Not an engineer. However I have been investigating a system for a 3 apartment (converted from a home) to upgrade the heating and hot water. Current setup is 2100 sq ft, old central heat furnace, and three hot water tanks. All very inefficient. Area is Colorado. My idea is to replace the whole mess with 199,000 50 GAL Polaris heater (257 Gal 90degree rise recovery) a hydronic variable speed blower. Diverters that point the ducting to only the units calling for heat. 850 sq ft 850 sq ft and 520 sq feet. Low gallon shower heads. My criteria is DHW prioritizes space heat. 3 showers simultaneous for 10 min then you get what you get (I pay the heat) no more than 15 min for system to recover for space heating. Building is old 2 story brick. I am told by heating specialists to consider it 35 BTU per sq foot. I will install NEST thermostats in all three units and limit cooing range from 76-80 degrees and heating season 62-73 degrees. I am told by some heating techs it won't work and yet they don't tell me what the shortcomings are. The system recovery metrics look amazing to me. Could use a little help from you to model if this is worth looking into a bit further. Thanks in advance

I would be upset if my neighbors took enough showers/baths that I had no hot water at 6am. But limit everyone's gpm of hot water and it should be OK.

As the resident mechanical inspector (eye rollers commence) my first reaction is to flip open the code book and see what provisions are there for your design. The good news is that you can make it codeworthy depending on local building codes are in place. I've not refreshed lately but ICC provides for a water heater that is approved for space heating to provide heat (as long as it also provides DHW). The MI state chief mech inspector stated emphatically that a water heater isn't a boiler (once again leaving the guys that enforce the code to scratch their heads).

Once that hurdle is behind you, you then have to look at the design side and be careful of the products you select. Most hydronic coils are rated for X btus at 180* so if you only make 120* you must ensure that you still have adequate capacity.
Also you generally prioritize domestic hot water as there is no thermal mass to compensate for a lag in delivery if everyone showers at once.
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Rules of thumb like "35 BTUs per square foot" are a piss-poor method of assessing the true design condition heat load, and are guaranteed to oversize the system. Crusty old-schoolers like that since it means they never get the 6AM call from the cold & irate customer, but it usually increases the cost and lowers the overall efficiency of the heating system. Heat loads are a function of exterior surface area and exterior surface materials, the difference in inside and outside temperatures, and air leakage rates, not the floor area of the conditioned space. A 1-story 2100' building has far more exterior surface loss than a 2-story. And the second story has much more exterior surface area than the first story does, etc.

There is no substitute for a room-by room and whole-zone heat load calculation (Manual-J method or similar) at the 99% outside design temperature for your location, which can be looked up here:

http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf

Some heating designers use much lower numbers, but that only increases the size & cost of the system, and does not increase comfort. There is margin inherent in Manual-J type calculations- IIRC bumping up by more than 15% from a Manual-J calc is even contrary to code in CA.

Ducted air has efficiency losses due to duct leakage and duct-imbalance driving outdoor air infiltration in the building. Hacking it up with zone diverters only increases those problems due to increased back-pressure driving increased duct leakage, but fully variable speed ECM drive air handlers takes some of the edge off those factors. Going with lower water temps in the coil requires increase air volume, which increases power use, and increases duct losses also. If you're going to use a HW heater for space and break it up into zones you'd get much higher efficiency and more reliable output with a fully hydronic (radiators &/or baseboard) system, and either abandon the ducts sealing them up, or keeping them for air-conditioning only.


If you insist on continuing to use the ducts for heating you'd get better efficiency at lower system cost by going with a multi-stage condensing hot air furnace with a variable speed ECM drive blower (in which case oversizing using the crummy 35 BTU/ft methods has little impact on efficiency, or comfort since it'll spend all it's time at low fire & low speed, only kicking into high for overnight setbacks, etc.).

If you're going to continue to use the ducts at all, sealing every seam & joint with duct mastic and insulating at least the supply ducts wherever the are outside of conditioned space is cost effective, and improves system efficiency. Typical duct losses are in the 15-25% range, but even 40-50% isn't rare. The AFUE efficiency of the furnace is only half the problem- distribution losses often dominate the system efficiency numbers (but only in a negative way.) An 82% AFUE furnace in a nearly-ideal duct system can easily beat a 95% AFUE condensing furnace in an average to mediocre duct system on minimizing fuel use. Sealing the ducts is far more critical than insulating them since duct leakage drives air infiltration, and it's driving it only when there's a call for heat, which is typically when the outdoor air is coolest, maximizing the increased heat load from those infiltration factors.


