Water Heater Dilema

[Julio Vargas]

Hi, I am looking at replacing my water heater. An energy auditor determined my water heater was back drafting and it is 15 years old so I'd like to replace it as well as address the air quality issue with back drafting.

I wanted to get a direct vent water heater but the height that the vent needs to go up to clear the ground level outside is too high. The mechanical room is in the underground basement. I'd like to stay away from power vented water heaters because of the added noise both inside and outside the house.

That leaves me with an atmospheric option. Would getting a shorter tank (current is 60" tall, short tanks are 50" tall) help out with the back draft issue I am having by increasing the slope of the flue?

Here are some pics for reference:
Flue going across mechanical room:
Flue tying into flue from the furnace and going up to the roof:

 

[Jadnashua]

When attaching photos, don't try to paste them in...use the 'Upload a File' option to embed them.

 

[Sponsor]

 

[Dana]

A number of factors affect backdrafting.

Total stack height is more important than the slope of the connector to the stack, as is stack diameter.

Adding 10" to the stack height isn't going to do very much at all, but adding 1o' would.

If the major flue is too big the surface area contact with the walls of the flue is high, and the combination of more volume per vertical foot and the cooling contact work in tandem to keep the stack velocity low. The additional surface area cools the exhaust more quickly, and running the same volume rate of exhaust into a wider path is slower even without the cooling effects. The cooling even shrinks the volume of the exhaust gases, all of which adds up to low stack pressure and higher backdrafting risk.

With an"orphaned water heater" vented into a stack formerly shared with heating appliance that is no longer being used backdrafting issues would be common.

If it's just an oversized stack problem, a flue liner properly sized for the burner size & vertical distance from the water heater can solve a lot of backdrafting problems.

The air tightness of the house can be another factor, but there's no need to make the house more air-leaky. The AO Smith Pro Line GDV series are atmospheric drafted unpowered direct vented water heaters suitable for side-venting.

Per the instructions, the maximum vertical vent distance is 72” above heater before 90 degree turn through exterior wall. That's six feet. The 50 gallon version is five feet tall, so with the water heated mounted on a basement slab the termination can be as much as 11' above the slab. Just how deep is this basement?

<<GDV50

If the house is heated with a hydronic (pumped hot water) boiler, an indirect fired water heater operated as a zone is usually the solution with the best water service and higher combined net efficiency, unless the boiler is truly decrepit and ludicrously oversized for the space heating load. (Merely ridiculous oversizing is OK. )

 

[Julio Vargas]

Dana, that's some great info, some of it a bit over my head. I am going to call a plumber off the AO Smith website tomorrow and have them come out to take a look at what I have going on so thanks for the insight on the AO Smith heaters.

I am afraid that while the ability of the AO Smith direct vent heaters to have a really tall vent, I will still have an issue with the distance from the outside ground to the vent termination. I am also limited by the height of the second floor joists above the mechanical room.

I can provide some more details about the flue. It is 4" diameter from the heater to the junction with the flue with the furnace. Once it T's into the flue from the furnace it is 5" diameter from the mechanical room all the way up to the roof.

I can't seem to post pictures and I don't think you guys can see the pics I posted in the initial post. I will try to figure out how to upload the photos.

Thanks

 

[Dana]

Is it a hot air furnace (with a forced draft), or is it a hydronic boiler (some people use the terms interchangeably)?

What are the burner BTU ratings of both the water heater and the heating appliance?

It's better to wye rather than tee into shared flues (especially if one has a forced draft.) Hopefully yours is already a wye, not a tee. How tall is the stack from the where the water heater ties in, to the very top of the stack?

 

[Julio Vargas]

Is it a hot air furnace (with a forced draft), or is it a hydronic boiler (some people use the terms interchangeably)?

What are the burner BTU ratings of both the water heater and the heating appliance?

It's better to wye rather than tee into shared flues (especially if one has a forced draft.) Hopefully yours is already a wye, not a tee. How tall is the stack from the where the water heater ties in, to the very top of the stack?

