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

[Dana]

The UFTC-140 has 4 gallons/ 33lbs of water in just the heat exchanger (distinct from the one gallon domestic hot water tanklet) compared to about 11lbs for the AB-120.

UFTC-140 combis also have an internal primary pump and MUST be plumbed primary/secondary on the heating side (unlike the UFT heating boilers), despite the low-head heat exchanger, though you could probably cheat that if you run the math on heating system loop's pumping head, using ONLY the internal pump (set to 3, it's maximum speed), given that it's one zone and probably fairly low head. The manual for the Westinghouse branded version of the UFTC-140 lives here. I believe they're identical under the paint, but the manual for the UFTC-140 appears to be updated and slightly different. It's worth looking over both manuals for figuring out the system design. In a quick scan I didn't see a maximum length specified for the length of plumbing between the boiler and the closely spaced tees hydraulic separator, but maybe it's in there. (Look for it.)

There is no particular advantage to wall mount vs. floor mount versions. The wall mount units are usually cheaper, but not by much when factoring in the cost of the wall-bracket for mounting it.

If 87% efficiency is acceptable cranking it to ~155-160F out on 27' of baseboard would guarantee nearly continuous burns during calls for heat without buffering. The max space heating output temp on the thing is 180F, so you really CAN get there without buffering, but you can also probably inch it down into the higher temp end of the condensing zone on space heating calls without short cycling it:

With even 50 feet of 3/4" plumbing + fin-tube you'd be looking at another 8 lbs of thermal mass beyond the 33lbs of water in the heat exchanger. Assuming so with ~50lbs total mass to work with, an output temp of 140F, AWT of 135F the 27 fin tube would be emitting ~8100 BTU/hr and running at close to 90% efficiency. At 90% efficiency the UFTC-140 would be delivering 12,600 BTU/hr into the system, for about 4500 BTU/hr ( = 75 BTU/minute) of excess heat into the system. With 50 lbs of thermal mass to work with the temperature would be slewing 75BTU/50lbs= 1.5F per minute, so it's highly likely you'd be able to get at least 3 minutes of minimum burn time out of it without external buffers.

The differential temperature between burner-off and burner-on doesn't seem to be programmable (it might be- read the programming details in the manuals), so the minimum temperature before it short-cycles may have to be determined empirically. With 3+ minute burns and much fewer than 10 burns/hour during continuous calls for heat (<5 burns would be better) it won't be putting huge amount of wear and tear on the boiler.

 

[Jeff Daley]

Well...maybe just scratch my entire previous post. As I was looking into various schematics for plumbing a combi boiler with a buffer tank, most of the ones I've seen include the DHW heating accomplished via an external BPHE...and I guess that makes sense, otherwise you're drawing your DHW off an open loop/petri dish tank situation. I guess you could plumb it so that the combi's DHW output bypasses the buffer tank and goes straight to fixtures, but then you lose benefit of the tank for limiting short cycling for DHW use. Am I wrong about this?

If I'm not, then going this route I would still need the combi boiler, the buffer tank, BPHE and all the associated valves and pumps that goes along with it. I might as well go back to your original idea of using a condensing storage tank heater for both space heating and DHW. Not only would that save money, it probably saves room depending on the size of the storage tank heater I get. The HTP Crossover is certainly modest enough in footprint to fit in my utility room. You suggested the RGH20-75 - but is 75,000 BTU sufficient on the DHW side of things? Earlier, you indicated the 80,000 BTU Weil McLain combi boiler was woefully undersized for DHW. I guess with the 20 gallon storage and fairly fast recovery, though, then the 75,000 BTU Crossover should be able to keep up with a standard shower and maybe a load of laundry at the same time? We are not luxurious shower-ers...certainly 10 min and less.

With such a small tank such as the Crossover, what happens to the radiant heating side of things while someone is in the shower? I imagine there would be a non-trivial dip in water temperature in the tank during a 10 minute shower. Since the Crossover isn't necessarily made to serve double duty, I imagine there isn't a similar ability for it to prioritize DHW calls and temporarily cease pushing water through the radiant system as most combi units have?

 

[Sponsor]

 

[Dana]

Well...maybe just scratch my entire previous post. As I was looking into various schematics for plumbing a combi boiler with a buffer tank, most of the ones I've seen include the DHW heating accomplished via an external BPHE...and I guess that makes sense, otherwise you're drawing your DHW off an open loop/petri dish tank situation. I guess you could plumb it so that the combi's DHW output bypasses the buffer tank and goes straight to fixtures, but then you lose benefit of the tank for limiting short cycling for DHW use. Am I wrong about this?

