What drives the astonishingly high costs of installing a combi boiler?

[Jeff Daley]

Current situation: 850 sq ft, 1-story house near Seattle. I have a Rinnai tankless water heater and a Navien heating box that diverts heated water from the Rinnai into 6 smallish baseboard radiators around the house - it is a true "open loop" system and has basically failed. I'm planning to tear out the tankless and the heating box and replace with a combi boiler. All the necessary plumbing, gas lines and venting already exist. All that's needed is the purchase of the combi boiler, a few valves and expansion tank, and then installation. Depending on the model of the boiler, it's possible the exhaust ducting will need to be modified.

I've gotten a few estimates which have called for different makes of boiler (HTP, Noritz, Navien) and have researched them all, and they appear reasonably sized and of decent quality, and they all are about $2500 (civilian price). All of the estimates have indicated the job could be done in a day. Making a few charitable assumptions like 1) the installer can't get a better deal on the boiler than I could get online; 2) another $1000 in valves and other parts will be needed; and 3) it requires 20 man-hours to install the boiler - that still comes out to an hourly rate of $325 before tax. With more realistic assumptions, it will probably end up at $400/hour or more for the install.

That seems insane to me, and triple what other professionals such as plumbers and electricians in the area are charging. Can someone shed some light on why boiler installation seems to fall into the same sort of hourly rate that named partners at downtown law firms are charging? Thanks for any thoughts.

 

[hj]

It is whatever the companies believe YOU will pay for it. Here, there are plumbers who charge $2500 for an installed 75 gallon gas heate and others are charging$4800 when they can convince the customer it is a good deal. BUT, I doubt you will find anyone who will give you a "deal' for installing a unit YOU purchased on the Internet.

 

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[Jeff Daley]

Thanks for the comments HJ - and fair enough. I get that the price is defined as whatever someone will pay. I guess I was hoping there might be some additional underlying logic that drives that price - along the lines of "this is really complicated stuff and only a handful of licensed plumbers are good at it" or something else that would support why the hourly equivalent is 3-4 times what normal plumbing (or electrical or any other licensed professional work) is costing these days. In my earlier post, I neglected to mention that the quotes I've received are $10,000 and up. I'd be happy to let the pro purchase whatever make/model they feel comfortable with, I wasn't asking for them to install something I purchased myself. I'm just not all that stoked about absorbing a massive markup when I can get the same unit online.

In my situation - where the radiant plumbing is already in place is a good example - I think all the plumber has to do is cut the current tankless and heating box units off the wall, bolt the new one to the wall, connect the gas and the main, DHW, and radiant in/out lines, and the gig is over. Maybe a system flush beforehand and tweaking some of the valves. Homeowners are sort of over a barrel here - manufacturers require licensed professionals do the install or risk voiding the warranty, and the licensed pros appear to be taking advantage of that. I say "appear" because I was hoping to hear someone tell me that because of certain complexities that I fail to appreciate, there really is some basis for the $10,000 price quotes I've received thus far for a job that can be done in a day or less.

 

[Dana]

An 850' house in Seattle (or even Fairbanks AK) is a downright lousy fit for any low mass combi boiler. Low mass combis are only a GOOD fit for houses with very high heat loads, and only moderate domestic hot water loads. Your house is the opposite, with heat load that's probably well under 20,000 BTU/hr, but with moderate hot water needs (due to the fairly cool incoming water temps in Seattle.)

It's highly likely that your design heat load is still smaller than the minimum fire output of the boiler, and that there isn't even enough total baseboard in those "...6 smallish baseboard radiators ..." to emit even 10% of the boiler's output at condensing water temperatures. That's a recipe for short-cycling the thing into an early grave, which is probably what happened to the Rinnai. Take a big step back, and run this math on the radiation when considering any boiler solution, combi or otherwise.