This is a 88,000 btu natural draft gas furnace and it's from 1987. The hot water heater is 40,000 btu 40 gal unit, also natural draft.

The hot water heater uses a T to tie into the flue for the furnace. My guess is that the stack goes up about 2.5 stories, maybe 35 feet? Really guessing here.

 

[Dana]

Take a look at the multiple appliance venting size specs in this document starting on p.15 and Table 4B on p.16. You have 40K + 88K of natural drafted burner. With both appliance tied into the same flue 5" is clearly the right size. But with just the water heater it would be oversized.

If you look at table 4A, even with just a 1' rise the connector between the water heater & tee would want to be 4".

If that's what you have, there's not a whole lot you can do to fix it utilizing the same venting. If you move the water heater off that stack the 5" stack at 30'+ of height is ~2x oversized for an 88K burner, and it too might be more prone to back drafting (though never as badly as the water heater on it's own), and reducing the stack to 4" might be in order (unless 5" is spelled out in the manufacturer's installation instructions.)

A 30 year old furnace has already hit it's normal lifecycle, and even though it's working fine and might continue to work fine for years, it's not insane to think about replacing it. If doing it at the same time as the water heater it may make more sense to install a hydro-air air handler right-sized for your heat load, and run the air handler off a condensing water heater. An 88K, 80% efficiency furnace (~70K out) is way oversized for the heat load most normal-sized houses in the US. It's enough furnace to heat my sub-code 2x4 framed 2400' (+ 1600' of insulated basement) antique from the 1920s down to an outdoor temp of about -50F. ASHRAE recommends no more than a ~1.4x oversize factor from the heat load at the 99% outside design temperature. (eg, if your heat load at the 99% outside design temp is 30,000 BTU/hr, don't install anything that puts out much more than 1.4x 30K= 42,000 BTU/hr.)

With a hydroair solution the actual output is tweakable with water temperature and pumping flow, but there's almost always a solution. A ~75K burner on a condensing water heater is enough for both heat & hot water for 9 out of 10 houses out there, as long as the air handler coil isn't oversized for the load (and thus hogging the burner output), and a 100K burner is enough for 19 out of 20 houses.

If you have an air conditioner sharing the same ducts that may put other constraints on the air handler size, but if you want to contemplate this approach, start by running a fuel-use based load calculation (wintertime use only) to get a handle on what your heat load and oversizing factor is.

 

[Julio Vargas]

Take a look at the multiple appliance venting size specs in this document starting on p.15 and Table 4B on p.16. You have 40K + 88K of natural drafted burner. With both appliance tied into the same flue 5" is clearly the right size. But with just the water heater it would be oversized.

If you look at table 4A, even with just a 1' rise the connector between the water heater & tee would want to be 4".

If that's what you have, there's not a whole lot you can do to fix it utilizing the same venting. If you move the water heater off that stack the 5" stack at 30'+ of height is ~2x oversized for an 88K burner, and it too might be more prone to back drafting (though never as badly as the water heater on it's own), and reducing the stack to 4" might be in order (unless 5" is spelled out in the manufacturer's installation instructions.)

A 30 year old furnace has already hit it's normal lifecycle, and even though it's working fine and might continue to work fine for years, it's not insane to think about replacing it. If doing it at the same time as the water heater it may make more sense to install a hydro-air air handler right-sized for your heat load, and run the air handler off a condensing water heater. An 88K, 80% efficiency furnace (~70K out) is way oversized for the heat load most normal-sized houses in the US. It's enough furnace to heat my sub-code 2x4 framed 2400' (+ 1600' of insulated basement) antique from the 1920s down to an outdoor temp of about -50F. ASHRAE recommends no more than a ~1.4x oversize factor from the heat load at the 99% outside design temperature. (eg, if your heat load at the 99% outside design temp is 30,000 BTU/hr, don't install anything that puts out much more than 1.4x 30K= 42,000 BTU/hr.)