If going with a condensing furnace solution, at current (and projected) natural gas pricing there's little economic incentive to go to a condensing water heater, but if you're consolidating three heaters (one per unit) into a single, a bigger burner and higher volume is called for, and going with the 100KBTU/hr version of the condensing ~50 gallon Vertex would likely have a comparable or cheaper installed cost than three pretty-good 0.60EF 40 gallon tanks. The 100K condensing burner with 90+ KBTU/hr output has enough output to run a 2.5gpm shower literally forever with some margin, and the recovery rate after a tub fill is very fast. The only time you might run short is if all three units are filling large tubs at the same time. With three simultaneous 2.5gpm showers (7.5gpm flow) and a 130F storage temp it would make your 10 minute spec, but it wouldn't make 20. Most low flow shower heads are even lower than 2.5gpm in practice (measure them with a bucket and a watch) so you'd still have some margin.

Hydronic coils are often rated at a couple of temperatures not just at 180F- many will have spec for both 180F & 140F, but the 140F number is typically on the order of half it's output at 180F. A storage temp on a HW heater of 140F will still yield some condensing efficiency, but though a plate heat exchanger and given the hysteresis of the tank's aquastat the temp at the coil would average at least 5F lower, with somewhat lower output from the coil. Like most heating system designs,getting satisfactory results and reasonable efficiency (including electric power efficiency) will require actually doing some math- really designing it, and designing to a heat load calculated by something better than crude rules of thumb.

I'm starting to wonder if this is a square peg in a round hole solution. Part of me looking at this is that I didn't want to buy "two" combustion units and ducting for two etc. However if the system controls and blower are not less expensive perhaps I should use a traditional 2 stage variable speed furnace at 96% efficiency and use a separate DHW solution. I still think there is value consolidating to one tank to share efficient infrastructure. I am looking at hydronic baseboard but look cost prohibitive. Someone already converted this into three apartments and all the ducting but only ONE furnace and One thermostat. So I will also proceed with zoned forced air. I really like the NEST thermostats for reports on usage remotely. I may develop a system to award tennants a bonus for good behavior at the thermostat ! Thoughts about aborting my plan of single heater ? Also frustrated with those bidding on the job, not providing details and just throwing out a bottom line number. I'm going to ask for line item detail and break out parts / labor etc.

Fin-tube baseboard is pretty cheap heat-emitters at ~$12-15/foot but it takes about twice as much of it to get heat into the space with 130F water compared to 180F water. See:

http://www.slantfin.com/images/stories/Technical-Literature/ratings_fineline30_r.pdf

Let's say the whole-house has a design condtion heat load of ~50KBTU/hr at design temp (about 25 BTU/foot it could be less, probably not much more) - you'd be able to deliver that much heat using 130F water with, ~200' of baseboard at a material cost under $3000, maybe even under $2000 if you shop around, distribution plumbing included ( installation extra). Cutting it into 3 zones may add another ~$500 per zone in associated hardware and controls, you can probably get the whole thing in for about 4 grand including installation.

But without a room-by-room heat load you won't even know if you can fit enough baseboard to handle the load for every room. A small corner room with a lot of window area may not have enough free wall-length to put in the requisite amount of baseboard for 130F operation, and you'd have to come up with a (more expensive) panel radiator or something for that room.

An air handler with an ECM drive blower that can deliver 50K with 130F water like the Rinnai 37AHB75xxx would run about $1500. (see the specs for output at different temps: http://www.alpinehomeair.com/related/Rinnai_37AHB_Spec_Sheet.pdf), but the system efficiency would be lower than on a baseboard system, and power use will always be higher when moving the heat with air rather than by pumping hot water. Then there's the zone valves & controls, installation labor, it's till going to run you at least 3 grand by the time it's said and done.

A 2-stage condensing gas furnace with 50K+ of output at the high end and a multi-speed blower is going to run about 1.5 grand with shipping, eg:

http://www.pexsupply.com/Goodman-GMV950704CX-Goodman-70000-BTU-95-Efficiency-Two-Stage-Burner-Variable-Speed-Blower-Upflow-Horizontal-Application-Gas-Furnace-12610000-p

Sure, there will be another grand or two in zoning & installation costs, but it would still come in for less money overall than a hydro-air solution (and would likely deliver higher efficiency than a hydro-air solution based on a Polaris.)

Most HVAC contractors won't have a detailed design at the point that it's bid, and won't be able to line-itemize. Most will at least spell out which major components they'd be using, but it takes time to actually design it, and they won't unless they have the signed contract. Another approach is to pay a competent designer for the detailed design, then put the installation out for bid. That lowers the design risk/responsibility for the installer, and the bids should be more competitive.

