Convert forced air to hot water heating?

kdo

I'm thinking about converting from forced air to hot water heating and I
wonder if this is a crazy idea.

Here's our situation:

6000 square foot home on 3 floors, built 1903, in eastern Massachusetts.
We've put a lot of money and effort into insulation and weather sealing,
but still it is an old, leaky house.

We currently have two separate forced air systems, one for the ground
floor and one for the two upper floors. The ground floor is heated by a
Lennox 100kBTU/hr gas furnace. This is insufficient to our needs. We
turn down the first floor thermostat to 50F at night when we're not
using it. On a cold winter day (say low 0F, high 20F) it will take
maybe 6 hours to get back up to the daytime setpoint of 65F.

For upstairs we have a Lochinvar 150kBTU/hr high-efficiency gas boiler
which provides DHW through a storage tank with heat exchanger and
heating through a hydronic air handler. We have separate
air-conditioning units for the two floors with outside compressors and
heat exchangers in the furnace and air handler.

It's nice to have A/C available, but we don't use it much. We use the
upstairs unit maybe 15 days per year and the downstairs unit maybe 5
days.

We often have quite a lot of our upstairs closed off, so we don't need
to heat or cool it. This works poorly because shutting off vents is not
very effective. Thus my interest in hot water heat, which would be much
easier to control. Presumably it would also not dry our bedroom air so
much in winter. The downstairs is open plan so there is no issue of
closing off rooms there.

The only equipment that is in good shape is the boiler and the DHW
tank. Everything else is old and needs replacement. The downstairs
A/C is not working. The downstairs furnace heat exchanger has a small
rust hole. The upstairs A/C works (but it's 35 years old) except for
the system that collects condensate water, which instead drips all
over everything else. So the hydronic heat exchanger is badly rusted.

This seems like a good time to think carefully about what we want. My
thought was to run the hot water from the boiler around the upper
floors for heating, and use the upstairs ductwork only for cooling.
I'm imagining either individual thermostats to control hot water in
each room or at least manual valves to shut off the rooms we don't
need heated. But maybe this would be insanely expensive.

For downstairs heat, I guess we just need a new gas furnace. But
I wonder whether there's some way to have a single air-conditioning
system, or at least a single compressor, that could be used for both
upstairs and downstairs. We don't use A/C all that much and we
certainly don't need to cool upstairs and downstairs simultaneously.

This is my first post to the Heating Help forum. It looks like a great
resource. Thanks so much for your help.

Ken

Jamie Hall

A few thoughts here, and I'm sure others will have many others -- and probably better.

First, on hot water vs. hot air vs. what you have. As it happens, I'm working just now on a similar situation in a church I care for. Forced air heat, separate air conditioner, etc. After a good bit of head scratching -- and some "interesting" vestry meetings, the option which penciled out as the most flexible and best was to keep the forced air ducting and all, but split it into two real zones, and provide the heat through heating coils in new airhandlers. The heating coils will be powered by two wall hung boilers, sized to meet the load. Which is what I would suggest -- no hassles or mess in the living spaces, much smaller units and much more efficiency, and better flexibility.

Now on the heating load. You want to be able to match the heat loss of the building to the heating capacity of your air handlers and boilers. The close you can come on a design day, the better overall the system will be. That means you really need to do a room by room heat loss on the building before you begin. There are on-line calculators to do this (Slant/Fin has a good one) but any competent professional can also do it.

You mention it takes 5 hours for the system to come back from a 15 degree setback on a cold day. I'm not a bit surprised. It doesn't mean that your heating system is undersized. It does mean that that much setback is way overkill for a daily setback (it's OK for a vacation). If your system were sized to recover more quickly, it would be way oversize for normal use, and your efficiency would suffer greatly. The fact that it can recover from that at all indicates that it is about the right size, or possibly 20 to 30 percent bigger than need be. I suggest -- even with hot air -- no more than 5 degrees.

