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Q&A Spotlight

Designing a Combined Hot Water System

A reader looks for advice on designing a system that provides both domestic hot water and space heating

This 75-gallon Vertex water heater from A.O. Smith is one of several options for a “combi-system” providing both domestic hot water and heat.
Image Credit: A.O. Smith

Vladimir Polyakov’s two-story colonial is undergoing a down-to-the-studs rehab, with insulation upgrades and a new heating system on the way.

“I am trying to put together a system that can deliver domestic hot water and heat the house,” he writes in a Q&A post at Green Building Advisor. “The colonial house is in Brooklyn, New York, 29 feet by 33 feet, and consists of two floors plus basement. I am doing a complete renovation. I already removed all walls throughout the house and will use closed-cell foam insulation in all walls and roof. I plan to run PEX in joists for both floors and I am thinking of using baseboard in the basement.”

That’s the rough outline of his plan, but Polyakov is concerned about oversizing the boiler. He’s come across several companies online that design and prefabricate systems that produce both domestic hot water and hot water for heat.

One of the systems uses a Takagi on-demand hot water heater. “Will this water heater short-cycle in my scenario?” Polyakov asks. “Is it better to use a dedicated boiler instead?”

His quest for an efficient combi-system is the topic for this Q&A Spotlight.

Some cautions about designing the system

System design is complicated, and the idea that Polyakov is taking on this challenge makes GBA senior editor Martin Holladay a little nervous.

“This is a big topic,” he writes. “The nature of your questions makes me wonder whether you will be able to design your own hydronic heating system. I suggest that you consult with a mechanical engineer or a reputable HVAC contractor with experience designing hydronic systems, because you probably don’t want to learn everything about this topic, starting from scratch.”

GBA has published a number of articles in this general topic (see the “Related Articles” sidebar at left), and Holladay points Polyakov to a number of manufacturers who sell heating appliances for combi-systems, including A.O. Smith, HTP, American Water Heater, and Bradford White.

But before he gets that far, Polyakov should check with local building officials to make sure this type of appliance can be used for space heating; some jurisdictions require a separate boiler. In addition, Polyakov should do a room-by-room heat load calculation to make sure the heating system he installs is sized correctly.

Low-mass combi boilers are “terrible solutions” in homes where heating loads are low, adds Dana Dorsett, because they don’t modulate to an output that’s low enough except in mid-winter when outdoor temperatures are lowest and heating loads are at their peak. Plus, he says, they have a “fairly lousy” domestic hot water performance.

“Tank-type combis can work out well, as can a site-built combi-system based on something like the HTP Light Duty stainless-steel tank modulating water heater,” Dorsett says.

A mod-con boiler with indirect tank would work

There are modulating-condensing (“mod-con”) boilers on the market that work just fine in homes with low heating loads, Dorsett says, “but there are no wall-hung combi boilers that are a good fit for both the space heating and potable hot water loads.”

The most common solution would be a small mod-con boiler with an indirect hot-water tank designed as its own zone and a zone controller that gives priority to the hot-water tank when it’s calling for heat, he continues. Space-heating would be put on hold until the water heater has done its job.

“Mod-cons work fine with radiant floors too, provided the system is designed correctly,” Dorsett says. “Designing heating systems via web-forum almost never results in an optimal design — it really takes a hydronic designer with the proper design software, and the experience to make judgement calls.”

“Something like the NTI TX51 can modulate down to under 6800 BTU/hr out at minimum fire yet can also deliver more than 50,000 BTU/hr when needed (say, for heating hot water),” Dorsett writes. “But to get it to behave correctly in your system requires some system design, as well as post-installation adjustment. It’s not just a plumbing job.”

Likewise, a Polaris model can be made to work with some warranty-busting alterations, but it’s easier to start with a modulating hot-water tank such as the Phoenix Light Duty, Dorsett says.

Is there a case for ductless minisplits?

Steve Knapp has another possibility: ductless minisplits. “Wouldn’t you like to have AC on 90-degree days?” Knapp asks. “You could use a heat-pump water heater to produce hot water very efficiently.”

As it turns out, Polyakov already has a minisplit installed in the basement, and admits he likes it. “But,” he adds, “if I consider having them heat the whole house, that means having multiple units constantly on, and I am not sure what my electricity charges will be.”

The cost of electricity in Brooklyn is “much higher” than in most locations, Dorsett says, adding, “it’s hard to make a financial case for heating with minisplits rather than condensing gas based on operating costs alone.

“The marginal cost of heating with gas is quite a bit cheaper than heating with minisplits in that local market,” he continues. “You can still factor in the lower cost of the equipment though, since the minisplit would cover both air conditioning and heating. It’s not a simple equation.”

Dorsett suggests that with sufficient access to the sun, the levelized cost of solar energy can be much lower than retail residential rates for power.

“But depending on where you are relative to the substation you may be able to get quite a bit in subsidy from the state and utility to offset the upfront cost of the solar array, bringing the average cost of power to something like half,” Dorsett says. “Under the rapidly evolving New York ‘Reforming Energy Vision,’ the utility is quite interested in avoiding the cost of upgrading the Brooklyn Queens substation by having a lot more solar, demand response, and microgrids.