But without a real heat load calc the different approaches, sizes and system costs will be all over the place.

This is all VERY helpful information. Here is an example of the lack of detail that I get with many bid and in my opinion excessive labor charges. This is a bid just for the furnace and zoning, no hot water electrical etc. Anyone in Colorado have real pricing ? +++++++++++++++++++++++++++++++++++++ Proposed Scope of work: Remove and dispose of existing furnace; install new condensing (high efficiency) furnace per options below; add supply air run into front room of back apartment; re-route main return air from front hall to main floor apartment; convert added supply in upstairs apartment to return air; install Reliable zoning to separate apartments. Option 1: Carrier model 59TP5A100E21-16 2-stage 96% furnace (recommended): $12,881.00 Option 2: Carrier model 59SC2A100521-16 1-stage 94% furnace: $11,816.00 Parts and Material included: Furnace; Zoning system; controls; all material necessary for above describe installation. Material, Tax, Labor and Permit included. Excluding: Electrical line voltage; water heaters; ducr cleaning. Terms: 50% down; balance due and payable to installer upon completion.

The 59TP5A100E21-16 has a lo-stage input of 65KBTU/hr, high-fire input of 100KBTU/hr, which is probably significantly oversized.

Assuming a hi-fire output of 90,000BTU (it's probably somewhat higher), that would be 43BTU/foot for your 2100' place, which is crazy-high, probably close to 2x the real load at the 99% outside design temp. It would have quick recovery from setbacks, but would probably only hit high-fire after DEEP setbacks. When very oversized for the load it becomes essentially a 1-stage unit at the low-fire spec, but it will probably meet or beat it's AFUE numbers (which asssume 1.6x oversizing), since it's likely to run at the higher-efficiency low-fire a greater fraction of the time.

59SC2A100521-16 isn't a real model number- the "5" is probably an "S", and the "16" is probably a typo. The three digits after the "A" is the burner size, eg: a real model number in that series, 59SC2A100S21-14 is a 100K 1-stage as is the 59SC2A100S21-20.

Again, probably something like 2x oversized (but you won't know without a heat load calc), and will probably meet it's AFUE numbers.

I'm not sure duct-cleaning is worth paying for, but duct SEALING definitely is.

I'm not sure what the HVAC market is like in CO, but those quotes look somewhat oversized to me. A typical furnace-swap near me would be well under half that, give them a couple or even three grand for the bit of duct runs plus zoning & controls & electrical work and I still can't see how it breaks 10 grand. The 100K-120K furnaces of equivalent efficiency & blower quality would cost at most $500 more than a 70K-80K furnace, not 2x. (eg: http://www.pexsupply.com/Goodman-GMVC951155DX-Goodman-115000-BTU-95-Efficiency-Two-Stage-Burner-Variable-Speed-Blower-Upflow-Horizontal-Application-Gas-Furnace ) The 50% upfront more than covers their material cost, and probably their labor too. How much they need to charge for overhead, design, & travel time will vary, but it just seems a bit rich at first glance- there's plenty of margin in it for push-back, if you think they're the dickering types.

A point of reference (but maybe apples to pears): a couple years ago (in Worcester, MA) my niece had a complete 140F hydronic baseboard heating system with a ~60KBTU 87% boiler (about 1.5x oversized to the load- the more appropriate 50K boiler would have taken another 2 weeks to get) PLUS an indirect hot water heater installed for a bit under $8K that included installing the baseboards, split into two zones. It also included removing the steam radiators & steam plumbing, but not removing the asbestos-covered steam boiler. (That part required a specially licensed contractor to be legal from a contamination risk point of view.) That's a more complicated installation, with more expensive hardware than you're talking here. To have taken it to condensing efficiency rather than 86% would have brought it up to ~$10K, but condensing boilers are much more expensive than condensing furnaces. Different HVAC markets &/or market conditions, mayhaps? Keep shopping it around, or push back on both burner output size and price.

I wouldn't sweat the lack of detail in the quotes so much- that's pretty typical. The quality of the design is important, and that begins with a credible heat-load calc. Spending $300-500 for a real heat load calc could save you more than that just on the raw furnace hardware cost when you find out that the true heat load at design condition is 43K (about 20 BTU/ft) , not ~74K (the 35 BTU/ft value) or 94K (the output of the quoted equipment). And a right-sized unit would just plain work better, running fewer but longer cycles.