Replacing 3 furnaces - Does new hydronic heat install make sense?

yekimouse

Hey HeatingHelp Braintrust,

I bought a home last fall and have a number of remodel projects planned for the near future. I will need to commit to a new heating and cooling plan, but the existing setup is kind of a mess.



I currently have 3 gas furnaces varying in age from 30 years to 10 years old, though they're all very basic, 80% efficient units with single speed fans. My house is mid-century style with high ceilings and thin (poorly insulated) roofs. The largest of the three furnaces, which is the one that services the bedrooms, has a 2.5ton A/C as well, but none of the other furnaces have any cooling or dehumidification. The previous owners said this setup was basically fine, that indoor humidity wasn't an issue for them in the past.

I'm on the shore of Lake Michigan at ~43º latitude (zone 5) but being that close to the lake provides some moderating of the temps. Gas bills from October when we moved in through March were: $30, $50, $175, $200, $275, $250, and $240, paying $0.80/CCF in April -- and we keep the place pretty cool.

Obviously all 3 furnaces are oversized and inefficient so the 'obvious' answer is to just replace them where they sit with more efficient / modulating / variable speed units and move on to some other project. This is what the 'comfort specialist' from the local HVAC company suggested.

I can't help but think though that this would be a pretty good setup for hydronic air / radiant floors though? Furnaces A & B are ~10ft apart but on opposite sides of a cinderblock wall below grade, so I could definitely run piping between them, but the shared wall isn't large enough to 'just' connect the ductwork instead to zone a traditional furnace. Furnace C is on the other side of the house across the bridge between the two main living areas. The ceilings in area C are very high (14ft) and the room has a lot of single-pane glass. We're going to replace the flooring on that side as well across the whole 'bridge' (Area D in the picture) so it would be an opportune time to add in-floor radiant as well to make that whole section much more comfortable.

I ran my own Manual Js based on the current structure and came up with the figures in the picture. We have some exterior projects planned where we'll insulate / air seal better and I'm going to add more foam to the roof when that's replaced in a few years, so the overall load on everything should decrease.

Using something like a Combi-boiler could replace the HWH as well, so we could go from 4 gas-burning appliances to 1 which definitely appeals to me..

Any thoughts/guidance?

Hot_water_fan

Interesting! Modulation would be much better with hydro-air, especially if you use an indirect instead of a combi (which have to balance central and domestic heating). Oversizing furnaces might be uncomfortable, but it’s not much of an efficiency hit and even modulating furnaces aren’t that great at modulating. Cold climate heat pumps could give you both AC and heating at those loads, with better modulation. 

What’s the total CCF used within that date range?  

I personally wouldn’t value this switch enough to do it, I’d probably replace the furnaces at end of life with heat pumps. 

Jamie Hall

What is your design temperature? If it's something reasonable -- say in the teens or even around 5 or so -- I would definitely look at air to air ducted heat pumps sized to the heating load. Several of the newer units claim to give adequate performance, in terms of COP, even down to 0, though I'd be a bit wary. Properly sized and installed, though, you'd get heat and cooling and dehumidification all in the same systems. I'd then just leave the gas hot water heater.

yekimouse

@Hot_water_fan

What’s the total CCF used within that date range?

About 1,500 CCF for the season so far - still have the heat running though, likely ~1,800 CCF when all said and done. Would surely be over 2,000 if we made full use of the house though - we've been doing some work on the "Area C" so didn't really use it this winter. Hope to make it our primary living room soon.

I personally wouldn’t value this switch enough to do it, I’d probably replace the furnaces at end of life with heat pumps.

Unfortunately the "main" furnace for the bedrooms seems pretty close to end of life, and 'Furnace C' will have to be upgraded fairly soon too - It shares a chimney stack with the existing fireplace but we're needing to replace that whole chimney due to failed masonry, so going to remove it completely and frame in a gas insert. Will need to put in something that can be vented out the wall (or not vented in the hydronic scenario).

Cold climate heat pumps could give you both AC and heating at those loads, with better modulation.