With a combi boiler the buffer tank would only be on the heating side. If storing hot water on the potable side a standard boiler + indirect would be a simpler solution. If all the thermal mass is in a tank isolated from the heating side it does nothing to suppress short cycling from limited radiation.

You suggested the RGH20-75 - but is 75,000 BTU sufficient on the DHW side of things? Earlier, you indicated the 80,000 BTU Weil McLain combi boiler was woefully undersized for DHW. I guess with the 20 gallon storage and fairly fast recovery, though, then the 75,000 BTU Crossover should be able to keep up with a standard shower and maybe a load of laundry at the same time? We are not luxurious shower-ers...certainly 10 min and less.

An 80K combi is BTU delivery rate limited by the burner- it has to deliver 100% of the heat in real-time. With even a 20 gallon tank the peak BTU delivery rate is limited by the water flow, and can exceed the burner's output by many times, but just not continously. So even if the laundry is drawing 5 gallons of hot water in 1 minute, the temperature at the shower is not affected. But if the laundry is drawing five gallons per minute for 3 minutes (15 gallons) the shower temp is at risk. But laundry does gulp anywhere near that much water.

With such a small tank such as the Crossover, what happens to the radiant heating side of things while someone is in the shower? I imagine there would be a non-trivial dip in water temperature in the tank during a 10 minute shower. Since the Crossover isn't necessarily made to serve double duty, I imagine there isn't a similar ability for it to prioritize DHW calls and temporarily cease pushing water through the radiant system as most combi units have?

As the temperature in the tank drops, the heat rate out of the radiation drops too, but so what? Your house won't get cold by a 10 minute reduction in heat rate, and the tank will be fully up to temp by the time you are dried off. Most boiler + indirect solutions suppress the heating calls entirely (zero heat rate out of the radiation) whenever the indirect is calling for heat. It's not a problem.

In my house I have enough radiation to emit ~45,000 BTU/hr when the systems buffer tank is at 125F. The buffer tank is a 48 gallon "reverse indirect", serving up domestic hot water from an internal heat exchanger. During continuous calls for heat from all zones while someone is taking a long shower the tank temp can sometimes drop below 110F, but the output of the burner serving the tank never delivers more than 70,000 BTU/hr. Even though it takes longer for the 48 gallons to recover, the reduced heat rate from the radiation doesn't affect the room temperatures. This is really NOT a problem to go with a 75KBTU/hr 20 gallons solution on your system, unless you're running two simultaneous 10 minute showers. But even if the tank gets depleted to a tepid-shower 95F by the end of the gulping dual showers, the reduction in heat rate out of the radiation won't be long enough for the room temperatures to suffer.

 

[Jeff Daley]

Really appreciate all the help here. I'm going to start talking to a few pros and kick their tires about this basic approach and using the Crossover as the heating unit. Though I'd like to avoid taking on this job myself due to bandwidth issues and for the peace of mind of having a pro who understands it doing any periodic servicing, if I can't find someone professionally, I can probably handle it though it won't be fast. Is there a place where the parts in the diagram you cited earlier are fully spec'd out?

A couple questions - the cold water feed in the top right of the diagram is just to fill the radiant loop part of the system? Any other reason for it? What would the optional high temp supply/return be for? Will I need any dirt filters, sediment traps anywhere?

I saw a similar system in a YouTube video from Bailey Line Road - I imagine as a pro you are familiar with it.

I know the open "simple" loop is frowned upon by most, but the closed loop system appears to be very similar to the diagram you showed. I liked the idea of installing purge valves above and below the BPHE on both the tank and the radiant loops in order to facilitate cleaning.

Is there any value to the "heat trap" series of 90 degree fittings where the DHW leaves the tank?

Finally, what is the work around as far as valving on the Crossover to plumb the BPHE loop? The Crossover looks like it just has a basic cold/in and hot/out pair of valves. In some of the diagrams I've seen, there is a second pair to facilitate connection to the BPHE loop, often with the cold/in and BPHE loop/return at the bottom and DHW/out and BPHE/out at the top. I think a lot of the Polaris and Vertex tanks have these.

