jbaron,

How do you heat your hot tub with your boiler?

I use (2) small Maxi-flo heat exchangers. I use two because ONE wasn't sufficient - I found the Maxi-flo heat documentation to be incorrect, and was not able to get all 70K of my boiler into the hot tub with only one. No mind, the pressure drop on either side (especially the hot tub recirc side) isn't bounded by the heat exchangers.

It would be difficult to overstate how much I like my setup - no outside heater and no noise from the hot tub, as all of the mechanical stuff is in the basement.  Only a supply pipe and a return pipe, with a temperature sensor in the basement.

Jeff

I have better energy numbers from now.

What should I post?

Based on my reading it looks like heat load looks is aprox 35k
Upstairs 14k
Downstairs 21k

35K seems about 2x too high for 2700' of 2x6 construction conditioned space in Santa Cruz unless you have a LOT of glazed area (like a greenhouse), or you intend to keep it 90-95F indoors all winter. 35K is about my (measured) heat load at -5F in a ~2200', 1920s antique 2x4 stick built house with ~1500' of semi-conditioned basement (that stays 65F or above all winter.) Your 97.5% percentile outside heating design temp is probably in the mid or high 30s(?).

I suspect reality is more like a 15-18K heat load for the whole house, or even less if you've built it tight, used foam or better-quality sprayed/blown fiber insulation, and have only moderate glazing fractions. If so, this is definitely hot-water heater territory- your peak load would be barely above the minimum fire of the very smallest mod-cons. The KBL150 probably has a min-mod of about 30K, which is well oversized even if your 35K numbers were true (but they're not.) If you go mod-con, the -50 is more than enough, and would (almost) never need to go above min-fire to keep up with space heating loads.

Hybrid electric hot water heaters might do it- it depends on the size of the resistance heating elements. But there's little point of going with hydronic radiant if you're using an electric hot water heater since it would be all resistance-electric supplying the space heating. The operational cost would be comparable to electric-mat radiant. Gas hot water heaters would be cheaper all around as a combi-heating system.

Hydronic air-source heat pumps for heating, cooling & DHW might make sense from a lowest operating cost and total-systems point of view. It's significantly more money than a mod-con, but perhaps not such a huge premium over a mod-con + central air conditioning system. If you're not cheaping out and heating with a hot water heater it may make more sense than separate systems. (Dew points are low enough in Santa Cruz that cooling with the floors may also be an option.)

No cooling will be needed here in Santa Cruz. Plus we won't have a duct system at all.

The coldest peak day listed 27 degree January 1 - which is actually lower than any low temperature we have had in a few years. In general we never go much below freezing for a low - and if it happens it is only for a handful of days per year. Last week with the potential for our first snow flurry in years it didn't even get below 34 degrees here. Some cold clear days we might get a 31 or 30.

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Hourly heat loss for downstairs 14,903 - upstairs 20,232

Looks like we have 331 sq/ft of windows downstairs - 607 sq/ft upstairs

I have the predicted yearly electric and gas use as follows: 7658 kWh, 473 therms - Demand 9kWh, 76 kBtu/hr
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So can I assume that we should spec a much smaller mod-con then - like a 80 at the biggest, and a 50 as the optimal??

Initially I wanted a tankless with a small buffer tank inline - but was told the mod-con would be a better solution.

I heat my house on a kludged up tankless-based combi- when the hydro-air zone kicks on it's putting out ~42K,(measured by flow x delta-T on the boiler loop) and was WAY more than enough to keep up when it hit -8F earlier this season. A 50K mod-con at full fire puts out 45K at 90% efficiency- it's actually already extreme overkill for what your actual heating loads will be. An 80K boiler would be ridiculously oversized. The key number to getting the efficiency out of a mod-con is it's minimum fire output, relative to your average winter heat load- they're great when the min-mod is well under your average. But it's my belief that a 50K's min-mod is close to your peak, not average heat load.

My ASHRAE 25 year binned hourly 99% design temp is 0F, the 97.5% temp is +4F. If your heating system can keep up with either number you still won't be cold except during record-breaking 100 year cold snaps. Even though you'll get temps 10F or more below your 99% design temp a handful of times in a 25 year period, they're typically short in duration (a few hours) and the thermal mass of the house keeps any shortfall on interior temps well bounded. (I've seen it as cold as -18F here). The design temperatures for San Fransisco are ~35F- Santa Cruz has to be similar, possibly warmer.