“This, too, is not a simple equation, but if your roof has reasonable shading factors for solar, it’s worth looking into before deciding how to heat the place. If you can get the installed cost low enough with subsidies and the net-metering or other remuneration is favorable, heating with minisplits can end up being quite a bit cheaper to run than heating with natural gas.”

Now, about the insulation

Polyakov’s plan is to use closed-cell foam in both walls and roof, where framing is 2×4 and 2×6 respectively. Dorsett thinks he’s making a mistake.

“Why closed-cell foam?” he asks. “It has twice the polymer per R as half-pound open-cell foam, and uses much more damaging HFC blowing agents (instead of water), and adds very little to the ‘whole wall’ performance when used as cavity fill in a wood-framed structure, less than R-2. For less money you could do the cavity fill as open-cell foam, and buy back the whole-wall R with fan-fold XPS siding underlayment on the interior side, behind the wallboard.”

Closed-cell foam does make sense on the inside of foundation walls, Dorsett adds, where its low vapor permeance is helpful, “but almost everywhere else there are greener methods of achieving the same performance.”

In Climate Zone 4, where Polyakov’s house is located, only about 30% of the total R-value of the roof insulation must be low-permeance foam, Dorsett says, but the whole roof also could be done with open-cell foam if he used a interior vapor retarder such as CertainTeed’s MemBrain, or vapor-retarder paint.

“For 2×6 rafters the depth is 5 1/2 inches, and with 2 inches of closed-cell (R-12) foam you could then fill the remaining depth with R-15 rock wool or fiberglass. The R-value under the roof deck between the rafters would be R-27, but the foam would be 32% of the total, which is more than 30%, and sufficient for skipping the interior vapor retarder.” Standard latex paint would be enough.

Polyakov also could install open-cell foam between the rafters, which would provide sub-code R-values by itself. But it could be augmented by rigid insulation above the roof deck when it was time to re-roof.

“At a 7% framing fraction (typical for 24-inch o.c. rafter spacing) and half-pound open-cell foam, with typical roofing and ceiling gypsum you’d be at about U-0.05,” Dorsett says. “If you later added 3 1/2 inches of fiber-faced roofing polyiso above the roof deck it would squeak under the code-maximum requirement of U-0.026. (You can often get used roofing polyiso for one-fourth to one-third the cost of new goods from reclaimers.)

“That is a lot cheaper than 5 to 5 1/2 in. of closed-cell foam, and a lot greener too,” he says “Polyiso is blown with relatively benign pentane, and using reclaimed goods would be even greener. (I can usually find used but nearly-perfect 3 1/2 inch or 4 inch roofing polyiso for about $20-25 per 4×8 sheet from reclaimers near me.)”

Our expert’s opinion

GBA technical director Peter Yost added this:

I had my own top-of-the-list recommendation on this one—combining space heating and domestic hot water for Climate Zone 4: a modulating-condensing boiler with an indirect tank. But I figured I should check in with a real expert, so I called Tim Jeffers, a leading HVAC firm in southern Vermont. Tim read through the posts and then called me back.

Here is his reply:

“I have installed pretty much every system discussed in the Q&A and my top recommendation would be a mod-con boiler with an indirect tank. But there are an awful lot of others in this field who would go with a tankless system. It all boils down to doing a full set of calculations, knowing the water quality because that is so critical for tankless performance, and making sure the turndown ratio [an expression of modulation range; learn more here] fits the situation, in this case likely at least 5:1.

“In this specific situation the HPT UFT 80 which has a 10:1 turndown ratio, would be a good choice. True, it has not been on the market all that long, so it would be nice to have a bit more history, but it is a fine piece of equipment.

“Put the correct footage of baseboard radiation or European style radiators such as Bosch or Runtal in the basement sized per radiant design temperature. Use a smart circulator for each space-heating zone and for the indirect water heater. The thermostats would work as a high limit only. Your boiler will run as high as 98% +/-.

“On a call from the water heater, your space heat circulator will drop off and the boiler will run up to 180° at 100% firing rate. When the water heater is satisfied, you will go back to the heat curve. The boiler will drop down to between 10% and 40% firing rate at a low condensing temperature for ideal comfort and efficiency. Choose a SuperStor Ultra with a heat trap and insulate the hot water pipe for at least the first 6 ft.”

And per usual, the exchanges on insulation for this project driven by Dana Dorsett are right on the money. I would just add that the recommendation for using fan-fold XPS on the interior of a wall system means essentially no drying potential to the interior. When XPS is 1/2 in. or thinner (as any fan-fold XPS is), they laminate a polypropylene skin on each side to make it stronger, rendering the insulation a Class I vapor retarder.


  1. D Dorsett | | #1

    Most fan-fold XPS is Class II , not Class-I @ Peter Yost
    "When XPS is 1/2 in. or thinner (as any fan-fold XPS is), they laminate a polypropylene skin on each side to make it stronger, rendering the insulation a Class I vapor retarder/"

    Rarely (if ever) are they Class I. The thin polyolefin skins on fan-fold are typically in the 0.6-.1.5 perm range, with a few exceptions that are both higher and lower. Manyr are in the Class III range, only unperforated aluminum clad fan fold reached Class-I performance. When it matters it's important to read the specifcations, but with inherently back-ventilated vinyl siding it's fine in zone 4A to simply let it dry toward the exterior.