JakeCK

Adding to what Jamie said about too large of a setback, you have to remember when you're recovering from that you're not just heating the air in the house but also the thermal mass of the house. Which in this case is a lot if only because of its age. Older houses had denser old growth lumber and heavy think plaster walls and typically thicker floors. Thats a lot of mass to reheat. And the house won't be comfortable until all those surfaces are emitting just as much thermal energy as they are absorbing.

pecmsg

You say the air handlers have to be replaced anyway so have heat pumps installed with slightly larger HW coils and a resettable circulator like the Veridian!

Agreed 15 - 20°f is way too much!

Steamhead

If you're wiling to spend the money for comfort, hot-water or steam with radiators, baseboard or other non-forced-air distribution is the way to go. As previously mentioned, you can use your ductwork just for A/C.

Jamie is right about the heat loss calculation. That's the only logical starting point.

Jon_blaney

What is your budget? You can do anything if you have the $$$.

If it were me, I would go with a central boiler and two air-handlers. Maybe the boiler you have would heat the whole house now that you have insulated and air sealed. Keep the indirect HW tank.

Think about using window a/c. They are lighter and more attractive than the were years ago. You can add a/c to the air-handlers latter if you really need them. Makes zoning easy. Put one or two were you want it cool. I use them and it takes just a couple of hours to put them in and take them out.

Set the thermostat at a comfortable temperature and leave it alone. Setbacks are not worth it and just make you uncomfortable. Make sure the thermostats are located where the temperature is most important.

You have to know your heat loss.

kdo

Thanks for all the comments. But there are some things I don't understand.

I can certainly do the heat loss calculation, though some guesswork would be required for insulation and infiltration figures. But why is this the right thing to do? Wouldn't it be better to use the historical information of how much gas we used per degree day? Then we'd be working with our actual house, rather than a theoretical model of it.

Why would a larger system be less efficient? Our current boiler is the opposite of this. If you want only a little heat it heats the water to a low temperature, thus extracting the most energy from the combustion gases.

I don't understand why setbacks are not useful. I figure that the colder the house is the less heat loss there is and so the less energy I need to use compensating for that loss.

@pecmsg suggested heat pumps. It would be nice to reduce our fossil fuel consumption. But I guess in this climate it would be prohibitive to heat entirely with heat pumps. Could we use a heat pump in combination with the existing boiler? I don't really understand how this would work. Heating/cooling coils from the heat pumps plus heating coils from the boiler in each air handler? I guess we'd have to give up the idea of baseboard hot water, unless there are heat pumps that can heat water in winter and cool air in summer.

Sorry for all the naive questions. Thanks again for your help.

Steamhead

Some answers:

1- Gas usage not only depends on the building's heat loss but also on the system's efficiency. Forced-air, with its leaky ductwork, is less efficient than a water-based system would be. You could, of course, use your consumption figures (and run-time figures if you have them) as a comparison, but a good system design will always start with the heat loss.

Another thing a heat loss is good for is projecting the benefits of more insulation, better windows etc. If your present system pre-dates when you "put a lot of money and effort into insulation and weather sealing", it's less efficient than it should be, since you have to heat up all that extra metal before the heat gets to the rooms.

2- Setbacks are only useful if they save more energy than it takes to heat the building back up. This is often true of a small overnight setback, no more than five degrees, assuming the system responds quickly in the morning. But it takes more energy to recover from a deeper setback, so this would only be useful if you're away from the house for, say, a week or more.

pecmsg

Heat pumps with gas back up gives the best of both worlds. Efficiency of a heat pump from 70* down to let’s say 20*.

pecmsg

Look up Duel Fuel or Hybrids

Brewbeer

@kdo I converted my (formerly) forced air home to hot water baseboard. A room-by-room heat loss calculation is necessary to figure out much radiation each room needs. If you click the link in my sig, you will get the thread where I posted a spreadsheet heat loss calc for my house, which was the basis for selecting, and sizing, the radiation.