With a hydroair solution the actual output is tweakable with water temperature and pumping flow, but there's almost always a solution. A ~75K burner on a condensing water heater is enough for both heat & hot water for 9 out of 10 houses out there, as long as the air handler coil isn't oversized for the load (and thus hogging the burner output), and a 100K burner is enough for 19 out of 20 houses.

If you have an air conditioner sharing the same ducts that may put other constraints on the air handler size, but if you want to contemplate this approach, start by running a fuel-use based load calculation (wintertime use only) to get a handle on what your heat load and oversizing factor is.

Wow that is some great info and timely as I am looking to do a complete overhaul of all the mechanicals in the house. Like you said, everything is really old and now is the time to get everything updated including water heater, furnace, and AC. The AC and furnace do share duct work but I will look at the documents you sent.

One thing I have found is it is difficult to get someone that will take a look at the updates I want to do in my mechanical room as a whole. By this I mean water heater, furnace, and AC all in one. This is probably why I haven't heard of the hydroair option. I have had HVAC guys come out, and plumbers come out, but no one looks at the systems as a whole. At the very least I would think that there needs to be coordination in venting the units. I guess that is why my current venting has issues, it was all an afterthought after each trade came in and did their thing.

Thanks for taking the time to provide this info, I am sure I will have a lot more questions.

 

[Dana]

It may be worth paying an engineer or RESNET rater to run a formal room-by-room Manual-J heating & cooling load calculation, using AGGRESSIVE assumptions (per the Manual) before making any decisions on this. (That blog bit is behind a paywall, but there's a free 10 day trial, if you want to look at that and other articles. That site has ample freebie blogs & other info too all targeted toward energy nerds and high-performance house architects, etc.)

Most HVAC contractors fly by the seat of their pants using archaic and oversizing rules of thumb, and those who even bother with a Manual-J more often than not use overly conservative assumptions (in contravention of the actual instructions.) It's likely that the AC is as oversized as the heating (or more). There are several free online Manual-J calculators, none of which are perfect, all of which oversize by a bit (not usually by 2x), but way better than a contractor's WAG.

It's also likely that a right-sized modulating heat pump would be more comfortable and cost about the same or less than a gas-burner, but you'd then have to figure out what to do about hot water. Heat pump water heaters do double-duty drying out the basement in summer and are often cheaper to operate than gas, but are a bit on the noisy side, and have slow recovery rates that can be a real issue if you have fairly high hot water needs.

But run the fuel-use heat load numbers first. That will put a firm stake in the ground not to be ignored, and can be used as a sanity check on anybody's Manual-J. The cooling season is probably already done, but with a data logger it's possible to measure the duty cycle of your existing AC on days where the temps will be in your 1% outside design temp range too.

There are HVAC contractors' rules of thumb, but it's also possible to ball park it with other rules of thumb that are more aggressive, but there are enough exceptions to the rule that they should not be counted on. Most contractors will use a ton per 750' of conditioned space for newer construction, 500' for old stuff. For most houses a ton per 1000' or even a ton per 1500' isn't going to under size the AC (at least not by much, unless you live in an uninsulated tarpaper shack, or a greenhouse with huge solar gain).

Similarly, many of the hacks use 25 BTU/hr per square foot of conditioned space for newer construction, 35 BTU/hr per foot for older stuff (sometimes even higher), whereas for MD type design temperatures 12 BTU/hr per foot usually works for 2x6 framed houses, 15 BTU/foot for 2x4 framing. So a 70K output furnace would cover the load of a ~4500' 2x4 framed house, whereas the typical 2500' 2x6 framed house would be fine with a 30K-out furnace. There isn't a big efficiency hit when oversizing a hot air furnace by 2-3-4x , but it is a comfort issue. When it's really cold out the heat comes on for several minutes of hot-flash, followed by the longer chill. With a right-sized furnace it's running more than half the time on cold days and at lower cfm / less draft.