 

[Dana]

It's true that the Crossover doesn't have purpose-made side ports for space heating applications, but the drain port the bottom can be used as the return from the heat exchanger, and HTP supports use ad a combi. HTP sells a fittings kit for making those heating connection modifications to a Crossover (~$90 from distributors), but it's not hard to DIY it from fittings purchased elsewhere. See Figure 6 in the kit instructions- replace "air handler" with "heat exchanger".

Househeeds (where I clipped the diagram) makes parts-shopping for those types of system fairly easy with links from this page. Anybody's cheapest smallest 10 plate heat exchanger is going to be more than adequate from a heat transfer capacity point of view, as long as it is potable-compatible.

The pump on the potable side the pumping head of a plate heat exchanger also needs to be potable-compatible (stainless, or potable grade bronze). The pumping head of a tank type water heater is really small, but the pumping head of the heat exchangers can be substantial at high flow- you probably don't need more than 2-3 gpm. (3gpm at a 10F delta-T is 15,000 BTU/hr.)

If there is 100' or less of total plumbing on your heating loop, and the heating pipe & other components are potable-compatible materials it's reasonably safe to run it open loop, as long as there is a timer to give it a minimum daily purge, even when there is no call for heat. (I'm not crazy about this approach but it's legal in my state, and probably yours too.)

HTP's Phoenix Light Duty PH76-50 also has side taps for space heating and a modulating 76,000 BTU/hr burner, but it's a 50 gallon tank, and like the Polaris, overkill unless you have large tubs to fill. The Vertex is a non-modulating glass lined tank version, but in other respects comparable tot the Light Duty. The Polaris water heaters have a tendency to short cycle in heating applications unless you do a resistor swap on the control board, voiding the warranty, but your radiation is small enough that it's probably not going to run in to that issue.

 

[Jeff Daley]

Well, just had an interesting chat with the techs at HTP. I wanted to get their thoughts on using the Crossover in double duty as DHW and space heat, and also ask them about the differences between the RGH20-75 vs the 76 (I still don't know what they are - it can't just be 1,000 BTU output) and received some surprising information: both the 75 and the 76 have been discontinued and the 100 soon will be. He strongly advised me against buying any of the Crossover series due to serious design flaws. Apparently they also have no short term plans to develop a small tank substitute and suggested I look at the Phoenix series if I want a tank system.

Secondarily, he also strongly frowned on the idea of using one of their water heaters (of any type) to operate a closed loop radiant heating system. He said the only space heating application they approve of for any of their water heaters is an open loop system using an air handler. He was 100% unambiguous about this. He also thought it was generally a bad idea to use them as a space heater even in an open loop air handler application, but obviously since that application is expressly sanctioned by the company, he didn't outright say not to.

Not wanting to put me or an HVAC pro in a position where I'm using an appliance in a way that the manufacturer expressly says not to, that likely puts me back to where I was a couple steps ago - looking at a combi boiler with as low a min firing capacity as I can find such as the HTP UFTC 140 or the Weil McLain AB-120C and then likely adding a small buffer tank.

 

[Dana]

Well, just had an interesting chat with the techs at HTP. I wanted to get their thoughts on using the Crossover in double duty as DHW and space heat, and also ask them about the differences between the RGH20-75 vs the 76 (I still don't know what they are - it can't just be 1,000 BTU output) and received some surprising information: both the 75 and the 76 have been discontinued and the 100 soon will be. He strongly advised me against buying any of the Crossover series due to serious design flaws. Apparently they also have no short term plans to develop a small tank substitute and suggested I look at the Phoenix series if I want a tank system.

Sorry to hear that news about the Crossover design flaws (they were looking like a great solution for the low loads of apartments!)

The difference between the RGH20-75 and -76 has to do with water heating efficiency testing and labeling. At 75K and smaller it is required to be UEF tested as a residential water heater. At 76K and up it doesn't- the steady state thermal efficiency is a sufficient labeling spec. (That's why the Phoenix Light Duty and Vertex come with 76K and not 75K burners.)

Secondarily, he also strongly frowned on the idea of using one of their water heaters (of any type) to operate a closed loop radiant heating system. He said the only space heating application they approve of for any of their water heaters is an open loop system using an air handler. He was 100% unambiguous about this.