    For instance, Pactiv's thin-XPS board products are specified at 0.8 perms, the vapor-open end of Class-II vapor retardency:

    But their fan-fold siding underlayments are between 1.3-1.7 perms, Class III vapor retardency:

    The reason manufacturers make fan-fold siding underlayments more vapor open is precisely to allow more drying because it's on the exterior and fairly low-R, not enough to provide IRC prescriptives for dew point control from zones 4-8

  2. Kohta Ueno | | #2

    Siegenthaler on Hydronic Heating in Low-Load Homes
    Echoing other contributors--a low-mass boiler combined with a low-load home hydronic system has lots of potential problems. If anyone wanted to delve into more detail, John Siegenthaler did a nice writeup in JLC on this topic:

    Hydronic Heating for Low-Load Houses
    With the right hardware and layout, hot-water heating saves energy and makes ideal use of the sun

    One particular problem is if you add zoning to your low load home--your emitter/heat shedding equipment becomes even smaller than a low-load home's equipment. A key pull quotes:

    In the past, a typical residential boiler might have contained 450 pounds of cast iron and perhaps 10 gallons of water. Those materials made such boilers self-buffering, and thus short-cycling was not a big problem. However, many current-day high-efficiency modulating/condensing boilers have much lower metal and water content. Even with the ability to modulate down to 20 percent of their rated capacity, these boilers can still experience short-cycling when connected to highly zoned distribution systems.

    Of course, per the discussion above--a hydronic system is lovely and elegant, but provides no option for cooling, and is likely to be more complex and expensive than a MSHP system.

  3. User avater
    Jon R | | #3

    these boilers can still

    these boilers can still experience short-cycling when connected to highly zoned distribution systems.

    Another solution is to put bypass valves on most of the zone valves. Then the zone valves are a high/low selector instead of on/off. The result is much more minimum output. And less noise.

  4. Andy Kosick | | #4

    Navies NCB-180, Fan
    I've been involved in a couple of projects that used a Navien NCB-180. They are heating well insulated slabs and that side of the system seems to be fine. My question is that there is no clear data on the DHW efficiency of these units. An internal flat plate exchanger is used to heat the DHW, and basically I had to use the efficiency of the boiler and the approximate efficiency of a flat plate exchanger to come up with a number. Anybody know if there's real DHW numbers out there on these, if not, why they're not required to provide them. Also, general feelings on these unit and the set up are welcome. So far they seem to be working fine.

    Also, please do not mention the existence of fan-fold let alone its use. It serves only as excuse for contractors to do poor quality work on the exterior of homes. There is no useful application of this product that can make up the disservice it does the industry. In it's typical application it has almost no r-value, does not provide an air barrier, or a barrier to liquid water. I'm sure somebody is installing it correctly but I haven't seen it. For an interior application how about a 1/2" of EPS, better blowing agents as well. Obviously this is my opinion and up for debate.

  5. D Dorsett | | #5

    Fan-fold was comparison, not a recommendation. @ Andy Kosick
    ""Why closed-cell foam?" he asks. "It has twice the polymer per R as half-pound open-cell foam, and uses much more damaging HFC blowing agents (instead of water), and adds very little to the ‘whole wall’ performance when used as cavity fill in a wood-framed structure, less than R-2. For less money you could do the cavity fill as open-cell foam, and buy back the whole-wall R with fan-fold XPS siding underlayment on the interior side, behind the wallboard."

    The point is to directly compare just how LITTLE is gained un studwalls by using R6/inch closed cell foam insulation compared to R3.7/inch insulation, and the extremely poor R/$. The gain in performance with closed cell foam cavity fill vs. open cell foam is comparable to what is gained by 3/8" of continuous layer of XPS, which is very small, yet comes with a HUGE cash upcharge,and dramatically larger environmental footprint.

    At the bottom line: Save the higher-impact high R/inch stuff for where it is not thermally bridged by framing so that the full thermal performance can be realized.

  6. Andy Kosick | | #6

    response to D Dorsett
    Apologies, I obviously misread the intent of the statement. Forgive me, fan-fold is an acute pet peeve of mine.

  7. Scott Goodman | | #7

    Rinnai Q Premier
    THE RINNAI Q PREMIER might be what your looking for hot water and heat

  8. User avater
    Dana Dorsett | | #8

    Too much min-fire output, not enough buffer @ Scott Goodman
    There's only 24 gallons of domestic hot water in the Rinnai QP130, and only 1.4 gallons in the HX. This might be enough for a showering-only single bath unit in NYC, but could be tough to fill a soaking tub. We'd need to know a lot more about the domestic hot water loads. The min-fire output is probably higher than his design heat load too.

    The QP85 would be even sketchier on the domestic hot water delivery end same 24 gallons, but only 85K of burner, and still more than half his likely design heat load at low-fire.

    Like most combi-boilers, it's tough to find a good fit.

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