 

[Julio Vargas]

Take a look at the multiple appliance venting size specs in this document starting on p.15 and Table 4B on p.16. You have 40K + 88K of natural drafted burner. With both appliance tied into the same flue 5" is clearly the right size. But with just the water heater it would be oversized.

If you look at table 4A, even with just a 1' rise the connector between the water heater & tee would want to be 4".

If that's what you have, there's not a whole lot you can do to fix it utilizing the same venting. If you move the water heater off that stack the 5" stack at 30'+ of height is ~2x oversized for an 88K burner, and it too might be more prone to back drafting (though never as badly as the water heater on it's own), and reducing the stack to 4" might be in order (unless 5" is spelled out in the manufacturer's installation instructions.)

A 30 year old furnace has already hit it's normal lifecycle, and even though it's working fine and might continue to work fine for years, it's not insane to think about replacing it. If doing it at the same time as the water heater it may make more sense to install a hydro-air air handler right-sized for your heat load, and run the air handler off a condensing water heater. An 88K, 80% efficiency furnace (~70K out) is way oversized for the heat load most normal-sized houses in the US. It's enough furnace to heat my sub-code 2x4 framed 2400' (+ 1600' of insulated basement) antique from the 1920s down to an outdoor temp of about -50F. ASHRAE recommends no more than a ~1.4x oversize factor from the heat load at the 99% outside design temperature. (eg, if your heat load at the 99% outside design temp is 30,000 BTU/hr, don't install anything that puts out much more than 1.4x 30K= 42,000 BTU/hr.)

With a hydroair solution the actual output is tweakable with water temperature and pumping flow, but there's almost always a solution. A ~75K burner on a condensing water heater is enough for both heat & hot water for 9 out of 10 houses out there, as long as the air handler coil isn't oversized for the load (and thus hogging the burner output), and a 100K burner is enough for 19 out of 20 houses.

If you have an air conditioner sharing the same ducts that may put other constraints on the air handler size, but if you want to contemplate this approach, start by running a fuel-use based load calculation (wintertime use only) to get a handle on what your heat load and oversizing factor is.

Dana, I am absolutely intrigued by this hydro-air solution for my house. I had 3 contractors come out to my house to have a look at my situation and one of them suggested I consider hydro-air. He explained to me how it works and how it would fit into my solution. I have done some reading about it and my concern is that there are not a lot of contractors familiar with these setups in my area (suburban Washington DC area).

I am also nerding out on the GBA article you sent and did one of my favorite things to do....excel spreadsheet calcs. I ran numbers for 3 months of last winter season and wanted to see if you could do a sanity check?

My house was built in 1972, 2x4 studs with mediocre insulation. It is only about 1,700 sq. ft. My numbers for a 65/60 deg delta on the highest gas bill last season were:
65 deg delta - 41278 BTU/hr
60 deg delta - 32250 BTU/hr

My issue is that I don't know what the exact output of my furnace is. I know input is 88,000 BTU but the label doesn't have output and I couldn't find anything online.

Here is a link to my spreadsheet on google drive: https://docs.google.com/spreadsheets/d/1O9iX3sqyrZ6la1kLgYG0oAPR_UzAr21mP6OrBDeQNf0/edit?usp=sharing

 

[Dana]

For a 2x4 framed house that was kept at 68-75F use the base 65F BTUs per degree day constant. If you set back to 60F and only heat to 65F use the base 60F derived constant, but you'd have to add a few heating degrees to come up withe load at a code-min 68F indoor temp to ensure code compliance.

The Delta 65° F and Delta 60°F labels sort of threw me off at first. That's not a temperature delta, that's using the presumptive balance point. The delta is the difference between the presumptive balance point (either 60F or 65F) and outside design temperature (12F). But that's just a labeling issue- the arithmetic looks right.