You're probably better off going with a condensing tank water heater and an exterior potable compatible plate heat exchanger isolating the potable water from heating system water in roughly this configuration:

 

[Jeff Daley]

Dana - thanks very much for the advice - I was not at all familiar with the setup you describe. As you mentioned, I wondered whether perpetual short cycling played a role in the relatively short lifespan of the current Navien tankless...but this was the system that came with the house, so I didn't have a whole lot of choice. Because I already have the radiators and plumbing in place (and don't have ducting for a forced air system), it seemed like a combi system was the best and cheapest way to go for my heat and DHW needs.

In talking with various contractors, no one offered any advice about whether a combi system was appropriate for my house - they just provided the estimate for putting in a new one. I did ask about short cycling and I recall the pro that recommended the HTP had said that better technology in how that unit modulates reduces the likelihood for short cycling. I have no idea if that is true. At any rate, I will look more into the type of system you describe, but one concern is being able to find a HVAC or plumbing professional that is familiar with it and could service it. Thanks again for the comments.

 

[Houptee]

Is the Rinnai not heating any water at all or you still have domestic hot water?
Or is the Heating box not sending hot water thru the baseboards?
If you ca determine what is failed you might be able to repair the Heat Box or the Rinnai yourself, or if the Rinnai is beyond repair, just replace it yourself with the current model that uses the same venting material as what is there now and it should be a straight forward swap.
Or another option is look into mini split heat pump system so you have ac and heat and leave this system as a backup heating system.

 

[Dana]

In talking with various contractors, no one offered any advice about whether a combi system was appropriate for my house - they just provided the estimate for putting in a new one.

Experience has proven that a large number of people installing this stuff haven't a clue about hydronic heating design or condensing boilers. Don't leave it up to them. Do at least the napkin-math version on the radiation to vet any proposals.

First, measure all of the baseboard radiators/convectors. How many running feet total? (It really matters! )

It's at least all operated as a single zone, hopefully?

What model pump is driving the radiation loop?

I did ask about short cycling and I recall the pro that recommended the HTP had said that better technology in how that unit modulates reduces the likelihood for short cycling. I have no idea if that is true.

The smallest HTP wall hung combi has a minimum-modulation rate of about 14,000 BTU/hr-in, which at condensing efficiency would be about 13-13,500 BTU/hr out, not less. The one advantage HTP's combi boilers have over the others it the fire tube heat exchanger that has at least some water volume/thermal mass to work with, but it's not a huge amount- only ~ 3 gallons/ 25 lbs. The fire tube heat exchangers also have much lower pumping head than water-tube types, and can usually be operated with a single pump pumping direct, with simpler near-boiler plumbing. But even with those advantages over a BTU-equivalent Navien it still needs a minimum amount of radiation to keep it from short cycling. A tank type water heater has more than 10x the thermal mass, and with reasonable controls becomes inherently short-cycle proof.

With typical fin tube baseboard it takes about 65 feet of baseboard to emit the HTP's minimum ~13,000 BTU/hr at temperatures low enough for 95% efficiency. If you don't have at least 50 feet the cycling is going to be pretty bad.

There are low mass condensing boilers (not combi-boilers) &/or tankless water heaters that put out 7- 7,500 BTU/hr at minimum fire, which would balance just fine at condensing temps with only 35-40 feet of baseboard, and could be reasonably made to work with as little as 25'.

There's nothing magic about Navien's heating box that's going to save you from the "BTU-in must equal BTU-out" fundamentals, but there's nothing really wrong with it, and makes it easier to use a water-tube heat exchanger burner(like a Rinnai or Navien water heater) to function as a boiler. Designing an boiler & radiation-specific system would perform better though, especially now that appropriately sized very flexible & efficient "smart" pumps have become affordable.

 

[Houptee]

For such a small house check out using a Takagi TK Jr 2 in place of the Rinnai (if the Rinnai is your current problem and not the Heating Box). The TK Jr is not a condensing unit and requires Category III venting made of stainless steel but you do not have to worry about it reaching condensing temperatures. I have 3 of them heating my 3 apartments the largest apt is 1400 sq feet with 74 feet of fin tube baseboard set temp at 160 deg in NJ with outdoor design temp of 14 deg and it works awesome for 6 yrs so far. And if it does fail they are only $600 so toss it and slam another one in re-use the venting it owes you nothing after 5 yrs and save the old one for parts.

