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

[Houptee]

You could put something like this in. It is a "slim duct" split system heat pump and a/c system. Takes up much less attic space than a air handler system.
It has adjustable static pressure so you can dial in the correct flow for the amount of duct work you have.
Other brands of slim duct systems are not able to handle any duct work at all, and only use the outlet as a supply register.

They also have a more conventional style air handler heat pump a/c system but the outdoor unit has inverter compressor technology to make it more energy efficient than a normal unit such as a Goodman or Carrier etc.

https://www.pioneerminisplit.com/blogs/products-lineup/pioneer-ryb-ceiling-concealed

https://www.pioneerminisplit.com/collections/ryb

https://www.pioneerminisplit.com/collections/dyc

 

[wwhitney]

Any other possibilities in terms of high thermal mass combi boilers?

Perhaps I'm missing something, but if you can find a combi boiler that would provide a low-enough heat output rate for your heating system, and the only problem with it is insufficient thermal mass, couldn't you just add a small water storage tank (2-10 gallons) to increase the system thermal mass?

Stainless steel tank electric point of use water heaters are an economical source of insulated stainless steel water storage tanks.

Cheres, Wayne

 

[Sponsor]

 

[Dana]

Aside from the aesthetics of not having a large device hanging on the wall, are there any inherent benefits to a ducted system vs ductless? If not, I'm not sure it's worth the premium in installation cost for the low profile aesthetics.

Yes.

It's primarily about getting adequate heat/coolth distribution into every room that needs it. Going solely ductless has both comfort and efficiency consequences, and in your case may not be cheaper up front. To go ductless and still achieve peak load distribution to every necessary room in houses with heat loads as small as yours requires multiple, ridiculously oversized heads, all of which will spend the vast majority of time cycling on/off taking an efficiency hit with every cycle, and yielding less stable room temperatures. Even the smallest output LG ductless units are 3/4 tonners that run about 1000 BTU/hr out, and would cost north of $3K per head, each with a maximum rated output big enough to cover your whole house load. The smallest Mitsubishis are half ton units with a minimum output 1600 BTU/hr per head.

The smallest 3 zone multi-spits are 2 tonners, with minimum output in the 5000-7500 BTU/hr output on the compressor, resulting in less modulation range on the heads when tied to those compressors then when on dedicated single zone compressors.

Only fairly high performance high-R houses with open floor plans can do well with a single ductless unit. If you're planning on adding 3" of continuous rigid polyiso on the exterior of your walls, insulating that miserable crawlspace, and installing <U0.20 triple-panes you can probably do it all with a single half ton Mitsubishi or 3/4 ton LG and not have comfort issues, but that's quite a bit more expensive than a ducted mini-split.

Regarding the primary/secondary loop plumbing question, I was wondering if it mattered which direction (up, down, or left/right) the T's supplying and returning the secondary loop flow were oriented.

In pumped low mass systems gravity has only a 4th order effects on the orientation of the hydraulic separator, just ahead of the Coriolis effects of the earth's rotation. Don't sweat those details.

Any other possibilities in terms of high thermal mass combi boilers? [/QUOTE]

HTP's Versa Hydro series has a fairly long and good history, but are extreme overkill for your loads. The smallest in series PHE-130-55 can put 100,000 BTU/hr into the heating side at max fire, but only about a 29,000 BTU/hr minimum output (more than twice your design heat load). But with the 460lbs of buffering thermal mass of the water in the tank it simply can't short-cycle. It's expensive, but it'll work, is fairly easy to install, and doesn't take much floor area compared to a boiler + indirect. See page 11 of the manual for the dimensions to see if it will fit.

A vertically mounted -9RLFCD only takes up about 7 square feet of floor air, and can sometimes be mounted horizontally under ceiling level in closets etc rather than in the mechanical room. This is even bigger, the 3/4 ton version taking up something like 8-9 square feet of floor being built into a tiny mechanical cabinet/closet heating and cooling a house in Berkeley CA:

The big grille is the common return & filter. The 18RLFCD cassette is the first box above it, with the refrigerant & drain lines hooked up, the box above that is a short penum where the flex duct takes off to other parts of the house.

This one is tucked in the back of a coat closet in an entry hall:

 

[Houptee]

Way lower cost mini splits out there.
Pioneer, Senville, Gree, YMGI, Thermal Zone and hate to break the news but they are all made in same factory in China as the big name brands.

 

[Jeff Daley]

Looking at some of those mini split systems, knowing that I won't be doing any of the installation, I can't see getting out of a full-home system (outdoor unit, 4 indoor units, ducting, and associated installation) for less than $12K, and probably more than that. For some context on the cost of professional trade work in Seattle, the going rate for the installation of a single electrical outlet is about $500 once permits and taxes are factored in. So, $12K (and likely more) for a brand new heating system, and I still need some type of DHW system to go inside a tiny utility room (55 tank system will not fit), and I'm in it for another $3K-$5K on top of the >$12K.