The BTU/HDD number took a huge spike in January compared to the December & February numbers but I haven't dug into it to figure out why. It's usually a bit lower ratio during the cooler periods when there's a steady heat load, and has bigger errors related to solar gain or hot water use during the shoulder seasons. Did you have a bunch of guests over showering or partying with the windows open? Were you surfing in Belize with the thermostat turned down for a couple of weeks in February? The BTU/HDD numbers at my house never vary by more than about 5% month to month in the winter. Yours appeared to have HUGE swings.

Either way, the 672 - 779 BTU per degree hour numbers are credible with a middle of the road 1700' house with 2x4 framing and no foundation insulation, but a bit on the high side, indicating possibly fair amount of leakage, maybe duct balance/leakage issues or ducts in an attic outside the thermal envelope(?), or maybe single pane windows (?). If the furnace is old enough to be getting AARP card solicitations it might only have a steady state efficiency of 70% rather than the 80% you used.

The Base 65 derived 392 & 322 BTU per degree hour numbers for December & February in a 1700' house would only be credible for a reasonably tight 2x6 framed house with low-E windows and an insulated foundation. Those houses exist- is yours one of them? (If so, what happened in January?)

Most 2x4 framed 1700' houses with clear-glass double panes would come in around 500 BTU per degree hour (give or take 50), maybe 425 with foundation insulation and much tighter than average. So at base 65F and a 12 degree design temp you have 53 heating degrees, and at a more typical 500 BTU per heating degree the load would be design load of 26,500 BTU/hr, and a 1.4x oversizing the equipment would need to deliver ~37K. A 40K condensing furnace would be work fine, a 30K condensing furnace would just make it but not with a lot of margin.

Even a 70% efficiency an 88K furnace would have an output over 60K, a bit above the ASHRAE recommended 1.4x even at your worst-case 41K calculated load.

There's not much rocket science to a single zone hydro-air running off a water heater. In most states there doesn't even have to be isolation between the potable and heating system water on one of those. In those that do require isolation it's just an extra pump and a plate type heat exchanger, and a small expansion tank for the isolated loop on the heating side. The pumps get activated at the same time by the same zone relay, the hydro-air handler blower can turned on by either internal controls or an external strap-on aquastat on the supply plumbing near where it enters the air handler. Water heater driven hydro-air are pretty common all over the southeastern US where heat loads are typically pretty low, and it's not rare in California either. Firstco (an air handler company in Texas) has a wide range of hydro air handlers suitable for everything from tiny apartments to pretty big systems, but the bigger ones need more burner than typical 76,000BTU/hr condensing water heater.

 

[Jadnashua]

Hydro air solutions can work well if you already have the ductwork for air conditioning and the requisite fan, etc. My condo has a combination of central air, some hydronic in-floor heat, and hydro-air. I just added the heat exchanger in the top of the air handler, and plumbed it up as a zone. The hassle with ducts designed for a/c is that they are often in the ceiling, and adding heat there isn't as comfortable as bringing it in nearer the floor level, but if the return ducts are well designed, can still work for both. FWIW, the response with the hydro-air is faster than my radiant if I've been away for awhile and use setback. Normally, when I'm home, I don't, as getting the thermal mass warmed up in the floor takes a long time, the hydro-air is much faster. My normal water setpoint is fairly low, so the hydro-air doesn't get as hot as a conventional furnace, but with good placement, that's not an issue. It's probably closer to what you'd get with a heat pump heating system.

 

[Master Plumber Mark]

Hi, I am looking at replacing my water heater. An energy auditor determined my water heater was back drafting and it is 15 years old so I'd like to replace it as well as address the air quality issue with back drafting.

I wanted to get a direct vent water heater but the height that the vent needs to go up to clear the ground level outside is too high. The mechanical room is in the underground basement. I'd like to stay away from power vented water heaters because of the added noise both inside and outside the house.

That leaves me with an atmospheric option. Would getting a shorter tank (current is 60" tall, short tanks are 50" tall) help out with the back draft issue I am having by increasing the slope of the flue?