If you want to put in mini split heat pump a/c system check out the multi heads from brands like Senville, Klimaire, YMGI, Thermal Zone, Kingsfin direct (ship from Oregon), Pioneer, Blueridge, and there are several others too once you start looking. The majority of the mini splits are all being made by Gree and Midea with various names slapped on the sheet metal.

 

[Jeff Daley]

houptee: From what we can tell, it looks like there might be multiple failures. We've had one pro determine that the pump inside the Navien Heating Box has failed. It is receiving electricity but isn't pulling water, so nothing is circulating through the radiators. At the same time, the Rinnai tankless is no longer providing in DWH to anything. Occasionally, you can still hear the burners kick on, but if any water is getting heated up, I have no idea where it's going. We are considering a mini split system, but estimates are above $10K, and that of course doesn't include the cost of repairing/replacing the water heater. One concern I have about simply trying to repair the current system (if that's even possible or cost effective) is that the current system appears to be an open loop, and I would prefer to separate the heating water from the DWH out of health concerns.

Dana: We have about 22' of finned radiator length across the 6 radiators, and yes, from what I can tell, it is all one zone. But it's tough to tell exactly what is going on with the plumbing as this system was courtesy of the previous homeowner who was the architect of a number of DIY cob-jobs throughout the house from electricity to plumbing. One thing I have not checked out is the plumbing connecting the radiators. All the connections coming up through the floor and from the wall in utility room are copper, but I don't fully trust that all the plumbing under the house connecting the radiators is new copper...I could easily envision a scenario where prior existing galvanized pipe might have been used to carry water between at least a couple of the radiators. Which leads to another question - why is only the finned radiator portion of the plumbing included in the load calculations? It would seem that uninsulated pipe, whether copper or galvanized, running through a cold crawl space would represent a significant "load" in terms of heat loss from the overall system.

 

[Houptee]

Ignore the piping since it is outside of the conditioned space so the BTUs are not helping to heat the house.
The fin tube is rated for BTU's per foot at various water temps.
In the Northeast the most popular brand is Slant Fin fineline 30 and is rated at 580 Btuh/FT at 180 deg. with flow rate of 1 GPM.
Not sure what brand they have on the West Coast do you know what it is?

If you do have iron pipe in the crawl you could replace it all with Oxygen Barrier PEX tubing and such a small amount you would need to daisy chain each baseboard you can use 1/2" Oxy Pex. My entire house all 3 apartments are run in 1/2" Oxy Pex with a Taco 007 circulator pump controlled by a Taco switching relay that turns the circulator on and off. The Takagi TK Jr senses flow and fires the burner.

 

[Jeff Daley]

I understand ignoring the piping when it comes to calculating the home's heat loss and BTU requirements to heat it, but with regard to estimating what the temperature of the heated water will be when it returns to the boiler after running through all the radiators and piping, it seems like the amount of heat lost in the piping running through a cold crawl space is at least as important as how much heat is lost by the radiators. Isn't this portion of the calculation essential for determining the potential for short cycling and for determining what size boiler I need and whether I would want a condensing or non-condensing unit? I admit I don't quite understand the balancing act here - where (for a non-condensing unit) you don't want the water returning to the boiler so cold that it starts to condense and corrodes the system, and yet, you don't want it coming back so hot that the boiler constantly short cycles...and conversely, with a condensing unit, you need sufficient heat loss from the system so that it does come back cold enough to condense and avoids short cycling. Seems like all that piping running through a cold crawl space must certainly play a role in whether a system short cycles and/or condenses, and whether or not I want it condensing depending on the type of boiler I have.

 

[Dana]

Ignore the piping since it is outside of the conditioned space so the BTUs are not helping to heat the house.
The fin tube is rated for BTU's per foot at various water temps.
In the Northeast the most popular brand is Slant Fin fineline 30 and is rated at 580 Btuh/FT at 180 deg. with flow rate of 1 GPM.

To approach 95% combustion efficiency in a condensing natural gas burner takes ~120F average water temp, a temperature at which Fineline 30 is rated 210 BTU/hr per foot.

