Help me understand pros/cons for my DHW options

[Jim K in PA]

I am absorbing as much as I can with respect to my setup options. I will be ordering my GARN this week. I am planning a number of details and setup options, but this is one I need some input from someone OTHER than the dealer selling me the unit and materials. I struggled with how to search for this subject, but gave up.

For this particular issue, we are just talking about the connection of my supply and return lines to my existing oil fired furnace. I have HWBB heat throughout the house. I have a DHW coil in the furnace. The GARN engineer suggested I eliminate the oil furnace altogether, as I have an alternate means of supply backup heat should we not be around to refire my GARN. However, if I eliminate the furnace unit and just connect my S/R lines directly to the S/R legs of my distribution manifolds, I need an alternate means of heating my DHW. The dealer is going to quote me a price for a Heat-flo indirect unit ((broken link removed)). This would run off another zone on the manifolds.

However, why would I want to do this? If I were to eliminate the oil from the furnace - pull the gun, remove the tank, and just leave the unit in place, I would be pumping the HW from the GARN through the furnace via the return line, keeping the DHW coil functioning just as it does now. Is there any substantial disadvantage to doing this? Obviously the dealer would like to sell more equipment, and I am sure the Heat-flo is a very good alternative, but for right now I would like to constrain the costs if possible.

Thanks for the input.

 

[WoodNotOil]

I think you are referring to an in line install which keeps the oil boiler hot at all times. This is often done in situations where the DHW comes from a coil. Mine is piped this way with a tarm and it works fine. There are those that argue there is efficiency lost by doing this, but in your case that might not be so bad (if it saves you money up front). Not sure if people do this with a garn usually or not. You could pipe it in such a way that with isolation valves you can direct the water through the boiler before zones or not at all. That way you could always add an indirect DHW later.

 

[Jim K in PA]

Thanks WNO. Yes, I am talking about a series installation as I understand it. I am not sure how you could isolate the flow, but I have no doubt there may be a way. Regardless, I don't think the efficiency loss would be much worse than with an indirect, other than the absorption of the heat exchanger. If/when I abandon the oil firing, I already planned to insulate the exchanger and obviously get rid of the flue (big heat sink). My EFM is a tube heat exchanger, so I could easily stuff closed cell into the tubes and blue board the ends.

 

[Eric Johnson]

I think that heating up the oil boiler vessel just to provide DHW through the coil would be a rather inefficient way to go. If you do that, be sure to plug up the chimney outlet and make sure the insulation is up to snuff on the old boiler.

Instead of doing that, I would simply install a sidearm heat exchanger on your existing (I would imagine) electric or gas water heater. The sidearm works just like another zone on your heating system, and the water is heated by circulating by gravity through the sidearm's inner tube. A sidearm will run you about $100 in materials if you build it yourself. Compare that to the cost of an indirect.

OTOH, since your zones are currently (presumably) supplied by the oil boiler, then why not just pump hot water from the Garn into the oil boiler and let it distribute the hot water into the zones? This would be the least expensive alternative all around, I think, and would give you a bit of "storage" in your basement. Under this scenario, leaving the coil and DHW setup like it is would make sense.

 

[Jim K in PA]

I think that heating up the oil boiler vessel just to provide DHW through the coil would be a rather inefficient way to go. If you do that, be sure to plug up the chimney outlet and make sure the insulation is up to snuff on the old boiler.

See my post above regarding the insulation. That is exactly what I plan to do when I deactivate the furnace.

Instead of doing that, I would simply install a sidearm heat exchanger on your existing (I would imagine) electric or gas water heater. The sidearm works just like another zone on your heating system, and the water is heated by circulating by gravity through the sidearm's inner tube. A sidearm will run you about $100 in materials if you build it yourself. Compare that to the cost of an indirect.

As I stated in my original post, I have a DHW coil in my furnace - no storage, no other water heater. I can certainly build a sidearm exchanger for a tank, but I would need to buy a tank and build the exchanger, and at that point I might as well step up to the indirect Heat-flo unit.