Yours isn't a radiant heating system- it's fin tube baseboard, and has no more plumbing than many air-handler systems. Radiant heating systems have a lot more thermal mass in the plumbing, and even more with concrete slab radiant, which is part of why they're reluctant to sign on to that. I suspect they would also run into all sorts of problems with idiot backyard designers over/under pumping systems with heat exchangers in the middle and blaming HTP for the system design flaws tying up tech lines with a lot of hours of pure BS. Your system is sufficiently low mass and simple that even with large errors it could be made to work.

He also thought it was generally a bad idea to use them as a space heater even in an open loop air handler application, but obviously since that application is expressly sanctioned by the company, he didn't outright say not to.

Not wanting to put me or an HVAC pro in a position where I'm using an appliance in a way that the manufacturer expressly says not to, that likely puts me back to where I was a couple steps ago - looking at a combi boiler with as low a min firing capacity as I can find such as the HTP UFTC 140 or the Weil McLain AB-120C and then likely adding a small buffer tank.

As pointed out earlier, there is enough thermal mass in the UFTC-140 (and not enough in the AB-120C) to be able to run it at near-condensing temps without short-cycling. With loads as low as yours there's zero payback on adding a buffer tank to be able to run the UFTC-140 at condensing temps to save 5-8% on the already low heating bills. The heat exchanger in the UFTC-140 is pretty much the same as a 4 gallon buffer tank.

If you're going to spend the kind of money it takes for a properly set up buffer tank on the system, replacing some or all of the baseboards with equivalent (or larger) output flat panel radiators would add both comfort and thermal mass to the system. The 12" tall x 2.5" deep Buderus series 21 rads come in some shorter lengths and a 24 incher has the output of 3 feet of fin-tube baseboard, but has 0.66 gallons or 5.5lbs of water, and another ~2lbs of water-equivalent mass in the steel, compared to about 0.6lbs of water-equivalent mass for the 3 feet of 3/4" piped fin tube it replaces (half that if it's half-inch fin tube.) If you replaced the whole 27' of fin tube with 27' of 12" panel rad it would be even better, but at $100+ per foot you might want to be selective, and only install them in rooms where comfort matters, and only ~2/3 as long as the fin tube that it's replacing.

There are cheaper,options for panel rads than Buderus though. The 12" tall x 5" deep EcoStyle rads put out about 4x as much heat per foot as fin tube (compared to 1.5x as much per foot for the 21 series Buderus), at comparable thermal mass per foot as the Buderus, if a 5" depth isn't a deal breaker. The 12" x 24" EcoStyle runs a bit more than $200, which is 15-20% cheaper with 35-40% more output (equivalent to about 4.5' of baseboard) than the 12" x 24" series 21 Buderus, and about the same thermal mass. Spending a grand on EcoStyle panel rads would roughly double the thermal mass of a UFTC-140 based system and increase comfort levels (and if done right, increase low-temp output), whereas spending it on a buffer tank only increases the marginal efficiency.

There are others, at even lower price points, if replacing all of it is something you'd consider.

Case in point: The ~8" tall Myson CV21 and CV22 series run ~$30/$45 per foot respectively. The CV21s have only slightly more output than fin tube per foot, the CV22s have 1.5x as much. So replacing 27' of fin tube with equal lengths of CV22 is would run about $1200 and would emit about 8000 BTU/hr at an AWT of 120F (instead of ~5500 BTU/hr with fin-tube or CV21). CV22 has about 0.2 gallons or 1.7lbs of water mass per foot, and another ~1 lb of water-equivalent thermal mass in the steel, 2.7lbs per foot total. So for 27' you'd be adding about 73 lbs of thermal mass to the system. The UFTC-140's heat exchanger has 4 gallons/ 33lbs of mass, the rest of the interconnection plumbing even more but rounding down to 100lbs to be conservative.

At 95% efficiency and 14,000 BTU/hr-in the UFTC is dumping 13,300 BTU/hr in to the system, and at an AWT of 120F the 27' of CV22 would be taking about 8000BTU/hr out. That leaves 5300 BTU/hr 88 BTU/minute of excess going into the system. That means the temperature is then slewing 88 BTU/ 100 lbs= 0.88 F per minute. Over 5 minutes it would only slew only 4.4F. Your minimum burn time would almost certainly be running well over 5 minutes, with no more than about 5 burns per hour during continuous calls for heat if the output temp was never much lower than 125F or so. That's going to be edging into mid-90s efficiency, and a 5 minute burn is definitely NOT a short cycle for a mid-mass boiler like the UFTC.