Given an array of solutions that are all flawed in some significant way, I tend to agree with Wayne - that adding thermal mass to the system and using a combi for DHW and space heat is the best approach all things considered. Regardless of heating system, I still need a source of DHW, and I already have the fin tube baseboards and plumbing in place, so I can't see anything other than a combi being able to address the issue given utility room space and cost constraints. Which is funny in a way, I guess, as I started this whole thread concerned about the $10K+ costs associated with going with a combi install.

Getting into the buffer tank approach opens its own can of worms - 4 pipe vs 2 pipe? Like pretty much everything, ask 10 people, get 10 different opinions. I can tell you this, paying $1,000 for a specifically-designed small (~20 gal), 4-pipe buffer tank is insanity. So, going with a using a small point of use tank heater like Wayne suggests seems like the way to go, and those all tend to have only 2 connections so looks like it'll be a 2 pipe approach for the buffer thank. Then, there's the issue of how exactly you plumb the 2 pipe buffer: 1) on the primary loop and before the heating load? 2) on the primary loop and after the load? 3) in series or in parallel on either the supply or return side? 4) and then do you pipe the input at the top and output out the bottom or vice versa? In my research, I've found pros far knowledgeable than me arguing passionately for every single one of those permutations. I found this thread https://forum.heatinghelp.com/discussion/104424/buffer-tank-question from back in 2007 pretty interesting as it contains a post (5th in the thread, from user MikeL_2) who apparently tried multiple buffer tank piping approaches collecting data on cycle length and determined that on the supply side (after boiler, before load), in series, and with the input at the top and output at the bottom, to be the most efficient approach.

John Siegenthaler appears to be someone whose opinions carry a lot of weight related to HVAC engineering, and on this page: https://www.pmmag.com/articles/100544-the-finer-points-of-applying-a-2-pipe-buffer-tank he weighs in on the merits for a 2-pipe approach in such a way that the buffer tank itself makes up the bulk of the primary loop with the supply and return T's for the secondary loop closely adjacent to the input/output of the 2 pipe buffer:

He also appears to suggest using a specifically designed buffer tank that will suppor the installation of air escape valve at the top - not something that a repurposed point of use tank will support, but I imagine there must be other ways to release air trapped in the buffer tank during filling. I have also read that "supply houses" carry tank "carcasses" that haven't yet been outfitted with the heating element and other components for less than an actual functioning tank, so I might look into that. As a place to start, the Rheem 10 gallon point of use heaters like this one:

seem promising, so I will start looking there.

 

[Dana]

Way lower cost mini splits out there.
Pioneer, Senville, Gree, YMGI, Thermal Zone and hate to break the news but they are all made in same factory in China as the big name brands.

Midea makes a lot of products for those brands, but Gree has traditionally had a lot of non-Midea mini-splits. I'd be surprised if any Pioneer units were 100% Midea under the hood, built in a Midea factory.

Midea's Premier Hyper series cold climate mini-splits are pretty good, and some are being marketed in the US under Carrier's nameplate. But SFAIK all Midea ducted units need to be mounted horizonally, which may be a limiting factor in tight spaces. But maybe it could be mounted below ceiling level in the utility room, and still have enough space for a water heater(?).

The 40MBDQ09---3/4 ton mini-ducted Midea Excel series unit married to a 38MAQB09R--3 compressor would do just fine in this house, if you can find a competent person to install it. It's minimum modulation at +47 is only slightly (negligibly, really) more than a 3/4 ton Fujitsu 9RLFCD, so it should be able to modulate nearly continuously with load without excessive cycling.

[edited to add]
I've discovered in the past couple of days that indeed Pioneer is now selling OEMed Midea units- I was mistaken! I'm not sure if all current Pioneer models are relabeled Mideas, but at least some are. There seems to be some major consolidation going on in that industry in China!

 

[Dana]

At your flow rates using an unpowered electric tank water heater as the "closely spaced tees" would work, even if you have to tee at the tank's hot output and drain connections. If it's dual heating element unit using the heating element ports for boiler/primary loop, and the hot-out and drain port at the bottom for the heating system loops is a pretty standard hack. Be sure to size the expansion tank on the heating system correctly for the water volume in the tank.

^^^^^ this could be an unpowred electric water heater^^^^^

 

[Houptee]

This online trade mag has a lot of good articles about hydronic heating.
The "Glitch and Fix" articles are always interesting to read.
https://www.pmmag.com/radiant-and-hydronics