So with 22' of Fineline 30 it would only be good for 22'x 210BTU/ft-hr = 4620 BTU/hr, which is a heluva lot less than the min-fire output of an HTP UFTC-140W combi boiler (or any other wall hung combi I'm aware of.) It's also a heluva lot less than the minimum firing rates of most tankless water heaters.

At high temp the UFTC-140W runs ~87% efficiency, and at minimum-fire now only put ~12,200 BTU/hr (sending all of the water in the exhaust out the flue rather than reaping it's heat of vaporization and putting that heat in the system.) For 22' of Fineline 30 to balance with 12,200 BTU/hr (~555 BTU/hr per foot) takes an average water temp of about 175F, an average water temp that is only barely achievable, since it's maximum output temperature is 180F.

It's highly unlikely that the place needs anywhere near 12,200 BTU/hr to stay warm (otherwise the Rinnai wouldn't have kept up), and while blasting 180F water at it would work without short cycling the boiler, hot flashes followed by extended chills isn't exactly the epitome of comfort. A much lower water temperature taking much longer run times to satisfy the thermostat is noticeably more comfortable. Running it at a temperature such that as the heat load increases with cooler outdoor temps the duty cycle hits more than 70% @ +25F outside would be nearly ideal, and would still cover those cold snaps when it's significantly cooler than that.

Plumbing running through the crawlspace is likely to account for less than 10% and certainly less than 20% of the total temperature drop on a loop that small. So at an entering water temp of 180F you might get return water of 170F if it was all inside of conditioned space, 169-170F due to the radiation off the plumbing.

 

[Houptee]

If it was me I would put a non-condensing tankless water heater dedicated for domestic water, and another one next to it dedicated for the heating system. Paying extra for a condensing unit that will never run at condensing temps is a waste of money.
You would no longer need the Heat Box anymore.
How many bathrooms I assume only one in a 900 sq ft house.

 

[Dana]

If it was me I would put a non-condensing tankless water heater dedicated for domestic water, and another one next to it dedicated for the heating system. Paying extra for a condensing unit that will never run at condensing temps is a waste of money.
You would no longer need the Heat Box anymore.
How many bathrooms I assume only one in a 900 sq ft house.

The cost of the stainless Z-vent exhaust often makes the total installed cost of a non-condensing unit higher than a condensing unit.

Pay attention to the minimum firing rate of the tankless too. You may do just fine with a condensing tankless that maxes out at 140F, if the minimum firing rate is under 9000 BTU/hr. It won't actually operate in condensing mode with 140F output, but you'd be able to get 5500-6000 BTU/hr out of the baseboards. At non-condensing temps 9000 BTU-in translates to about 7800 BTU/hr out, and if at least 2/3 of that is being emitted by the baseboard there is probably enough thermal mass in the system to keep it from excessive short cycling, even though it would be guaranteed to cycle some. HTP's small & cheap ($1600-1700) mod con boiler the UFT-80W modulates down to ~7600 BTU/hr in condensing mode, ~6500 BTU/hr above the condensing range.

But a condensing AO Smith Vertex GPHE-50 (50 gallon) tank or if space is an issue, HTP's tiny 20 gallon "Crossover Floor" RGH20-75F isolated from the heating system with a plate type heat exchanger (configured as in response #4) is probably a cheaper solution for providing both hot water and heat, and would still deliver the "endless shower" experience for just one shower. The thermal mass of the water in the tank suppresses short cycling. The max storage temp in the RGH20 is 147F, not sure about the Vertex, but it'll do at least 140F.

 

[Jeff Daley]

Thanks again Dana and houptee - this is great information and I really appreciate it. Also a lot to digest.