Here are some pics for reference:
Flue going across mechanical room:
Flue tying into flue from the furnace and going up to the roof:

Rheem makes a smaller sized 50 gas heater that would probably solve your issues
without going too crazy... they cost a few hundred dollars more than a standard unit
but you can put a 4 inch flu on the unit and increase the pitch dramatically ...

it would probably solve all your issues....

 

[Julio Vargas]

For a 2x4 framed house that was kept at 68-75F use the base 65F BTUs per degree day constant. If you set back to 60F and only heat to 65F use the base 60F derived constant, but you'd have to add a few heating degrees to come up withe load at a code-min 68F indoor temp to ensure code compliance.

The Delta 65° F and Delta 60°F labels sort of threw me off at first. That's not a temperature delta, that's using the presumptive balance point. The delta is the difference between the presumptive balance point (either 60F or 65F) and outside design temperature (12F). But that's just a labeling issue- the arithmetic looks right.

The BTU/HDD number took a huge spike in January compared to the December & February numbers but I haven't dug into it to figure out why. It's usually a bit lower ratio during the cooler periods when there's a steady heat load, and has bigger errors related to solar gain or hot water use during the shoulder seasons. Did you have a bunch of guests over showering or partying with the windows open? Were you surfing in Belize with the thermostat turned down for a couple of weeks in February? The BTU/HDD numbers at my house never vary by more than about 5% month to month in the winter. Yours appeared to have HUGE swings.

Either way, the 672 - 779 BTU per degree hour numbers are credible with a middle of the road 1700' house with 2x4 framing and no foundation insulation, but a bit on the high side, indicating possibly fair amount of leakage, maybe duct balance/leakage issues or ducts in an attic outside the thermal envelope(?), or maybe single pane windows (?). If the furnace is old enough to be getting AARP card solicitations it might only have a steady state efficiency of 70% rather than the 80% you used.

The Base 65 derived 392 & 322 BTU per degree hour numbers for December & February in a 1700' house would only be credible for a reasonably tight 2x6 framed house with low-E windows and an insulated foundation. Those houses exist- is yours one of them? (If so, what happened in January?)

Most 2x4 framed 1700' houses with clear-glass double panes would come in around 500 BTU per degree hour (give or take 50), maybe 425 with foundation insulation and much tighter than average. So at base 65F and a 12 degree design temp you have 53 heating degrees, and at a more typical 500 BTU per heating degree the load would be design load of 26,500 BTU/hr, and a 1.4x oversizing the equipment would need to deliver ~37K. A 40K condensing furnace would be work fine, a 30K condensing furnace would just make it but not with a lot of margin.

Even a 70% efficiency an 88K furnace would have an output over 60K, a bit above the ASHRAE recommended 1.4x even at your worst-case 41K calculated load.

There's not much rocket science to a single zone hydro-air running off a water heater. In most states there doesn't even have to be isolation between the potable and heating system water on one of those. In those that do require isolation it's just an extra pump and a plate type heat exchanger, and a small expansion tank for the isolated loop on the heating side. The pumps get activated at the same time by the same zone relay, the hydro-air handler blower can turned on by either internal controls or an external strap-on aquastat on the supply plumbing near where it enters the air handler. Water heater driven hydro-air are pretty common all over the southeastern US where heat loads are typically pretty low, and it's not rare in California either. Firstco (an air handler company in Texas) has a wide range of hydro air handlers suitable for everything from tiny apartments to pretty big systems, but the bigger ones need more burner than typical 76,000BTU/hr condensing water heater.

Dana, thank you for taking a look at my numbers. I think I can provide some context that can shed light on the situation.

House details:
- 1970's 2x4 framed mediocre insulation, no insulation on basement poured concrete walls, half of the house has a sloped roof with minimal insulation, single pane wood framed windows, furnace is from the 1980's, blower door test resulted in 2900 CFM @ 50Pa, half the duct trunk is in the un-insulated crawlspace has insulation wrap but it has seen better days), basement is ~4 degrees warmer probably due to the trunk that runs through the drop ceiling that is leaking air.

