Boiler to Forced air plumbing help/suggestions needed

I need some help! To start I'll let you know that I don't need separate zones nor do I need A/C and I'm not oversensitive to hot/cold spots in the house. I'm hooking my boiler up to my existing hot air furnace to avoid the expense of buying baseboards for 12 rooms and to avoid buying a separate air handler unit. At this point radiant floor heating isn't an option due to cost. 1st question: The ductwork on top of the existing furnace is 19"x20" with that exiting into 2 branches that which are 8"x16" each. Would it be better to install 8x16" water/air coils in each duct or use one central coil? 2nd question: the boiler has a TACO 007 circulator on it, I was thinking of wiring it so that the circulator runs continuously and the forced air duct fan also blows continuously at a low RPM via a larger pulley, gear reduction, or some type of resistor. As it is set up now the house cools off, the fan kicks on 1500CFM and makes a bunch of noise until the house heats up, then it shuts off. When it's VERY cold, say below 0 degrees f., it probably needs to be on high but for 95% of the time it doesn't. On/Off, Hot/cold, repeat. I think that a continuous 24/7 300-500CFM low speed would make the house nice and toasty without all the noise but I'm not sure why it wasn't hooked up like that originally. 3rd Idea: The other option I dreamed up is to get some big x huge industrial size cast Iron radiators and heat the basement to 90+ degrees and remove all the ductwork and just keep the floor grates and let the heat radiate upward naturally. I have 3 iron water tanks already in my basement that probably will hold 300 gallons total, they were used as a cistern originally but I was thinking of running boiler water through them.

I think the idea of a continously running low-speed fan is a good idea. One of my main dislikes for forced air heat is the scenario you described witht he wide swings in temperature and comfort.

You probably will be better off with one water-to-air heat exchanger. I don't think two separate ones will cost less unless they are a more common size than one large one. Plus the plumbing will be simpler.

With a constant running fan, you will have to control the temp in the house by adjusting the water temp in the exchanger. You can buy a mixing valve, either three way or a four-way valve, and I think there are electic-controlleved versions of these valves as well.

Or you can manually adjust the the water flow with a couple of ball valves. A good temperature guage or two in strategic places will make it much easier to adjust for the weather conditions.

Can you get by with manual adjustment or will you need electronic control?

You can electronicly control the Taco pump, but even with a constant running fan, you will get hot and cool peaks unless you have a constant amount of heat in the exchanger. At least I'm pretty sure you would have a heat spike until a thermostat is satified.

I'd search Ebay for 'outdoor wood boiler' and see who the sellers of heat exchangers are. I may still have some links to various sellers, I'll have to look.

Greg L

Greg's stuff is all good. I would add, get yourself a set of stepped pulleys for your current blower and motor. This will allow you to fine tune your fan speed to suit your needs to a T.

Someone correct me if I'm wrong here.
Disconnect (I have a three way switch between my two boilers) the thermostat from your current furnace and connect it to your boilers aquastat. This will run the circulating pump off the aquastat on a call for heat. The blower on your existing unit can be run with the pump or continuously as described. The boiler will build a little temperature between heat calls and the house will recover quickly, lose heat as it runs and taper off before shutting down, making it a little more comfortable. I would think a bypass at the heat exchanger could be used to regulate it's temperature in the hood.

Here's a link to the McQuay brand of booster coils. http://www.mcquay.com/mcquaybiz/literat ... 2-6biz.pdf
There is a lot of design information in the catalog. Look at the data for the coils that would fit your plenum and/or duct(s). If a single coil would having enough heating capacity that would be the least expensive way to go. You do not have to use the McQuay brand, any coil of similar size will have similar engineering data. The additional resistance of the blower coil will reduce the available air flow. This will work the blower motor harder and depending on how marginal the current blower is may require an upgrade. Be sure all the supply air actually flows through the water heated coil by blocking off any air flow paths around the coil. There are any number of ways to control the boiler water circulation pump and blower. I'd get it working first with simple manual control switches first before doing anything special.

Thanks for the info and links. Using the calculations I have determined that my existing furnace, while rated at 144k BTU's can only supply about 100k BTU's. The two openings coming off the plenum are 8"x16" and with a 1500cfm blower it can't, according to engineering charts, supply 144k btu's. A 200k BTU coil is huge. I did some research on outdoor wood boilers and most people run them into a standard 16x16" heat exchanger which, thoretically, negates the need for a huge outdoor boiler as it can only supply about 90k BTU's. There is something here that I am missing because my house was warm last year even in the coldest temperatures as long as I was willing to burn the propane, the duct size wasn't the limiting factor; my wallet was. According to my calculations using internet programs my house should require ~160k BTU's to warm it. So back to square one with the alternatives. Using radiant floor heating guidelines at 40 BTU's per foot of underfloor pipe I would need about 4,000 feet of pex pipe and 4,000 feet of insulation to cover the pipes plus the manifold. What if I ran 1 1/4" steel pipes under the floors instead of pex pipe and didn't use any insulation around the pipes? That would heat the basement and the floors and I can get all the steel pipe I need for almost free. For the second floor I would install either cast iron radiators (also almost free) or 3/4" copper baseboard radiators (which aren't free but easier to handle). A plumbing parts installer I know says the easiest solution is to just install 3/4" baseboards in every room, connected in series so they form one big loop. blah. What it is boiling down to (pun intended) is I can't see the sense of installing a $600 water-to-air coil if it isn't going to work. I have plenty of time to do plumbing and I do enjoy doing the work this time of year so labor isn't a factor. What I've also learned is that whenever there is a conversion there is an efficiency loss. Just using a water/air heat exchanger will lose energy.