Steamhead

@Brewbeer , how much fuel did you save?

Brewbeer

I didn't see a significant difference between how much fuel we used when we had the furnace and water heater, compared to the mod-con with indirect. The old furnace (installed mid 1980s) was vented through a 2-inch PVC pipe, so I'm assuming that is a 90%+ efficient furnace, but it was grossly oversized by at least a factor of 4.

The big difference was the immediate improved level of comfort in the house; the bottom floor has an open stairwell and was always cold unless the furnace was running. Since the mod-con is always running at the "correct" level of output, the heat in every room is always at set point and doesn't vary like it did with the furnace (on warm, off cold, on warm, off cold, 4 times per hour). I also eliminated the set back I had been using with the furnace, so instead of a 69 day/64 night swing, the house is a constant 69.

Voyager

It isn’t crazy if you want hydronic heat. However, moving from forced air to hydronic isn’t going to fix a leaky house.

I am probably the only one who participates in this group who has forced air and likes it. The main reason is that I have a well-designed forced air system and, unfortunately, most FA systems are not well designed or well installed.

If you go with hydronic, forget about doing nightly setbacks. If you think your forced air takes a long time to recover, you will really not want to see hydronic recovery times. The forced air system in my home will recover from a 6 degree setback in 20 minutes or so depending on OAT. If it is below zero out, then it obviously takes longer. I would not recommend setbacks much more than 6-8 degrees overnight as, as has been mentioned, you need an oversized system to recover from that on a very cold night. I suspect my system is slightly oversized, but forced air furnaces are still pretty efficient even if oversized as they have very little internal mass to heat and so they don’t have the short cycling issues that can be trouble with boilers. My FA furnace actually runs on very short cycles by design. I think it fires for 3-4 minutes 6 or so times an hour in cold weather. I did not design the system, but it has worked very well for nearly 20 years now.

I would let your pocketbook and preference drive the decision. If you really want hydronic, then you will always be unhappy if you stay with FA. If all you want is a comfortable and efficient heating system, then a new forced air system that is properly designed and balanced will be very efficient and comfortable and will recover from setbacks more quickly and almost certainly cost less money unless your current ductwork is a complete disaster. So, it all depends on what you prefer. A capable HVAC designer can design a good system with any technology and make it pretty darn comfortable, but each technology has its pros and cons and it all depends on what you like.

bob eck

Get a heat loss done on your house.
See how many heating BTU is needed for each floor.
With a condensing gas boiler you want to run as low as possible water temps going our of the boiler as possible so you can actually get the boiler to condense even when near design day outside temperature.
Take a look at zoning each floor by themselves.
Take a look at using a copper baseboard like heating Edge HE2 or HE3. You can run low water temps through this copper baseboard and still heat the house if you have enough wall space for the correct amount of each room.
The Ecostyle wall steel panel radiators would be a great choice for this type of job. Sized correctly you should be able to run low water temps to keep your boiler in condensing mode most of the heating season. You can put thermostatic radiator valves on each Ecostyle steel wall panel radiator and you could turn the temps down a little (not 15 degrees) in the rooms you do not normally use.
Check the Ecostyle steel wall panel radiators at
www.ecostyle.us
Click on Ecostyle CV Radiator Literature.
Have your heating professional call the local F W Webb company in your area for pricing on these steel wall panel radiators.
This company also offers Ecostyle wall towel bar heaters.
Good luck.

ChrisJ

What ever you decide, please do not go with window units.

They may be lighter than ever, not sure on that to be honest. But they're also built worse than ever. Not only are they still terribly inefficient, they let bugs in due to all of the gaps inside and they fit the window bad. I had units years ago that fit pretty good but the recent ones needed foam stuck all over the place.

I switched from cooling a few rooms with window units to cooling the entire house with a 16 seer system with duct work in the attic and my electric bill dropped. That's pretty sad considering all of the circumstances.

Window units are great if they're your only option but they aren't better than the alternatives in any way other than Initial price.


