convert existing NG forced air to hydronic

dwatson3

I've a forced-air NG furnace and contemplate removing the gas burners, purge system, pressure switches, etc and replace with a high quality heat transfer coil using water pumped from a high efficiency condensing combi boiler (space heating and hot water). That allows continued use of a good duct system. A second zone (addition) will pump hot water to two lower level radiant panels. The existing furnace is 60 KBTU gives way to the new boiler rated at 80 KBTU with a AFUE rating of 95%. Any thoughts / comments are appreciated.

ScottSecor

Some things to consider. The ductwork and fan are likely going to require a high temperature copper heating coil to supply enough energy to heat the space. If I am correct, this might mitigate the idea of a condensing boiler as they condense the most when they operate below 140 degrees. Personally, the dry air a warm air furnace produces dries out my nose to the point that it bother my nose. With a high temperature coil (if required) the warm air might have the same effect. On the other hand, IF you can run the system at a lower temperature, this might mitigate the "super dry air" that most people dislike.

As far as the second zone with the radiant, sounds like a great idea and the boiler will be in condensing mode all the time if done properly. We're not big fans of combi units, instead we prefer a small condensing boiler and a indirect tank piped in for dhw.

Jamie Hall

May I ask why you haven't considered a high efficiency hot air furnace -- or even a heat pump?

dwatson3

Prime objectives are: 1.) reduced carbon footprint 2.) one NG appliance only. Existing NG furnace is ~20 years old, in decent condition and operates at 90% efficiency. It's likely getting near the end of its lifespan but has served us well with no major repairs. (it's been maintained regularly.) We live in Southern Ontario, Canada where air-to-air heat pumps will require additional backup in winter months.

ratio

Instead of hacking up the old unit, I'd look in to a real hydronic air handler. You'll have a hard time finding someone to work on a homemade Frankenunit like you described, especially at 5 pm Friday, when those sort of things usually fail. You may be able to size it for cooler water too, although the ductwork will be the driving factor in the end.

Jon_blaney

I think you would be surprised at how long a furnace will last. I would have it inspected carefully before you condemn it. Also, how does the furnace output compare to your heat loss? If you decide to go hydroair, get a new air handler. You will be happier in the end.

JUGHNE

IIRC, some ModCons (heating only) can be programed with 2 different temps.
Higher for fan coil and lower for radiant. It depends which units are calling for heat.

Hot_water_fan

1.) reduced carbon footprint 2.) one NG appliance only.

Heat pump is for you then! They work down to -17 F or so, which might be enough for your location. If still adding a boiler, a hybrid gas/heat pump system would serve you well. 

JUGHNE

They will work down to -17F.....but enough BTU's delivered to cover the house heat loss at that temp in Ontario? And the COP might be 1.0?

Jamie Hall

JUGHNE said:

They will work down to -17F.....but enough BTU's delivered to cover the house heat loss at that temp in Ontario? And the COP might be 1.0?

This is largely correct. All the ones I know of would either a very low COP at your temperatures -- or require backup heat. I'm still not sure why you aren't considering a purpose built high efficiency gas furnace replacement, however. They have efficiencies up to what you are considering for your boiler -- at considerably less complexity compared to a hydro-air system.

Hot_water_fan

They will work down to -17F.....but enough BTU's delivered to cover the house heat loss at that temp in Ontario? And the COP might be 1.0?

We’re just guessing what the heat load is at this point. I doubt the design temp in southern Ontario is -17 F and it doesn’t particularly matter the COP for .4% of the year. It especially doesn’t matter if a boiler or furnace is used as backup. If you want low carbon emissions, you have to use at least a hybrid setup. 

bucksnort

dwatson3 said:

Prime objectives are: 1.) reduced carbon footprint

Then go 100% electric

JUGHNE

I just looked back at some files.
I installed several 90+ NG furnaces in 1991, they are still running. 3 of them were counter flow, Heavy as hell as I had to pick them up to put on AC coil boxes. Single pipe venting only.
That is 30 years ago....I feel really old now....
Repairs included only inducer fans and ignition module and some fan controls.
They were "Heil" brand.
Also a bunch of 80%+ of that vintage that are still humming along.

So if it made it 20 years it could have more in it.
Today's 95+ whatever furnaces are so "tweaked" for efficiency and thin as possible.
20 years out of one of them hard to imagine.
Repair costs out of the 10 year warranty will push you to buy another new one.

Just like a ModCon boiler 15-20 is almost unheard of.

Having to run boiler temp high for a fan coil will drop your efficiency from the boasted 95 or so down to less then 90% IMO.

And keeping the FAF would give you redundancy in the event of failure of a single NG appliance.

Just an old guy's perspective FWIW.

ethicalpaul

I installed one of these myself in place of an oil burning furnace. I contracted out for the engineering work which included really just a heat loss on my house, and the drilling. I had a single 500 foot vertical well drilled and the driller put in and grouted the loop, terminating it with a pressure test into my basement.