A couple other comments/questions - I actually got out the tape measure and it appears we have just over 27' feet of fin-tube radiator...not a huge difference from the 22' number I estimated earlier, but adds a bit more load. Along these lines, we have been considering upgrading from fin-tube baseboards to wall panel radiators, and a quick look at some of the ones that would fit in our existing locations would easily double the BTU-hr output of the baseboards at the same water temperatures. With our current baseboard setup, we're at about 15,718 btuh at 180 deg and about 5700 btuh at 120 deg and condensing temps. Upgrading to all panels with sizes that will fit the current locations would put us at well over 30,000 btuh at 180 (obviously way more heat than we need) and about 15-16,000 btuh at 120 deg. Factoring in what I have to believe is non-trivial btu loss through the unheated crawl space, it seems like we might be in the realm of the smallest condensing combi boilers out there, such as the HTP you mentioned earlier. And if we are just barely outside the threshold here for condensing, what about adding a small buffer tank to the system? Another question - doesn't much of Europe depend on small combi boilers in houses and small flats for their heating and DHW needs? Are these models not available in the US?

I'm also looking closely at the HTP Crossover Floor tanked approach you mentioned, as well as the idea of giving up on the idea of achieving condensing efficiencies and going with either a non-condensing combi boiler, or using a non-condensing tankless heater for both DHW and heat such as the Takagi that houptee mentioned.

Not sure we want to go the route of two dedicated non-condensing tankless water heaters for DHW and space heating - the costs in additional gas plumbing, plus upgrades associated with 2 new stainless ducts seem prohibitive.

Ducting question - if we install a condensing unit and the corresponding PVC exhaust ducting, am I risking sending superheated flue gases out of ducting that is not rated for those higher temperatures if the unit never actually condenses?

Short cycling question - how is the DHW side of a combi-boiler which is defined by short bursts of demand for hot water for stuff like hand washing and shorter showers - not putting a similar strain on the boiler along the lines of space heating short cycling?

 

[Houptee]

Are the existing units on a outside wall now? How is the Rinnai vented to the outside? Up thru the roof or 90 deg directly to outside thru the wall?

A low cost condensing tankless water heater to consider is the Takagi T-H3M-DV-N which vents with PVC and can be common vented with another unit if 4" pvc is used so only one pair of holes thru the house for 2 units. It specs at 15k min fire rate up to 120k max. More than sufficient to heat 27 feet of baseboard.
Street prices I see are just under $700 with shipping included if you search.
Thats why I say put in 2 units, one dedicated for heat, one for domestic water.
http://www.takagi.com/products/tankless-water-heaters/t-h3m-dv-n

You do not need to add more radiators (yet) first you need to do a "Heat Loss Calculation" and then have enough btu's capacity going in to match the "loss" of the building.

So look up online "manual j heat loss calculation" there are some free apps out there or you can do it on paper by downloading the actual manual j documents and worksheets.
Or go to Slant Fin website and they have a free online software at top of screen click www.hydronicexplorer.com at top of screen click "Heat Loss Calculator" and then you have to register (free) to log in and do the calculations.

 

[Jeff Daley]

The Rinnai is mounted on an exterior wall and vents through that wall.

Using the heat load calculator here: https://www.loadcalc.net/load, I came up with some surprisingly high numbers - over 19,000 BTU for heating and 15,000 BTU for cooling. Wasn't expecting that for an 850 sq ft house, but is reflective of the age, poor insulation, single pane windows, and generally loose construction of the place. We are working on improving insulation and replacing many of the single pane windows, so the heat load will go down over the next couple years. In an earlier post, Dana estimated that the heat load was likely 12,000 BTU and likely less "or else the Rinnai wouldn't have been able to keep up"...but the truth is, it really never did keep up. We have a gas fireplace that ought to be mostly for ambiance, but during stretches of the winter, it was part of our primary space heating. As things went along, the radiators kicked out less and less heat such that the gas fireplace was our primary source of heat.

Not ruling anything out at this point, but I would prefer to keep the solution as simple as possible, and it seems like if we can find a combi boiler that can modulate down to the relatively low BTUs this house needs, that would be preferable in terms of finding a pro who can install it correctly and also do any servicing and maintenance over the years. Goes back to my earlier point of homeowners being over a barrel here...regardless of one's skills, a homeowner is at risk of voiding warranties by going the DIY route, and at the mercy of whatever the market will bear for installation. Like I said - I'm looking at $10K and up. Going with 2 tankless systems for space heating and DHW, I'll be needing to modify the gas line plumbing, and potentially modification to the venting depending on the solution we go with. I imagine that using a dedicated tankless water heater would also require some sort of pump and valve setup to circulate water through the radiators as well?