The burner is probably rated at 144 BTUs, that would be input. If so your unit probably has an output rating of around 110-120K BTU output rating. Something is wrong with your calculations if they show you need 160K as you are heating your house now with a little over 100K BTUs. I would doubt your need for a coil bigger than 150K. This will work, I would stick with the plan as the alternatives are very pricey and involved. Just keep looking at it and asking questions and you will be fine. The devil is in the details.

e.alleg wrote: What it is boiling down to (pun intended) is I can't see the sense of installing a $600 water-to-air coil if it isn't going to work.

I think you can buy a coil for way less than that. I just purchased a 19 x20 inch coil for about $200 delivered.

Check here: http://stores.ebay.com/KJ-Motorsports-dotcom_Forced-Air-Plenum-Coils_W0QQcolZ4QQdirZ1QQfsubZ4221097QQftidZ2QQtZkm

e.alleg wrote:What I've also learned is that whenever there is a conversion there is an efficiency loss. Just using a water/air heat exchanger will lose energy.

What I have learned is heating your house with a coal boiler warms up your basement which warms up your floors which makes the rooms temperature more comfortable which makes you happy because your saving so much money you don't care how warm you are. The energy loses between the boilers water jacket and the coil in your plenum simply warm your basement.

The conversion here is coal into happiness, so efficiency doesn't enter into the equation.

A double row coil 19" X 20" X 3.5" thick will give you 160K BTUs.

Do you take off the boiler's insulation and covers for the winter and reinstall it for the summer months when using the boiler just for domestic hot water? Bob; thanks for the link! I'm going to buy a coil for my ductwork and see how it works, at $180 this is by far the cheapest and simplest way. Ebay is great. I just bought a lot of 14 brass gate valves, all brand new name brands for $27 total including the shipping. Sizes range from 1/2" to 1-1/2" I couldn't pass it up.

e.alleg wrote:Do you take off the boiler's insulation and covers for the winter and reinstall it for the summer months when using the boiler just for domestic hot water?

You have an EFM, right? I wouldn't worry about the insulation on the boiler just leave it. The boiler will give off a little heat anyway even with it. I would not insulate the loop from the boiler to the coil either. Depending on your basement, it may warm a little or a lot. If it isn't enough you can run a length of baseboard radiator in the boiler to coil loop to warm it more if you want. I would just set it up the way we have discussed and if you want it warmer down there add a length of finned copper to the loop later if needed.

e.alleg wrote:Bob; thanks for the link! I'm going to buy a coil for my ductwork and see how it works, at $180 this is by far the cheapest and simplest way.

Not to put to much cold water on your furnace retrofit project but there is a reason they are called booster coils. As your design analysis showed it takes a hefty multiple water loop coil to get all the needed BTU's into the furnace duct. The referenced Ebay coils are single pipe designs, the 1" pipe will not flow sufficient water to transfer the needed BTU. Note the Ebay ad says nothing about the thickness of the coil only its area. There will be a big mismatch between the EFM's BTU production capacity and the ability to get it into the air stream. At a minimum you will need bigger pumps. A bigger air pump (furnace blower) and a bigger water pump (Taco 007 is to small). You are trying to get heat transfer and you need lots of flow. The furnace blower was sized for the gas furnace heat exchanger. It's air flow resistance is much less than a blower coil. You are putting them in series. Much more resistance. The Taco 007 is a circulator pump. Nice slow, usually quiet water flow. You need lots of water flow. Flow that's restricted my the small, 1 inch inlet pipe. Much two small to flow the BTU's your EFM can produce. Compare the supply tapping on your EFM to 1" and you will see what I mean.

You need an appropriately sized air handler ducted into your existing furnace or radiation units (baseboard, radiators, etc.) located in each room. If all you want is a booster coil to reduce the propane use it might work. But as you have outlined your design it will be marginal at best. My engineering opinion.

Yanche wrote:Not to put to much cold water on your furnace retrofit project but there is a reason they are called booster coils. As your design analysis showed it takes a hefty multiple water loop coil to get all the needed BTU's into the furnace duct. The referenced Ebay coils are single pipe designs, the 1" pipe will not flow sufficient water to transfer the needed BTU.

I guess I come to a slightly different conclusion:

First the coil in the link I posted is three loops deep (see picture in the link).

If the current furnace NET OUTPUT is 100,000 BTU per hour that can be achieved with a water flow of 10.4 gallons per minute assuming a 20 degree temperature drop through the coil.

10 gallons per minute through 1" pipe is not unreasonable but will likely require a larger circulator pump than a Taco 007.

However, if you are considering purchase of a coil from the link I posted you should seek technical data from the seller--he offers design and planning assistance.