I'd personally go that route before adding a buffer tank. YMMV

Got a breakdown of the lengths of all your baseboard sections?

 

[Jeff Daley]

I've got five 5' baseboards - 1 each in a kitchen and 2 bedrooms, and then 2 in the living room; and then a small 2' baseboard in the bathroom. We hope to replace the baseboards with panels, though that will likely happen in a couple phases and probably not right off the bat when we replace the boiler.

Going to talk to HTP again tomorrow as well as a couple of the pros I've gotten previous estimates from. One guy bidded an HTP, but it was the 199K BTU combi; the other bidded a Viessman 100-W B1KA. I'd like to get their thoughts on the smaller HTP as well as ask why they never inquired about the home's heat loss before bidding.

I also probably need to get under the house in the crawl space and inspect the plumbing to the radiators. Given the quality of the workmanship of various systems that the previous homeowner installed (the heating system is one of them), there's no guarantee that the baseboards are plumbed correctly, and based on what the regular plumber said when he was under the house, he thought he saw some galvanized pipe under there that were not among the supply lines that he replaced with copper - so it is entirely possible that the plumbing between the baseboards in galvanized, and I hate to think what corrosion coming off those pipes might mean for a new boiler. Which leads me to ask - does plumbing for a relatively simple single zone baseboard system usually include things like sediment filters or dirt traps?

You mentioned that given to the relatively simple system in my home that I might be able to get away without using primary/secondary plumbing, but sounds like it would be advisable, so I will mention that when I talk to the contractors.

 

[Houptee]

You would solve the rust and sediment problem by replacing all the pipes with 1/2" oxygen barrier pex tubing.
Home run each panel rad back to a manifold so you get same temp water to each and then you put TRV thermostatic radiator valves on each rad so you can fine tune the room temps.

 

[Houptee]

You have no domestic hot water or heat at all since you started the thread?

 

[Dana]

I've got five 5' baseboards - 1 each in a kitchen and 2 bedrooms, and then 2 in the living room; and then a small 2' baseboard in the bathroom. We hope to replace the baseboards with panels, though that will likely happen in a couple phases and probably not right off the bat when we replace the boiler.

Going to talk to HTP again tomorrow as well as a couple of the pros I've gotten previous estimates from. One guy bidded an HTP, but it was the 199K BTU combi; the other bidded a Viessman 100-W B1KA.

The minimum modulated output of that Viessman 100-W B1KA is ~20,000 BTU/hr at condensing temps, still more than 18,000 BTU/hr at it's maximum (non-condensing) temp, and it only has ~8.3 lbs of thermal mass in the heat exchanger.

That's a terrible fit for a system with only 27' of baseboard. Keep in mind that that at temps that would deliver 95% combustion efficiency your radiation won't emit more than ~5000-5500 BTU/hr, so there has to be enough thermal mass in the system somewhere to deal with any minimum-fire output in excess of that number. The 100-W B1KA is just plain ridiculous (but of course most combi boilers are.)

I'd like to get their thoughts on the smaller HTP as well as ask why they never inquired about the home's heat loss before bidding.

HVAC contractors rarely ask about or calculate the heat load, and many/most of them aren't even competent to do that task. I'd never ask a plumber to to that math- it's outside their wheelhouse. But any boiler installer should at least be able to do the math on the radiation, but many/most of those that do measure up the radiatio assume 180F water, giving no thought to what it takes to get condensing efficiency out of it without short-cycling the thing into an early grave. A hydronic heating designer would/should know how to do it correctly, most boiler installers and very few general plumbers haven't developed those design skills. Larger "heating and plumbing" companies will have hydronic designers on staff or outside consultants that they use to specify the system components, but most of the mom & pop plumbers are just winging it, often with pretty bad results on tiny systems like yours.

Bottom line- don't even bother asking them why they never asked about the heat load. That would only put them on the spot and piss 'em off, which would be a waste of your time (and theirs).

I also probably need to get under the house in the crawl space and inspect the plumbing to the radiators. Given the quality of the workmanship of various systems that the previous homeowner installed (the heating system is one of them), there's no guarantee that the baseboards are plumbed correctly, and based on what the regular plumber said when he was under the house, he thought he saw some galvanized pipe under there that were not among the supply lines that he replaced with copper - so it is entirely possible that the plumbing between the baseboards in galvanized, and I hate to think what corrosion coming off those pipes might mean for a new boiler. Which leads me to ask - does plumbing for a relatively simple single zone baseboard system usually include things like sediment filters or dirt traps?