I did find this combi boiler from Weil-McLain: https://www.ecomfort.com/Weil-McLain-383100030/p94359.html which seems interesting - 80,000 BTU with 10:1 turndown ratio...so given my house's larger than expected heat load - it seems like it might be in the ballpark. It's right around $2,000 which is better than a lot I've seen, but I have no idea the quality of this brand. Seems like decent enough reviews.

 

[Houptee]

Combi's are a pain in the ass to work on and will cost you a fortune to have it repaired after warranty runs out.
Warranties on most equipment after 1yr is only for parts not labor.

How handy are you?
I thought you were trying to do this work as a DIY project yourself so you did not have to pay 10k or more.

Did you contact Terry Love the owner / admin of this website for a estimate he is in your neck of the woods?

 

[Dana]

The Rinnai is mounted on an exterior wall and vents through that wall.

Using the heat load calculator here: https://www.loadcalc.net/load, I came up with some surprisingly high numbers - over 19,000 BTU for heating and 15,000 BTU for cooling. Wasn't expecting that for an 850 sq ft house, but is reflective of the age, poor insulation, single pane windows, and generally loose construction of the place.

Even with aggressive inputs the LoadCalc tool usually overshoots reality (or a pro-tool operated by a pro) by more than 20%, often by more than 30%. How much does the load change when you select for "good" or " very tight" rather than leaky construction?

We are working on improving insulation and replacing many of the single pane windows, so the heat load will go down over the next couple years. In an earlier post, Dana estimated that the heat load was likely 12,000 BTU and likely less "or else the Rinnai wouldn't have been able to keep up"...but the truth is, it really never did keep up. We have a gas fireplace that ought to be mostly for ambiance, but during stretches of the winter, it was part of our primary space heating. As things went along, the radiators kicked out less and less heat such that the gas fireplace was our primary source of heat.

If the single panes are in reasonbly good/repairable shape, adding an exterior low-E storm window is a valid approach, costing far less than a replacement window. A tightened-up wood sash single pane with a low-E storm performs about as well as a code-min replacement window, but at a fraction of the installed cost. Perhaps the tightest/best low-E storm windows in the industry is the Harvey Tru-Channel w/the low-E glass option, but the low-E Larsons sold through box store are still pretty good. There are others. As a DIY it can be 1/3 the cost of a replacement window.

I did find this combi boiler from Weil-McLain: https://www.ecomfort.com/Weil-McLain-383100030/p94359.html which seems interesting - 80,000 BTU with 10:1 turndown ratio...so given my house's larger than expected heat load - it seems like it might be in the ballpark. It's right around $2,000 which is better than a lot I've seen, but I have no idea the quality of this brand. Seems like decent enough reviews.

AB-80C would be great if your incoming water temp year-round was north of 60F, but at Seattle water temps it would be woefully under-powered as a water heater. It would cycle but not short-cycle at condensing temps on your 27' of baseboard. But a 2 gpm shower with 40F incoming water takes 65,000 BTU/hr, and a 2.5 gpm shower takes over 80,000 BTU/hr. For a 1-bath house with mostly shower-bathers the smallest you would even consider as a water heater would be the AB-120C- even then you'd potentially be risking marital harmony if drawing hot water from other taps while your spouse was showering. It's very comparable to Navien's NCB-150E. Either one would require a thermally massive hydraulic separator (not necessarily a buffer tank) to keep the short cycling under control.

The bigger you go on burner size, the bigger the hydraulic separator or buffer would have to be, but it could still be "reasonable" with the AB-155C or NCB-180E. With it's own internal thermal mass of the UFTC-140W and a fairly tiny buffer tank might be cheaper/better than an NCB-180E or AB-155C. It has more than twice the water volume of the -155C.

A dual tankless solution would be truly insane. (Just sayin'...)