OTOH, since your zones are currently (presumably) supplied by the oil boiler, then why not just pump hot water from the Garn into the oil boiler and let it distribute the hot water into the zones? This would be the least expensive alternative all around, I think, and would give you a bit of "storage" in your basement. Under this scenario, leaving the coil and DHW setup like it is would make sense.

Correct, and that was precisely what I was getting at. The existing oil fired EFM furnace is realtively new (c.1997), in excellent condition, and is already piped for the 4 seperate zones of HWBB which I do not want/need/plan to change. This would be a primary/secondary setup, where the circ pump for the GARN loop in the garage would circulate the water to/from the furnace, and then the zone pump on the furnace would circulate the water within the zones as the t-stats called for heat, just as they do now.

I am going to tap the heads at GARN for guidance as far as pump selection and piping connections for the primary circuit.

I was just curious as to what the issues were other than some unkown difference in transfer efficiency beween the furnace coil and the indirect unit, and the potential losses into the heat exchanger if the furnace was not insulated and converted properly.

Thank for the input Eric.

 

[Eric Johnson]

That's how my system is piped, and it works great.

If you don't need a tank now with the coil in the oil boiler, why would you need one with the Garn? Same setup, just heated from a remote source.

BTW, I pipe the hot water from my EKO into the top of my gas boiler, and send the return water back from the return on the gas unit. That way the hottest water is always available to the zones.

 

[Jim K in PA]

That's how my system is piped, and it works great.

If you don't need a tank now with the coil in the oil boiler, why would you need one with the Garn? Same setup, just heated from a remote source.

Thanks Eric, that was the kind of feedback I was looking for. Piping questions aside, I could not see why it would be "necessary" to eliminate the furnace from the circuit. If it were old, leaky and ready for retirement, it would be a no-brainer to toss it and re-engineer the DHW solution. I will keep my unit, and if oil drops to $1.29/gal again then I can always convert back! LOL.

BTW, I pipe the hot water from my EKO into the top of my gas boiler, and send the return water back from the return on the gas unit. That way the hottest water is always available to the zones.

That is a piping question I have to work out. If you do that (output from GARN to the output side of the furnace), then you will have little flow into the furnace when the secondary (zone) pump is operating, especially of more than one or two zone valve are open. Piping the output of the GARN to the return side of the furnace circuit keeps the jacket full of hot water, keeping the DHW coil warm, and the hottest water will be going up and out of the output side to the zones anyway. I imagine there would be a couple of degree drop, but not enough to have a substantial effect. Of course, in practice I may be singing a whole different tune. Good thing I have a couple of months to get the piping figured out.

BTW, I already ordered Holoman's books on piping and pumping.

 

[Eric Johnson]

I used a separate 1" tapping on the top of the gas boiler vessel for the hot water input, so there's problem with flow, regardless of what's going on with the zone pumps. The return taps into the existing return manifold, which is about a 4" opening with plenty of capacity.

There's about a million different ways to pipe these things, and about as many different opinions on how best to do it.

 

[Jim K in PA]

I used a separate 1" tapping on the top of the gas boiler vessel for the hot water input, so there's problem with flow, regardless of what's going on with the zone pumps. The return taps into the existing return manifold, which is about a 4" opening with plenty of capacity.

Understood. I have a second, unused 1.5" return port on my furnace that I could use to connect the supply side of the GARN piping. Unfortunately I do not have a second outlet port, so I will T into the outlet pipe for the other (return) leg. Sizing of the primary pump is going to be somewhat critical, and the pump will go on the supply side of the GARN circuit so I do not create a pressure drop on the furnace outlet side pipe.

There's about a million different ways to pipe these things, and about as many different opinions on how best to do it.

When you have constraints in the eqiupment, you do what you have to do. I suspect not many people get to design a conversion where the entire existing system is scrapped. So, you wind up with compromises to accomplish the integration. I understand and accept that.

At least now I can cross the indirect water heater off the list when I get my quote from TWH.