At 35F my house is pulling ~15K, and yours will too, despite having somewhat more glazed area than I do. My windows are mostly circa 1923 double-hungs with circa 1988 exterior storms. Your windows have to beat mine efficiency wise by a real fraction. Most of my roof area is 2x6 rafter cathedral-ceiling style 250 square feet of which currently has less than R15 of rock wool (which will be significantly upgraded when I gut the attic rooms.) My walls are mostly full-dimension 2x4, 16" o.c. mostly insulated with dense-pack cellulose, but with a few yet-to-be rectified known gaps. You have 2x6, for 1.5" more insulation depth, yet 25% less framing fraction (if 16" on center, less if 24" o.c.) for lower thermal bridging. If your house doesn't outperform mine by good measure, there's some thing seriously wrong (like a missing door or window!)

To guarantee performance on new construction, pressure-door test it an air-seal at least to under 3 ACH/50. Under 1 can be tough unless it was built for air-tightness but is quite doable if the air-barrier plan is explicit in the design, and followed during construction. (The PassiveHouse spec is 0.6ACH/50) Only use sprayed or blown insulation- batts can generally only be installed perfectly and perform to in a lab. Use only awning or casement windows or fixed windows- no sliding doors, or windows, no double-hungs (which all leak like crazy, comparatively speaking.)

Where did your heat load number come from, and what were the design temperatures used? Even with standard 2x6 construction and 1000' of glass I don't quite believe them unless it's all single-pane goods. With U0.33 glazing 1000' of glass will be a bit over 10KBTU/hr at 70F indoors, 35F outdoors, but that would rise to ~30K with single-pane (is that even legal under CA Title 24?).

A hot water heater (even a $300 beater) puts out more than 25K, and there are plenty that are bigger or more efficient. Tank type hot water heater based systems are inherently self-buffered, and operate at close to the units combustion-efficiency in heating applications (ignore the EF ratings), and are impossible to short-cycle under even the lightest loads. Even, assuming your gas-use numbers are anything like reality, the difference between a mod-con and a cheap tank hot water heater's fuel use in a heating application will be on the order of 50therms/year. At a buck a therm you might think about doing something else with the up-front money. Such as...

It's actually pretty easy to high PassiveHouse type levels of energy use with 2x6 24" o.c. construction in your neighborhood and skip the heating system entirely (easier with new construction than as retrofit):

http://www.nabihtahanarchitect.com/rte/upload/download/hem_nd08_tahan_remodel.pdf

The upgrades on air-sealing & insulation, & windows, (especially if you insist on that much glass- cut back where you can) that get you there could end up costing less than a mod-con based full radiant system for two floors, leaving hot-water heating would be your biggest thermal load (and you could go solar on that one, if it's in the budget.) In your area it's mostly a construction detailing for air-sealing and thermal bridging issue, not massively more material. I wouldn't dream of building there without at least running some numbers for going PassiveHouse on it. (YMMV) High efficiency heating systems are for low-performance building envelopes.

Posted By Dana1 on 01 Mar 2011 03:25 PM
Where did your heat load number come from, and what were the design temperatures used?

The numbers came from EnergyPro software using the design of the house, including the walls, insulation, windows etc...

The current set of numbers is 7 pages or so with even more information on them. Temp was 27 degrees for peak heat load

Thanks for the reply's by the way - I appreciate the information and education.

2,700 sq feet at 13BTU load per square foot seems right in there.
Mod Con, if you use a mod con fro your domestic as well as your heat and you are going domestic on demand you will be sizing in part for the domestic load as well. We sell a lot of the new Mascot II mod con boilers and they turn down to 20%.
Domestic on demand is 3.3 GPM,
Now you would have on appliance and can meet both domestic and heat beautifully.
May be a bit high on the heat side, but again in Santa Cruze the main application 8 months out of the year will actually be on the Domestic H20 side.
Dan

I am using more condensing tank water heaters with outdoor reset for smaller or low load homes. No shortage of water, no noise, no high tech maintenance and 96% thermal efficiency.

Using a design-temp of 27F would be like a 99.5th percentile number.  That's probably ~10F colder than the 97.5% ASHRAE number for Santa Cruz. Designing for the 97.5 number would still provide adequate thermal input to never be cold in a house of average thermal mass, and certainly adequate for a home with a radiant slab.  I could design for -20F, since it's been that cold at my house at least once in the past 25 years, but there's really no point in a place with a 97.5% design temp of +4F. (It wouldn't take much to make up the difference with auxiliary heat sources.) 

If you have the software (and not just hard-copy) run the calc at 35F outdoor temp, and play around with the ventilation rate values. I've never used EnergyPro, but it still seems wrong using a 27F design temp unless you have a ridiculously high ventilation rate and U0.5 windows. 

In new construction you CAN control the ventilation rate, and the difference in cost between good windows & better windows (or where appropriate, smaller or larger windows to balance solar gains against heat loss) pays off in both comfort and utility costs.  (I'm not sure where Blueridge comes up with 13BTU/foot at 27F number.  Heat loss is relative the the surface area x U values of the exterior, the outdoor air temp, and air leakage factors, not the square footage of living space.  The shape of the house alone can move that BTU/foot number, since it affects the surface area, which is why BTU/foot is no substitute for a real heat loss calculation, even if it's close in some significant fraction of homes in a particular location.)