I've been having a hard time figuring out actual heating costs on a cold climate heat pump -- I pay ~$0.12/KWh for power if that's useful at all. I know they have dual zone mini-splits with, but does a product exist that would allow for one outdoor unit to serve two ducted AHUs? The two existing furnaces (A & B from my pic) are *so* close together, it seems like there must be some efficiency gained from combining them into a bigger unit?

yekimouse

Thanks @Jamie Hall

What is your design temperature? If it's something reasonable -- say in the teens or even around 5 or so -- I would definitely look at air to air ducted heat pumps sized to the heating load.

Yeah design temp according to the different tools I've seen are ~7º. Did my own little histogram and 99.5% of hours in the past 6 years have been over 5º... but 480 hours below that..



We will have a few sources of aux heat already - prior owners installed a pretty ugly electric fireplace in the area under the master bedroom, but it's good for 7,500BTU or something. In "Area C" on my original pic, we'll be adding in a gas insert, and eventually "Area B" will have a wood-burning fireplace too, so that *should* cover the colder scenarios I would think?

Any idea what I should expect costwise to install a ducted heatpump? Seems like 3 of those are going to add up..

pedmec

Honestly, there really isn't a wrong answer. One thing i have always done is when i comes to air and water is to split them. hot water for heat and air for cooling.

Radiant heating for the floors is great heat, the best heat, if you can afford it. And that's the installation cost.

I have always advocated for hot water baseboard with separate air handlers for a/c. again, its more money for installation but its the next best comfort and more affordable than radiant. its the one i put in all my properties because its has like a mid-price to high price point.

Furnaces with split system for cooling is probably the most affordable because your just doing a equipment replacement. not any major ductwork.

then you have mini split heat pumps that you can use, although they put out heat in subzero temps the air supply temps get lower the cooler it gets outside. takes a little getting use to when its cold.

This is just the tip of the iceberg. In our industry we are getting just as bad as cars salesman with all the different options we can offer. sometimes i feel like customers get information overload and there heads are going to explode. your gonna get so many different options and opinions. i prefer comfort over price. price is just once. comfort is everyday.

the one thing that you should definitely avoid is trying to heat and cool different levels using the same ducts without using zone dampers. (and i'm not crazy about zone damper system either but that's a discussion for another day). your airflow demands change depending on the season. warm air risers ALL THE TIME. so in the summer you will need more air upstairs than downstairs. in the winter the reverse is true.

Jamie Hall

I'm kind of new to the heat pump racket -- @Hot_water_fan will, hopefully, come in here with more knowledge. I do think they may be suitable. As to cost, it is a bit hard to pin down, but the key number for a heat pump is the COP, which, very roughly speaking, is the power out in BTUh divided by the power consumed. Unless fuel is very very cheap in your area, at $.12 per KWhour you're going to be well ahead of the game with a heat pump.

Jon_blaney

Is $.12 per KWH the full price for electric with all charges or just the power?

Keep in mind that the air temperature of the various alternatives is different, furnace warmest and HP coolest. This effects your duct sizing. My first reaction was hydronic air handlers with one boiler. Current ducting will probably work and you can have your radiant floor if proper insulation can be achieved.

Many choices. How much do you want to spend?

yekimouse

@Jon_blaney

Yep - Just checked my most recent bill, $15 monthly service charge + $0.1144/KWh. Local power company is pretty great, they just published a 5-year plan which is looking to lower rates as well (geopolitics dependent I assume).

I do worry a bit about duct sizing - the main part of the house was built 70 years ago and it shows in a bunch of ways. The previous owners installed an elevator for a handicapped family member (long story, but we're removing it) and in the process cut off one of the only supplies on the "Area D" bridge. In my pic that little orange branch heading North isn't actually attached at the moment.

I do worry about cost, but more about comfort I think.. we definitely design for heating load in this part of the country. If I were to do the Hydronic or furnace-swap plan, I'd likely keep the single AC unit in the bedroom area. The original HVAC guy who came out proposed a 3 furnace, 3 AC unit plan for ~$25k which seems really steep for treading water comfort wise.