@Voyager your recovery time is true but also not the entire story. While your system gets the air temperature up in 20 minutes everything in the house is still cold. Radiant systems handle this far better. They're slower but everything warms up, not just the air. I bet it takes hours for everything in your house to stabilize at the warmer temperatures.



Quick setbacks and recoveries is what the forced air community always falls back on. My steam system can do a very very fast recovery and it does it well without overshoots thanks to the Ecosteam. I have 104,000 btu output in a house that only needs 74,000 when it's -10 out. And I don't have much water mass. Steam transfers heat faster than almost anything else.

However I still don't do a setback because I have no reason to. My bedrooms stay 64 all night and day and the bathrooms, living room etc stay 72. TRVs are wonderful and they work with hot water as well. Of course, hot water also has wonderful zoning options as well.

Voyager

Yes, the air definitely warms up much faster than the massive objects. However, it does not take hours to get things stable. Maybe an hour, unless it is really cold (below 0), then maybe 1.5 to 2. Even in northern PA, many winter nights are in the upper teens and 20s and with mild setbacks of 6-8 degrees, the recovery is pretty good.

And if the sun comes out and the temp shoots quickly to 40, I don’t get overshoot which can happen if all of the massive objects are warm and the solar heat gain picks up quickly. Overall, my forced air system is excellent for my house and my needs, but may not be for someone else who has different needs.

As I have often said, every system has its pros and cons. There is no one perfect system. However, if you want AC also, a well-designed FA is pretty darn close. If you only need heat, then there are a lot more options that make sense. If is unfortunate that most here have not experienced a well designed FA system. The difference between that and a typical spec-home or mobile home FA system is night and day.

ChrisJ

@Voyager I believe most on here understand a well engineered forced air system will work very well.

The problem is a well engineered forced air system is far from cheap and it will still lack some efficiency that warm water can provide and it will almost always be felt to some extent due to no mass.

Unfortunately no matter what you do, where a good radiant / baseboard / cast iron system blends in and is transparent....... even the best forced air system is still Marcus Brody in the Last Crusade.

Voyager

We will have to disagree on that. I am frequently in three buildings heated with forced air, HWBB and in-slab hydronic. For heating alone, I would probably take the in-slab although the overshoot is annoying on days when it warms up quickly after a cold night. The least comfortable by far is the HWBB with separate mini-splits for AC. Overall, including AC, the FA is the most comfortable.

I have a fair bit of experience traveling in Europe so have experienced a fair number of panel rads of various styles and they aren’t bad, but again you need separate cooling or just don’t have it at all as in much of the UK and Europe.

And the efficiency difference in modern systems is nearly immeasurable as a few head to head studies have shown.

ChrisJ

Voyager said:

We will have to disagree on that. I am frequently in three buildings heated with forced air, HWBB and in-slab hydronic. For heating alone, I would probably take the in-slab although the overshoot is annoying on days when it warms up quickly after a cold night. The least comfortable by far is the HWBB with separate mini-splits for AC. Overall, including AC, the FA is the most comfortable.

I have a fair bit of experience traveling in Europe so have experienced a fair number of panel rads of various styles and they aren’t bad, but again you need separate cooling or just don’t have it at all as in much of the UK and Europe.

And the efficiency difference in modern systems is nearly immeasurable as a few head to head studies have shown.

You can disagree, you're certainly allowed to your opinion even if it's wrong.

kdo

Hi, everyone. Thanks for all the comments. We did have some installers come out and we asked about conversion to hot water, but it was prohibitively expensive. Part of our problem is that the boiler is in the basement and it's the second and third floors where we wanted to have hot water heat, making for a lot of plumbing. I suppose running the ducts originally was even more complicated, but that was done long before we lived here.

We are planning to get heat pumps with hydronic coils in the air handlers for times when it's too cold or we need more heat than the heat pumps can supply. I have some questions about that, but I'll ask them in a separate post in the heat pump section.