I used an atmospheric circulator pump from this same site that required no pressurization.

In a single Saturday afternoon I pulled the furnace and swapped in the heat pump. The location was southern Connecticut and I sized it on the small side for my heat loss which always gave me a good excuse to improve the insulation and tightness of the home. It kept up for the 3 years I had the house with no issues and provided amazing air conditioning.

If I would have needed supplemental heating, I would have put resistive strips in the ducts but it always kept up. Maintenance is zero and the life of the system is expected to be minimum 20 years. I don't see the loop failing in 100 years. My only regret is that I didn't live in the house long enough but I moved for other reasons.

https://iwae.com/shop/3-ton-28-7-eer-2-stage-geocool-geothermal-heat-pump-vertical-package-unit-ha18833.html

Jamie Hall

For reference, it makes a bit of a difference where in Ontario you are located, @dwatson3 . To give you -- and others -- an idea, here are several, sort of scattered around the Province. Toronto:-9, Ottawa, -18, Peterborough, -15, Sarnia, +6, Thunder Bay, -30, Sioux Lookout, -38.

And lest someone wonder, having relatives who live or have lived here and there in Ontario, those numbers are, if anything, conservative.

These are not temperatures which the folks south of the border are accustomed to looking at. Depending on just where you are, it is possible that an air to air heat pump with big resistive strips will work. It's going to be costly -- last I heard Ontario Hydro wasn't all that cheap -- but over a year your carbon footprint would probably be at least no worse than a high efficiency gas furnace, since about 60% of your power is nuclear and thus zero carbon. But that is true only along Lake Erie and over towards Sarnia. Go any further up country -- even to some of Toronto's northern suburbs-- and your winter electric costs for all resistive heating will be impressive and that is what you would be dealing with.

If you have the capital to invest and the land available, a geothermal heat pump, as @ethicalpaul mentioned, will also work.

ethicalpaul

Thanks @Jamie Hall ! The poster did say they were in southern Ontario so that is good.

I will just clarify one thing...land is not required for ground source heating (some questionably call geothermal). My well was vertical and was under my driveway. It could have been on a 1/3 acre lot.

dwatson3

We're located in London, Ontario..... By the way, .... thank you for sharing your impressive skill-sets. I'm learning much and have now started considering a hybrid system. 👍

Hot_water_fan

@dwatson3 excellent news! Incorporating a heat pump is the key to reducing emissions, everything else just nibbles at the margins. If you go hybrid, skip the geothermal, which is wildly expensive usually. 

hot_rod

Data is available to make a good decision on the type of equipment in your actual condition.
Low temperature heat emitters will be part of the calculation to keep A2WHP in the money

hot_rod

This program crunches all the numbers. Show me the money

mrhemi

ethicalpaul said:

Thanks @Jamie Hall ! The poster did say they were in southern Ontario so that is good.

I will just clarify one thing...land is not required for ground source heating (some questionably call geothermal). My well was vertical and was under my driveway. It could have been on a 1/3 acre lot.

Permitting and drilling in Ontario for this purpose is cost prohibitive. Been there, didn't do it!

mrhemi

dwatson3 said:

We're located in London, Ontario..... By the way, .... thank you for sharing your impressive skill-sets. I'm learning much and have now started considering a hybrid system. 👍

I live just down the road in Sarnia. Built my new home with a system similar to what you are contemplating, although no "Franken parts". Don't bother with an air to air heat pump, in time you will regret it. You don't mention A/C in your original post but I assume you have or will be installing it. A high temp hydronic coil air handler is your best bet. The one in my home has a 140-180 deg. F operating range. I also incorporates the A/C "A" coil.

By the way, in Ontario (and elsewhere), contrary to the "Green Energy Act" any electric power is "contaminated" with some carbon foot print.

archibald tuttle

hey guys. much as it pains me to say this, while you are right that optimized condensing furnance is going to be have service life closer to the 'stainless' burners on a gas grill, they aren't expensive, you can buy two or more for the price of a condensing boler, with a little sheet metal they pop right in where your old furnace is and the return temp for a condensing furnace is room temp. I turned on the first one I put in and it was dripping condensate with 15 seconds of start up, if you are focused on carbon (which I am not) all this hand wringing to get hydronic temps to condensing is a waste unless you need other aspects of hydronic and you simply are not going to get the operational efficiency associated with the theoretical maximum efficiency of condesnisng boilers. So hydronic has less issues with dry air, but of course they have legionaire spreaders, eer humidfiers. (that was a joke more than a caution, not too many people with humidifiers dropped off lately that i'm aware of) and it is a better flywheel and radiant source obviously so you can't get the comfort you could out of like a warm floor , but that is a whole different change than what the OP is talking about.

Without really affordable aftermaket and new install infloor or other low temp. emitter radiant tech, I wonder about the future of hydronic heat.

brian