate & efficient for the boiler, it has to be plumbed primary/secondary.

Navien designs their boilers & combis with an internal pump sized appropriately for the primary loop when used with their pre-engineered hydraulic separator manifold. The same function can be fabricated for less money from closely spaced tees and short sections of pipe, but the designer/builder has to know WTF they're doing. When the hydronic designer's or plumber's competence in doubt, buy Navien's manifold. (Note the relative placement of the expansion tank and radiation pump in the diagram for Navien's manifold. The configuration is more important for pump longevity and system efficiency for this boiler than some others, but often violated in the field.)

It's VERY likely that all zones (including an indirect water heater zone) could be run off a single smart pump using pressure or delta-T feedback and zone valves, but proving that is more than a napkin-math exercise suitable for "design by web forum". But knowing the load/radiation ratios for the fin tube zones as well as for the radiant zone would be a good start, since you may or may not need to run it as a dual-temperature system.

The UFT-080W (or the exact same unit under the Westinghouse label, the WBRUNG-080W, whichever is better supported in your area) is fairly inexpensive modulating condensing boiler, cheaper than a combi-boiler, cheaper than many mid-efficiency cast iron boilers, and usually a fairly easy drop-in replacement for cast-iron boilers. The fact that it comes pre-plumbed for an indirect water heater reduces the design & installation time. I'd be surprised if the total installed cost of that boiler + indirect even $500 more expensive than an NCB-240E, but it would be far more appropriate, and deliver better hot water service, with on order of magnitude fewer burn cycles in either hot water heating or space heating mode once the reset curve and other parameters have been tweaked in a bit. Fire tube heat exchangers can handle a wider flow range without internal damage or excessive pumping power.

FWIW, the UFT-080W and the Noritz CB199-DV were both designed and manufactured by Kiturami, a Korean boiler & water heater manufacturer that competes head-to-head with Navien in the Korean domestic market as well.