If it were all cast-iron baseboard and all iron plumbing that might make sense to install filtration systems, but it sounds like there's only a limited amount of iron plumbing in the system. I have several cast iron rads three zones on my system, but it's all being fed from a buffer tank at modest flows (which makes it behave a bit like a dirt trap), and I have only a small screen filter. Grit may accumulate in the bottom of the buffer tank to be purged every few years, but it's doing just fine. The most recent cast iron rad I put on the system had seen 40 years of service before I bought it last winter. In the first week after installation I must have cleaned flakes of rust/grit the screen filter 5-6 times, but never since. Unless you've been running potable water through the heating system the galvanized plumbing isn't likely to be corroding & spitting up more grit.

If you're concerned, using the smallest Spirovent VDR as the main system vent would also include a dirt trap.

You mentioned that given to the relatively simple system in my home that I might be able to get away without using primary/secondary plumbing, but sounds like it would be advisable, so I will mention that when I talk to the contractors.

Most contractors won't want to do anything that would violate the warranty. Some contractors might give you a deer-in-the-headlights stare if you use terms like "primary/secondary", or "thermal mass", but it's a good thing to ask them about these things, and how they are going to keep the boiler or combi-boiler from short-cycling at condensing temperatures given your limited baseboard and low thermal mass, to force them to think about it before making inappropriate proposals. Let them know that you know that the baseboard won't emit more than 5000-5500 BTU/hr at temperatures that deliver decent condensing efficiency, and that it's a problem that needs to be resolved in any proposed solution.

As I pointed out in one of the early responses, combi-boilers are always a lousy fit for the loads of normal sized houses and systems, though they can be made to at least work if you do at least a napkin math rough cut of the hydronic design and pick something that's at least remotely workable.

 

[Jeff Daley]

Thanks houptee. To answer your last question, the house has not had hot water or working radiators since even before the thread started, however, we have been staying in a rental place the past several weeks as we are having a lot of work done on the place so it is basically uninhabitable anyway. There is a small gas fireplace that we run when we are over there so that the pipes don't freeze. Speaking of which, I need to get into the crawl space and inspect the plumbing to the radiators. There are copper stubs coming up out of the floor at each radiator, but I don't have 100% confidence that the piping under the floor is copper or that they are plumbed in parallel off a manifold as you suggest...could be a linear straight line run from radiator to radiator.

If I determine the plumbing is subpar, would you advice going with oxygen barrier pex over copper? I imagine it would not be advisable at all to leave the galvanized in place (if in fact there is galvanized pipe in the heating loop)?

 

[Dana]

Thanks houptee. To answer your last question, the house has not had hot water or working radiators since even before the thread started, however, we have been staying in a rental place the past several weeks as we are having a lot of work done on the place so it is basically uninhabitable anyway. There is a small gas fireplace that we run when we are over there so that the pipes don't freeze. Speaking of which, I need to get into the crawl space and inspect the plumbing to the radiators. There are copper stubs coming up out of the floor at each radiator, but I don't have 100% confidence that the piping under the floor is copper or that they are plumbed in parallel off a manifold as you suggest...could be a linear straight line run from radiator to radiator.

If I determine the plumbing is subpar, would you advice going with oxygen barrier pex over copper? I imagine it would not be advisable at all to leave the galvanized in place (if in fact there is galvanized pipe in the heating loop)?

If replacing the plumbing, PEX is quicker/easier /cheaper than sweat copper. Insulating it to at least R3 would be a good idea (an IRC code requirement, actually) and easier to do while installing the pipe than retrofitting as an after thought. PEX can usually even take freezing without damage, whereas freezing will often separate sweat soldered fittings on copper.

