General questions on radiant heat and DHW setup

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Logan2015

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Looking for a little guidance in choosing general set-up for radiant heat/DHW on a new build single family home in Ohio.

I have read a number of posts comparing different set-ups, but can't quite find an answer to my unique (apparently) situation:

The home is approximately 5,000 sq ft with full basement (currently unfinished except for small home office) and 2 stories. The main floor, second floor, and over-garage bonus rooms are heated and cooled by three separate conventional gas furnace/AC units (based on heat loss calcs and installed by experienced HVAC professional).

I have installed radiant floor heat in the basement and garage and plan to heat both in addition using some combination of boiler+indirect, combi, tankless, etc. for heat and DHW.

I am having trouble deciding on the most efficient and economical way to provide the heat as well as ensure my DHW needs are met. I have a couple questions:

1. The radiant used to heat basement and garage is more of a comfort/luxury with the thought that it may help lower heating costs of the main house and above-garage space. My primary concern for the system is DHW with 2 near-teenagers and another close behind as well as a wife who is installing a new soaking tub as I type. I have seen several members indicate a Combi boiler is not likely the best choice when radiant heat requirements are low and DHW requirements are relatively high. Is this a general consensus?

I have spoken with a few others who have used a combi boiler for similar size home which appears to work well for them, but not sure how efficient it's working. Obviously, after reading a lot about short-cycling and shortening the life of equipment, I want to make sure I'm sizing appropriately, as well as maximizing efficiency. Seems like my needs for radiant are relatively low and something with a lower BTU and good TDR along with a decent size indirect tank may be the best bet, but want to be sure I'm not missing something.

2. I have not performed any heat loss calcs as I am unsure how to account for the specific areas (basement and garage) in the face of the rest of the house already heated. Are there general assumptions I can use for a partially insulated basement and fully insulated garage that I could use to appropriately size a boiler for my radiant needs? A quick snapshot of the areas:

Basement: 65 x 38 with R-5 exterior board below grade and R-10 rigid under slab
Garage: 30 x 38 with 11 ft ceilings, R-13 blown in cellulose in walls, R-18 garage doors, and R-10 rigid underneath

I do have access to free gas on the property, which also plays into the equation, but want to be sure I'm choosing the best set-up.

Any input on the situation or others that have had success in similar situations is welcomed. Thanks.
 

Dana

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Three separate gas furnaces + split AC are almost guaranteed to be way oversized for the actual zone loads. Who ran the load numbers? (I'm guessing it was the HVAC contractor, not an engineer.) Do you have copies of the Manual-Js? For the record, what are the BTU-out of each unit, separately?

Have a competent third party (not an HVAC installer) run a room by room, zone by zone Manual-J load calculation. A WAG is only going to get you into trouble. Don't use anybody's rule of thumb or "...general assumptions ..." for coming up with the load number or equipment sizing.

Most 5000' houses have more bathrooms than would be adequately served by a single tankless or combi-boiler. How many bathrooms (and at what distance) would the water heater need to support? Just the new soaker-tub alone becomes an exercise in tedium when using a tankless or combi-boiler, and you may have to schedule tub fills to never coincide with teenage showers to have sufficient showering capacity, even if installing a drainwater heat recovery unit to enhance showering capacity.

I suspect when it's all said and done you're looking at a big tank type water heater, or indirect served by small mod-con boiler to get adequate hot water service.
 

Logan2015

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Thanks for your responses. I appreciate the input. This will help when I'm talking to folks to get some quotes and making my decisions.

Three separate gas furnaces + split AC are almost guaranteed to be way oversized for the actual zone loads. Who ran the load numbers? (I'm guessing it was the HVAC contractor, not an engineer.) Do you have copies of the Manual-Js? For the record, what are the BTU-out of each unit, separately?

I was under the assumption that the load calcs were done by an engineer. Perhaps I'm mistaken. I don't have copies of them, but will try to get my hands on them. The largest unit, servicing mainly the first floor is 120,000, the second unit upstairs is 80,000, and the third unit servicing bonus room is 60,000. I guess it's a bit of a moot point now, as they are all already installed. Though I have noticed, the upstairs unit has not done much work over the winter (albeit somewhat mild winter here). I think part of the issue was inaccessibility of the rooms over the garage/limited space for ductwork and after discussion of separate furnace and AC versus mini split or other options, this seemed to be the "best fit"(?) Hopefully for the home and not his wallet.

Most 5000' houses have more bathrooms than would be adequately served by a single tankless or combi-boiler. How many bathrooms (and at what distance) would the water heater need to support?

There are 4.5 total baths, with potential addition of another half bath in basement once finished. Furthest bath is about 55' away (including vertical travel), additional upstairs baths are around 40' and 20'; first floor master about 30' away. Have thought about drainwater heat recovery, but not sure on specifics and need to look more into this to see if it's an option (or need) in my case. Thanks again.
 

Dana

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Wow- 260,000 BTU/hr of furnace for 5000 square feet! That is over 50 BTU/hr per foot, which at Cleveland's 99% outside design temp of +9F would be the load/area ratio of a house with only single pane windows, no wall insulation, and maybe 3-6" of fluff in the attic. Even if it were sized a the ASHRAE recommended 1.4x, that would imply a 99% design load of about 185,000 BTU/hr, a ratio of 37 BTU/hr per square foot. Even if your house has clear glass double panes, R11 fiberglass in the walls, R20 in the attic and no foundation insulation (sort of like a 1950s house) your real heat load would still be less than half that.

If your house has low-E double panes and 2x6/R19 or better you're probably looking at a 4x -5x oversize factor on average, some zones probably more ovesized than others.

Yes, it's a moot point- it'll heat the house, but it's the opposite of comfort. Heating duty cycles never hit 30% even when it's 0F outside, which means it's delivering the hot flash followed by the extended chill. At 1.4x oversizing the duty cycle would already be 71% when it's +9F outside, and increase as it gets colder, spending far more time in "warm summer breeze" mode than "cooling off draft" mode. If your "free" natural gas is metered, running a fuel-use based load calculation using the furnaces as measuring instruments logging fuel use against heating degree-day data. If they are all single-stage units it's possible to use a smart thermostat to just log the duty cycle of calls for heat against heating degree-day day data to figure the oversize factor of individual zones.

There is no way a single tankless or combi boiler is going to adequately serve 4.5 baths. If one sizes the tank (either an indirect or standalone water heater with a 75KBTU or larger burner) to fill the soaker tub with more than just a hair of margin it'll do fine for serving up showers. A single ~80,000 BTU/hr boiler can run a single 2 gpm shower 24/365 with a bit of margin, but sizing a burner to serve 2-3 simultaneous showers plus a tub fill tankless style would require 2-3 199,000 BTU/hr tankless units, which would be one way to go. Installing multiple water heaters in closer proximity to their loads would reduce wait times. A tank with a recirculation approach to reducing wait times works too.
 
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