The PassiveHouse tools are also a PITA to fill out in detail, but are useful in determining where the money is best spent even if you don't take it all the way to their design-standard for energy use.  Applying thermal breaks to all framing, and putting a bit of foam under the slab, and site-optimizing the thermal mass & solar gains gives the building a great deal of ability to coast through the brief peak heating/cooling load hours with very modest thermal input, which should be pretty effective in the Monterrey Bay area.  If Tahan can do it in Berkeley with cellulose insulated 2x6" framing and modest amounts of rigid & spray foam, you probably can too.

Posted By Blueridgecompany.com on 01 Mar 2011 07:48 PM
2,700 sq feet at 13BTU load per square foot seems right in there.
Mod Con, if you use a mod con fro your domestic as well as your heat and you are going domestic on demand you will be sizing in part for the domestic load as well. We sell a lot of the new Mascot II mod con boilers and they turn down to 20%.
Domestic on demand is 3.3 GPM,
Now you would have on appliance and can meet both domestic and heat beautifully.
May be a bit high on the heat side, but again in Santa Cruze the main application 8 months out of the year will actually be on the Domestic H20 side.
Dan

We are going to be using the Mod-Con for both DHW and Radiant.

Yes I do believe that with our well insulated home, that has some limited passive solar features (good south and south west window exposure) we will have limited months of heat needs.

Laars was recommended to me as well (I mentioned that above).


I am just trying to focus in on the size needed...

If mod-con, size==>>as small as possible, since the high-fire output of even the smallest mod-cons exceed your (probably overestimated) design day load by more than 25%. Size the indirect as-needed for your DHW requirements- you'll have burner to spare. The lower the min-mod, the better heating efficiency you'll get out of it.

Your hot water heating efficiency with an indirect will be in the mid to high-80s. Your your combined efficiency might even be somewhat higher if you went with MA's condensing tank HW heater combi, since the return water temp when heating the indirect with a mod-con is above condensing for much of the burn. But the difference in efficiency is "in the noise", and the total cost of the hardware has to be considered. (MA's combi probably wins there too.)

It seems from everyones help and advice here that I should also consider a condensing natural gas water heater with a tank...

This would be almost as efficient, and cost less than a mod/con with an indirect tank. Plus since the heat is not needed most months of the year - the majority of my use during the year is for DHW.

2 Showers at one time, plus laundry potential - and a large tub fill on rare occasion (not concurrent with a shower) so we might be at 5gal/min need for DHW at the peak...

Reason prevails again!

Thank you for the help everyone!

We are looking at a design similar to what is shown on the Warmboard site - not sure if Warmboard will be the choice yet for the upstairs, but they are local...

http://www.warmboard.com/wp-content/uploads/2010/01/hlm_6-1.gif

Now I just have to research the different options and brand of Condensing tank water heaters...yet more work!!!

Update here:

We have the first quotes now for our radiant install - this is before I told them to switch over to a Condensing Tank heater as per the help provided here.

Total installation and parts costs are around $24,500.00 which includes all pex piping in slab & Warmboard, 6 zones,  mixing valves, expansion tank etc... Spec includes a Munchkin 80 boiler, with an Indirect tank.

This all seems pretty high cost for our needs - especially since we only have heating needs a few months per year.

So with that in mind does anyone suggest anything lower in cost that would work for us?

I am looking at the idea of a mini-split system - but we don't need cooling - we also currently have no ducts or much room for them. I don't know the duct size for a ducted mini-split or a hybrid heat coil ERV...

It should be lower with a combi water heater. If you have a design you can let it out to multiple bidders. The number for described package seems in order.

You can probably build to PassiveHouse spec on the building envelope for less than $25K extra in construction costs, and skip the heating system entirely (or at most lay in some electric radiant mat in a few appropriate places rather than doing the typical resistance-elements in the ventilation air the way many PassiveHouse projects do.)

Just sayin'...

Total installation and parts costs are around $24,500.00 which includes all pex piping in slab & Warmboard, 6 zones, mixing valves, expansion tank etc..

That's the warmboard killing you. It's gotta be a third of the bid. I wouldn't use that there.

I'd tube the basement slab for heat and set it up to run off the same heat as your hot water- whatever has enough output to keep you in showers and heat that slab when necessary.

Then, I'd use a ductless mini-split heat pump with two units upstairs and one unit in the basement. That reserves another unit for where you might need it, up or down.

Tubing and controls: $3,000
Ductless Mini: $8,000
Hot water: ??

This system would easily accept solar hot water in the future and PV panels to offset electrical usage if you wanted.