- This was our first winter in this house so we were still trying to settle into the furnace settings. In December we were away on vacation for 1 week where I turned the heat down. Looking at my program for the thermostat we had it set 70 deg.

We would like to add a small addition to the house. There is currently a small addition (180 sq. ft.) built on slab on grade which is always really cold and drafty. We want to add to it for a total of 294 sq. ft. (adding 114 sq. ft.).

All of the equipment, furnace, AC, water heater, is ready for replacement. With all of this in mind, I don't know if I should wait to build the addition (1-2 years away) to upgrade HVAC. Or upgrade HVAC now knowing I will probably have to do a mini split for the addition due to the difficulty of running ducts to the addition....or maybe going hydronic radiant floor heat...but how would I do that on an uninsulated slab?

I also have complete access to the subfloor via the basement because of a drop ceiling and open crawl space....should I be considering hydronic radiant floor heating?

As you can see I have lots of questions. Thanks for all your help.

 

[Dana]

Many times adding a tight well insulated addition onto a house will lower the total heat load numbers, because it's replacing a wretchedly performing section of wall + window with something with the heat loss (despite being more wall area), and replacing it with only a modest increase of ceiling & floor loss. Even if it adds 50% to the window & wall area, a Uo.30 window only loses 60% of the heat that a U0.50 window does. A 2x6/R23 air tight wall loses only half the heat of a leaky poorly detailed 2x4/R13 wall.

Is there any slab edge insulation on the slab on grade addition? Any plans to air seal & insulate the basement? (Air sealing & insulating the basement of my 1920s bungalow to current code min-took more than 15% off my whole-house heat load- that was after blowing insulation into the walls. YMMV.)

With a water heater based hydro-air system you can tweak the output temp with the storage temp.

If you run a room by room I=B=R type heat load calc on the place in a spreadsheet it'll give clues as to which measures will have the biggest effects. The wild card is the air infiltration numbers on each room, but with the ACH/50 numbers you can estimate an ACH-natural number, and just apportion it to rooms based on the room's exterior surface area (not floor area.) eg:

The specific heat of air is about 0.018 BTU per cubic foot per degree F, so if a leaky-drafty room has, say 100 cfm (6000 cubic feet per hour) of infiltration and your 99% outside design temp is +15F, and the indoor temp is 70F, that's a 55F delta, and the infiltration losses are

6000 cfm x 55F x 0.018 BTU/ft^3= 5940 BTU/hr

 

[Julio Vargas]

Many times adding a tight well insulated addition onto a house will lower the total heat load numbers, because it's replacing a wretchedly performing section of wall + window with something with the heat loss (despite being more wall area), and replacing it with only a modest increase of ceiling & floor loss. Even if it adds 50% to the window & wall area, a Uo.30 window only loses 60% of the heat that a U0.50 window does. A 2x6/R23 air tight wall loses only half the heat of a leaky poorly detailed 2x4/R13 wall.

Is there any slab edge insulation on the slab on grade addition? Any plans to air seal & insulate the basement? (Air sealing & insulating the basement of my 1920s bungalow to current code min-took more than 15% off my whole-house heat load- that was after blowing insulation into the walls. YMMV.)

With a water heater based hydro-air system you can tweak the output temp with the storage temp.

If you run a room by room I=B=R type heat load calc on the place in a spreadsheet it'll give clues as to which measures will have the biggest effects. The wild card is the air infiltration numbers on each room, but with the ACH/50 numbers you can estimate an ACH-natural number, and just apportion it to rooms based on the room's exterior surface area (not floor area.) eg:

The specific heat of air is about 0.018 BTU per cubic foot per degree F, so if a leaky-drafty room has, say 100 cfm (6000 cubic feet per hour) of infiltration and your 99% outside design temp is +15F, and the indoor temp is 70F, that's a 55F delta, and the infiltration losses are

6000 cfm x 55F x 0.018 BTU/ft^3= 5940 BTU/hr

I have a feeling that what you just described with the addition will be our situation. The addition that is here now will be much improved when we go to 2x6 construction with much better sealing.