Any clue on a price range for 3 AHU + Boiler setup? If I just went with the AHU for now, think I could leave capacity/ability to do a few loops of radiant later on? One issue is that they'd be at the furthest point from the boiler so I couldn't do home-runs, would have to branch off near the "Furnace C" water supply.

The Steam Whisperer

Depending on ductwork setup...modulating hydro air could be good option ( they are not exclusive in themselves). I believe probably the best ductwork design for this option is low velocity high on all supplies and low on wall or floor returns. This way, when the water temperature is modulated down in warmer weather, you won't feel the draft as much from the cooler supply air. We've worked on a few first class homes built in the 1950s that were set up this way by the genius level engineer that worked in the area.

Hot_water_fan

I've been having a hard time figuring out actual heating costs on a cold climate heat pump -- I pay ~$0.12/KWh for power if that's useful at all. I know they have dual zone mini-splits with, but does a product exist that would allow for one outdoor unit to serve two ducted AHUs? The two existing furnaces (A & B from my pic) are *so* close together, it seems like there must be some efficiency gained from combining them into a bigger unit?

@yekimouse, yup you can get one outdoor unit to power 2 indoor ducted air handlers.

The calculation for heating costs is:

Heat Pump: $/kwh * (1,000,000/3412)/COP = $/MMBtu
Furnace/Boiler: $/CCF * (1,000,000/103,700)/COP = $/MMBtu

A COP of 2.5 for a heat pump is a good estimate. A furnace is somewhere between .8 and .99.

If the ducts are undersized, you'd run into similar issues with a high efficiency furnace/boiler, as they use lower air temperatures than low efficiency furnaces. A hybrid heat pump/(furnace or hydro-air) set up can remedy this, as you'll use the heat pump for the majority of the winter. An air-to-water heat pump would get you the radiant floor possibility, the hydro-air efficiencies and cooling all in one, but they're less common in the US right now.

All prices are elevated right now, no option will be particularly cheap here.

yekimouse

yup you can get one outdoor unit to power 2 indoor ducted air handlers.

Interesting, I see the multizone options now... I guess I had only associated those with the ductless.

The closest AHU would be ~8' from the outdoor unit, the second closest would be ~15', then the furthest AHU would be ~75' from the outside unit -- would it be hypothetically possible then to run all 3 from the same outdoor unit (I know elbows/drops etc impact this)? I hadn't once considered that.. All 3 AHUs are below the outdoor unit and the lines would run entirely through conditioned space if that matters..

The calculation for heating costs is: ....

Unfortunately with baseline numbers, it seems like a gas furnace has a ~30% cheaper operating cost. If I use a COP of 3.5 and a gas cost of $0.90/CCF, it's pretty close though.. Then again, the COP on all of these units is closer to 4.0 at 40º and only drops below 3.0 at 20º -- so for most of the year, it'd be cheaper.. and this would get me AC in each space too..

Instead of narrowing my search options from hydronic vs. furnace, did I just expand the choice to include a heat pump?

pecmsg

yekimouse said:

yup you can get one outdoor unit to power 2 indoor ducted air handlers.

Interesting, I see the multizone options now... I guess I had only associated those with the ductless.

The closest AHU would be ~8' from the outdoor unit, the second closest would be ~15', then the furthest AHU would be ~75' from the outside unit -- would it be hypothetically possible then to run all 3 from the same outdoor unit (I know elbows/drops etc impact this)? I hadn't once considered that.. All 3 AHUs are below the outdoor unit and the lines would run entirely through conditioned space if that matters..

The calculation for heating costs is: ....

Unfortunately with baseline numbers, it seems like a gas furnace has a ~30% cheaper operating cost. If I use a COP of 3.5 and a gas cost of $0.90/CCF, it's pretty close though.. Then again, the COP on all of these units is closer to 4.0 at 40º and only drops below 3.0 at 20º -- so for most of the year, it'd be cheaper.. and this would get me AC in each space too..

Instead of narrowing my search options from hydronic vs. furnace, did I just expand the choice to include a heat pump?

No, you've narrowed it to Heat Pumps with Hydronic coils. Now the only issue is if these coils are in cold locations AKA an attic then glycol MAY be needed.