 

[Houptee]

I did my 3 family house with all 1/2" oxy pex from a supply and return manifold. I have fin tube baseboard in 3 apartments each has their own Takagi TK-Jr (currently known as the TK-110U-I) as the heat source.
I split it up so there are separate loops to different rooms. The advantage is you get more uniform temperature water reaching each loop, and if there was a leak you could turn off one loop and still have heat in the house to the other loops.
1/2" oxy pex is really inexpensive I bought a 1000' roll online and the manifolds too.
Very simple system you just need a T stat, a Taco switching relay, a Taco 007 circulator, and the feed water pressure reducing valve and backflow preventer, a air scoop with automatic air bleeder, and a expansion tank.
I do not use closely spaced tees just piped the system to flow thru the Takagi as a closed loop.
When T stat calls for heat the relay turns on the Taco 007 which starts the flow thru the system.
The Takagi sees the flow and fires up the burners and the temp is set to 180, so the burner runs high at first then as return water comes back warmer the burner modulates down to lowest fire rate.
Systems normally run for a few minutes until T stat is satisfied and then it shuts off until the next call for heat.
Apt 1 is 950 sq ft
Apt 2 is 1200 sq ft
Apt 3 is 1500 sq ft

 

[Dana]

The Takagi sees the flow and fires up the burners and the temp is set to 180, so the burner runs high at first then as return water comes back warmer the burner modulates down to lowest fire rate.

I take with 180F output temps it you're not using the Takagi as a water heater, only a space heating boiler?

Older Takagi water heaters (TK2, TK1) shut down when the return water went over 130F, causing systems like that to cycle on/off a lot unless there was a substantial amount of baseboard.

 

[Houptee]

3 units are dedicated only for Heating each apartment.
But I also have 3 more that are dedicated for the domestic hot water, one for each apartment.

They just burn at low fire rate most of the time and output temp stays steady at 180.

These are non condensing models so the stainless venting has to be used but they are all in a row on a outside wall so I was able to come off the top with the Z Flex adapter, then the z flex elbow, a wall thimble, then the z flex vent hood. Venting was about $100 extra per unit but if these machines fail I can reuse the venting and replace the TK Jr with the newer model TK-1100U-I.

 

[Dana]

Mind you, the newer Takagi water heaters since the buy-out are reportedly not as robust as the TK-Jr & earlier vintage units are/were.

None of them were/are warranteed for use for any space heating uses other than "open" combi applications, even though they work just fine as modulating heating boilers if not over-pumped.

The minimum modulated output of the TK-JR at 180F is about 17,000 BTU/hr, which still takes something like 33-34' of baseboard to balance perfectly at an entering water temp of 180F, at least 25% more baseboard than Jeff Daley currently has to work with.

The TK-110U-I puts out about 13,000 BTU/hr @ a maximum output temperature of 140F, which would need about 50 feet of baseboard to balance. Is there enough baseboard in each apartment? Is the heat load of each unit less than 13,000 BTU/hr, the maximum that the baseboard would emit at the maximum water temperature?

In that series the TK-510U-I would be needed to take it higher than 140F, and then only to 160F (not 180F like the TK-JR) which would also not balance with his 27' of baseboard, but might still work with ~35, and may serve as a TK-JR replacement in your three family apartment application if it still keeps up with the load using 160F water.

 

[Jeff Daley]

Following up here...was talking to a boiler installation pro who had earlier given us a bid. He seemed receptive to my concerns about short cycling and was doing some of his own research. Focusing on the minimum BTU side of things, he suggested I look at the NTI FTV110C and FTV150C - both with 10:1 turndown ratios modulating down to 11,000 and 15,000 BTU, respectively. I took a quick look and they don't look terrible, but I doubt the 110,000 BTU of the FTV110C would be sufficient. In looking at total water capacity, it seems like they are a "medium mass" boiler at 2.0 gallon capacity for the FTV150C - not as good as the HTP UFTC-140W with 4 gallons, but better than comparable combis from Viesseman and Weil-McLain.

Then, this guy told me that he had heard that HTP was discontinuing the UFTC model line. I called HTP, and sure enough, both the UFTC-140 and UFTC-199 are going to be phased out. I asked him if this was due to design flaws or reliability issues, and he said, point blank "yes", but would not elaborate on that. This is on top of the discontinuing of the entire Crossover Floor tank heaters. So HTP is in a bit of a state of flux right now. After talking with the HTP guy, the replacement line is called the Elite Ultra (ELU) series. Here's the links: https://www.htproducts.com/eliteultra.html and http://www.htproducts.com/literature/lp-700.pdf The line includes the ELU-120WCN and ELU-150WCN combis that feature 11:1 turndown ratios that modulate down to 11,200 and 13,200 BTU respectively. However, it looks like HTP is moving completely away from the high mass fire tube heat exchanger approach to a totally different style of heat exchanger that is much lower mass: 1.1 gallon volume for the ELU-120WCN and 1.2 gallons for the 150WCN. So, despite the lower minimum modulation rates of these boilers (relative to the UTFC series), it would appear the tradeoff in terms of lower thermal mass may not be worth it. I told the HTP tech that there are still UFTC-140 models out there on the market, and if I had the choice to pick up one of those or one of the new ELU models, he emphatically said get an ELU. This could just be sales-speak, hyping up the new series.

