Converting hot air to water/hot air...?

Ted_5

call the rep in your area RST Inc. 781-320-9910

Ted

Holly Gates

What are the options?

Hello, I am wondering if there are any reasonable options for converting a forced hot air furnace to work off a boiler. Why, you might ask? Here is the situation:

I have a two family house, one apt on gravity hot water and an ancient boiler, one apt on an 80% forced hot air furnace about 13 yrs old. Next summer I want to repipe and replace the gravity system, and am currently thinking of using a Vitodens 300 (if I can find a place to buy one). That leaves the apartment with the forced hot air to deal with.

Because of basement remodel plans, it would be really convenient if I could eventually convert everything to side venting, and I like the idea of high efficiency, so 90%+ condensing equipment is attractive. I know I still have maybe 10 years left on the furnace, but replacing it might be worth it to me for the venting options and the efficiency.

I also want to add a few more zones to the hot air system, which will be annoying to run ducting for.

It seems like the ideal situation would be if I could find a big water/air heat exchanger which would allow me to replace my 80% furnace with another Vitodens 300. This would be nice since then I could also get rid of my other water heater (which is 80%, so has to vent to the chimney).

The furnace is rated at 115K BTU. I have not yet done calculations on what the actual heating load is. I will before actually doing anything, but I'm still in the brainstorming phase. Based on how much the furnace runs now, I suspect I could go with a 100K or a bit less for the new equipment, especially if it was 90%+.

So anyway, I thought maybe some of the pros that hang out here would have opinions on the feasability of running a central hot air system from a boiler.

Thanks for the advice,
-Holly Gates

flange

you could do this very easily, there are hydroair units out there already built to operate on hot water from your boiler. this could be one zone of your system. OR you could utilize a standard air handler with a very inexpensive hot water coil, sized to carry the load, with a pressure drop of around .1". most ahu's today will work in this app, you will need a little wiring experience and a couple of relays, but its preety simple.

Ted_5

Vitodens 200 is a good choice

for the boiler. Do you think you need 2? The largest Vito
is 230,000 btu and has a control that can do multible temps.
Add a reheat coil to your duct or coils to each zone. The
Vitodens has a max High of 167F so the coil has to be designed for that temp. About what area are you in? I hope
this helps.

Ted

Holly Gates

more info

I definitely need two heating systems because the utilities need to be separate for the two apts. One thought was that it would be sort of cool if both heating systems used the same boiler.

Sounds like there are components available to do what I am thinking of, which is great!

I am in Somerville, MA, which is across the river from Boston. Coldest temp in the winter is maybe -10F. The house is circa 1890 with no insulation except a bit in the attic. I've got spreadsheets for the heat loss for the downstairs apartment, which will be the first to get its system replaced, possible next summer. Then maybe the summer after that I will tackle the upstairs apartment system, which has the hot air.

I am an electrical engineer with experience building power plants and wiring houses, so relays and system design are no problem. I've also done quite a bit of plumbing, as well as maintaining and replacing circulators, etc. of a large circulated hot water system. Also done some hot water heaters, etc. Haven't done much duct work, but I am excited to try it out. I've already got a plasma cutter

Working mainly off stuff from this textbook called "Modern Hydronic Heating" and my college thermo, I am working out the heating loads of the building. Then at least for the downstairs I want to calculate how much radiation I've got with the current (gravity) radiators. I'm still not sure what the best layout is for the two systems; I really like the idea of TRVs on a two pipe reverse return parallel setup with a variable speed pump that responds to pressure differential across the radiators. Doesn't seem like the controls to do that are incredibly easy to setup, so maybe I'll have to do something else. Anyone know where I can get a copy of a document with recommended system designs for the Vitodens? I couldn't seem to find it on their website...

Thanks for the advice everyone,
-Holly