If there is more than 4' of vertical drain downstream of the shower a drainwater heat exchanger might be enough to make the AB-80C work without thermally massive hydraulic separators or buffering. That's not going to be the case if the shower is on the first floor over a crawl space. The VT1000 series Ecodrain are the current industry leader. They're not cheap, but cheaper than adding buffers or a second tankless. The biggest (tallest & fattest) that fits is the "right" one, both in terms of return efficiency and fuel savings/pay-off over time, the higher performance paying off sooner despite the upcharge for the bigger unit. I have the R4-48 ( 4" x 48") PowerPipe in my house (a bit less than 50% return efficiency) and even during an endless shower in mid-January the burner never goes more than 55-60,000 BTU/hr-out, even though it's not a low-flow shower. A VT1000-3-54 would beat it hands-down on return efficiency, with less pressure drop too.

 

[Jeff Daley]

Apologize for the delay in following up - been out of town for work. Have done a bit more research on the issue and as you and others have pointed out in various forums, one of the fundamental issues is a mismatch in the capabilities of what combi boilers provide on the central heat side and the DHW side. Most units are oversized for what is needed for heat unless you have a giant house or are located in the far north - and undersized for most DHW needs unless you have very warm incoming water temps which most of the country does not have.

Factoring in all the issues at play apart the technical details discussed here such as the amount of room one has to install the heating units, the amount of time one has to spend on research - particularly when it's cold outside and the problem needs to get fixed, the learning curve and warranty-voiding issues with going the DIY route, lack of pros who are familiar with all the details, and often lack of consensus even among the pros, and of course, price - at some point, you just have to research it as good as you can and make the most informed decision which is sort of where I'm at. For whatever reason, the Seattle area appears to lack experts in both the design and installation of these systems.

I think I'm down to going with a smaller combi system and adding as large a buffer tank as my utility room allows (about 20 gallons max). The 2 units I'm down to are the HTP UFTC-140 and the Weil-McLain AB-120-C. The Weil McLain has the benefits of a slightly lower min firing rate of 12,000 BTU (vs 13,400 for the HTP) and a slightly better warranty. The HTP has the benefits of a slightly bigger internal tank (2 gal vs 1.31 gal for the Weil McLain), slightly better DHW rating (77 deg rise for 3.2 GPM vs 70 deg rise for the same on Weil McLain). Their prices are similar. Really comes down to what's more important: a 1,400 BTU reduction in min firing rate (Weil Mclain) or an extra 0.69 gallons of water in the internal tank (HTP). Another pro wants to install the Viessman Vitodens 100-W B1KA, but at a 21,000 BTU min firing rate, terrible warranty, and $1200 more expensive, I think that is out of the running.

I see that HTP makes both a floor and wall mount of the UFTC-140 (floor mount is slightly bigger). Aside from space savings, is there any inherent advantage of one over the other, particularly when a buffer tank will be inline? Another question - will a combi/buffer tank arrangement require primary/secondary plumbing? This is another thing none of the pros has asked/mentioned.

Now I'm left with trying to find a decent 20 gallon (or less) buffer tank. Stratification in a buffer tank is good, right? I thought I saw some manufacturers advertising internal mixing which I thought should be avoided. Boiler Buddy is out - nearly $1000 for an 18 gallon buffer tank is a no-go. I will look into the HTP and Vaughn tanks. Any others I should look at? I see some people use a storage water heater for this, but if stratification is important, seems like I need one made expressly to be a buffer tank with the pairs of valves at the top and the bottom.

Over time, we hope to reduce heat loss from the house via better insulation and windows, and we also may consider replacing some or all of the fin-tube radiators with wall panels - depending on costs and also how the new system fares in heating the house with the fin-tubes and with regard to short cycling.

Final question - isn't a lot of DHW use (short showers, hand washing, filling a small dishwasher or front loader washing machine cycle) effectively the same thing as short cycling? Short bursts of very hot water, and then off. How come there isn't the same sort of concern about this pushing boilers into an early grave as with central heating?

Edit - Dana, thanks for the heads up on the drainwater heat recovery systems...though not appropriate for the house in question here (1 story, crawl space), they would be for a rental property, so I will definitely look into that.