There is no insulation on the slab on grade. I had an energy auditor out to perform an audit and that is what kind of kicked off this whole plan. I am using credits from my power company to seal and insulate. The issue is that for them to perform the work, I need to address the back drafting water heater. If I address the backdrafting water heater I might as well take a whole view of the mechanicals and see what makes the most sense. Do I go with atmospheric gas furnace and water heater that is there now? Do I do a tankless hydronic forced air system? Do I do hydronic forced air with a tank? Do I do hydronic radiant put in below the subfloor? Seems like contractors just come in and suggest what they know. The only contractor that really offered me options suggested hydronic forced air, using a mini split in the basement to separate it from the rest of the system, but he said wait until you do the addition. He did say I will probably need to do another mini split in the addition due to the challenge of running ducts to the future addition.

I think that for now I will fix the back draft issue with the current water heater by using a Y instead of a T in the junction between the flues, get the air sealing and insulation and crawlspace encapsulation done before winter, and do the same analysis this winter along with finalizing the plans for the addition.

 

[Master Plumber Mark]

Julio... you are gonna get contradictory
advice from people that is gonna drive you crazy.....

and some suggestions sound real fancy and hi-tech
but god help you if if it all breaks down 3 years from now..and their is no one
around any longer who knows how to get it up and running ........fast.....

the more simple you keep things the happier you are gonna be down the road.

no more said from me.......

 

[Dana]

The heating and cooling loads of most basements are WELL below that of a 3.4 ton mini-split. If you're going to go with mini-splits, replacing the furnace with a 2-3 ton 3 stage heat pump and using a mini-split for the addition probably makes more sense.

A hydro-air off the water heater is the furthest thing from "high tech"- it's dumber than a box o' rocks, but if tied to a condensing TANK type water heater (that's also serving up the hot water) it'll do about as well as a condensing gas furnace on efficiency.

A First Co. 36HBQBR will deliver over 40,000 BTU/hr with 140F entering water temp out of a water heater, enough to pretty much cover your worst-case fuel use calculated heat load. The 30HBQBR would deliver 37K @ 140F EWT, the 24HBQBR would deliver 31K, and is probably more appropriate if you're doing extensive insulation & air sealing. If you take a commercial condensing water heater that can be cranked up to 160F if needed (eg HTP Phoenix PH76-50 ), the 24HBQBR will deliver 40KBTU/hr at an EWT of 160F. There are other, cheaper First Co. air handlers and water heaters other than the PH76-50 that could work. I like the stainless steel construction an 3:1 modulation on th the burner of that water heater- set up correctly it basically never hits it's max-fire 76K burning rate unless you're taking an extra-long shower while the air handler is running. The 24HBQBR also has a 2-ton cooling coil- still oversized for your likely load, but not ridiculously so.

You combine that sort of setup without oversizing it, with the output of a 3/4 ton or half ton mini-split for the addition you're in pretty good shape overall, with plenty of margin for cold snaps. A 3/4 ton Mitsubishi FH09NA can deliver over 10,000 BTU/hr even at 0F, but can idle back to 1600 BTU/hr (much lower than most 3/4 tonners), which makes it more comfortable & efficient. There are cheaper mini-splits out there, but the minimum-modulation level may be well over the design heat load for the zone you'd be heating with it, and it's a first tier company with a good support network.

A cheaper mini-split that would also modulate well is the 3/4 ton LG Art Cool (silver or plain), which can throttle back to about 1000 BTU/hr @ +47F. It's pretty decent, but the LG's distributor support in the US isn't as well developed as Mitsubishi/Fujitsu/Daikin. (Fujitsu & Daikin have much higher minimum modulation numbers, and need more load than you'll probaly have in the addition to hit their efficiency numbers.)