So, now, once again, after thinking I had nailed down the "correct" course of action, a product line discontinuation from HTP has thrown up some red flags. And they didn't just make some minor tweaks, it looks like they basically blew up the entire general design of the previous UFTC product line. Are there are other combis with 10:1 turndown and high-ish thermal mass out there? Looks like the NTI with 2.0 gallons and min firing rate of 15,000 BTU might be the best of some not great alternatives.

Marginally related question - for primary-secondary loop plumbing, is there are a preferred orientation for the closely-spaced tees that connect the primary loop w/ the secondary? In other words, should the base of the two tees for the supply/return of the secondary intersect the primary loop vertically at the top, bottom, or horizontally to one of the sides? This may sound like minutiae but if there's one thing I've learned in this exchange, it's that every little detail seems to matter. Not sure if thermosiphoning would be an issue or not.

 

[Dana]

All low mass combi boilers are a lousy fit for home with low heat loads. Or did I mention that already?

When using closely space tees as the hydraulic separator the loop with the highest flow should be the straight path, and the lower flow loop the teed-off path. With your simple system the flows are probably going to be comparable and it may not matter.

Sorry to hear that HTP is giving up on the Kiturami-designed & manufactured combi boilers. I hope that's not also true for their Kiturami cousin space-heating only boilers, the UFT series.

With a heat load as low as yours probably is I'd be inclined to scrap the hydronic system, install either a condensing gas water heater or heat pump water heater, and install a 3/4 ton cold-climate (or at least vapor-injection scroll compressor type) modulating ducted mini-split for heating/cooling, if there is any reasonable way to route ducts. Installing ducts makes it a lot more expensive than throwing another low-mass gas burner at it, but it can be more comfortable than baseboard and will modulate with load. Without a floor plan it's hard to say if that's remotely reasonable or not. The Fujitsu AOU/ARU9RLFCD can deliver 15,000 BTU/hr @ +5F ( and 13,500 BTU/hr @ -5F, which is about what your 27' of baseboard delivers at an entering water temp of 180F), which seems like a pretty good fit. The same unit can throttle down to 3100 BTU/hr @ +47F, so it'll basically idle along at ultra-low ultra-quite speed at superb efficiency until it's really quite cold outside. There are others, but Fujitsu units have a bit more flexibility in how they're mounted, since they can be mounted vertically in tiny 7 square foot "utility closet", whereas most require horizontal mounting.

The problem may be finding someone competent to design & install the duct system, or who isn't going to try and twist your arm into installing something oversized, when you already KNOW you don't need more than 13,500 BTU/hr even when it's colder than your +25F outside design temp (since the current heating system that has been heating the place only delivers that much even when maxed out.)

 

[Jeff Daley]

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.

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. For example, I saw this diagram from an earlier post:

where the secondary loop T's intersect at the top of the horizontal primary loop plumbing. But they could just as easily be made to intersect at the bottom of the primary loop pipe or to the left or right of a vertically-oriented section of primary loop. So I was wondering if there was any preferred way to orient those T's with regard to issues such as thermosiphoning from one loop to another or gravity-related influence on issues like turbulence, head pressure, or flow differential. If there are no advantages to a particular orientation, then it seems like you just let available space and ease of plumbing dictate the actual layout.

Any other possibilities in terms of high thermal mass combi boilers? Given that the HTP UTFC series is going away, I am not aware of any other combis on the market with a heat exchanger capacity larger than the 2 gallons present in the NTI FTV series (150,000 BTU model). One model I cannot seem to find the heat exchanger water capacity is the AO Smith ProLine XE series. Since the house needs both a DHW system as well as heat, you can understand from a cost perspective why it's probably not feasible to install a separate tankless water heater and mini split system. There really isn't space in the utility room to install a 50 gallon tank system that does DHW and space heat double duty, especially with the additional plumbing that would be necessary.