 

[Nofossil]

The other reason for an indirect tank is so that you don't have to have a fire every day. By overheating it, I can have a short fire every three days during 'shoulder' seasons and have plenty of hot water out of my 40 gallon indirect tank. Indirect tanks are MUCH better at retaining heat than a typical boiler - 1/2 degree per hour loss in my case.

Heating the oil boiler with the Garn will certainly work, but there are plenty of benefits to indirect in the long run.

 

[Jim K in PA]

The other reason for an indirect tank is so that you don't have to have a fire every day. By overheating it, I can have a short fire every three days during 'shoulder' seasons and have plenty of hot water out of my 40 gallon indirect tank. Indirect tanks are MUCH better at retaining heat than a typical boiler - 1/2 degree per hour loss in my case.

Thanks for the insight, nofossil. Here is where the rest of my system become important relative to your point above about firing frequency. I intend to utilize the GARN tank to store heat collected from a couple of passive solar collectors on the roof. Again, this was a suggestion from the dealer and GARN (to utilize the storage capacity of the GARN for the solar collectors). I had been planning to utilize an 80 gallon electric tank with a sidearm with a smaller solar collector. I have not yet sized the collector I will build - probably 4, 36x72 panels with ~120' of 1" black poly coils in each one. Glycol in them utilizing a heat exchanger.

Heating the oil boiler with the Garn will certainly work, but there are plenty of benefits to indirect in the long run.

Please feel free to share some of the other long run benefits. As Eric pointed out, there are many ways to debark this tree, so fire away.

Thanks again.

 

[Nofossil]

Please feel free to share some of the other long run benefits. As Eric pointed out, there are many ways to debark this tree, so fire away.

Thanks again.

You asked.....

1) Low standby heat loss compared to other storage options.

2) Easily integrated with multi-source hydronic systems (wood/fossil/solar)

3) Can provide separate low-loss storage for a small volume of very hot water (more useful than a large volume of lukewarm water)

I've set mine up with a pair of mixing valves so I can overheat it. If I run it up to 160 degrees, I can get three days use out of it. I use it as a storage location for any spare really hot water - leftover oil boiler heat, the hottest water from the solar panels, etc.

Item 3 is really important. If you have a really large storage tank, it may be very difficult to get any part of it up to a useful temperature. A small high temperature tank is very helpful in that case. If my storage is down to 100 degrees, my solar panels can only raise it by about 10 degrees in a day, even though they're producing 170 degree water. If I can store some of that really hot water in the indirect tank, I have useful hot water even though the big storage tank is only at 110 degrees.

I'm currently using oil as my backup heat source. If needed, a 12 minute oil boiler cycle can provide nearly a day worth of hot water stored in the indirect tank.

I'm replacing my oil boiler with a tankless propane hot water heater, which will provide backup heat for my house. It can also provide a backup source for DHW by heating my indirect tank if the solar panels can't keep up. I wouldn't want to have it heat my big storage enough to provide useful DHW from there.

 

[Jim K in PA]

3) Can provide separate low-loss storage for a small volume of very hot water (more useful than a large volume of lukewarm water)

I've set mine up with a pair of mixing valves so I can overheat it. If I run it up to 160 degrees, I can get three days use out of it. I use it as a storage location for any spare really hot water - leftover oil boiler heat, the hottest water from the solar panels, etc.

3 days out of the 40 gallon storage tank? Amazing. Do you have teenagers . . . ?

Item 3 is really important. If you have a really large storage tank, it may be very difficult to get any part of it up to a useful temperature. A small high temperature tank is very helpful in that case. If my storage is down to 100 degrees, my solar panels can only raise it by about 10 degrees in a day, even though they're producing 170 degree water. If I can store some of that really hot water in the indirect tank, I have useful hot water even though the big storage tank is only at 110 degrees.

I agree, this is the key issue for my analysis. I have more calcs to do for the DHW Btu consumption for us on a daily/weekly basis. We use more DHW for clothes washing than people washing, and our dish washer is run 3-4 times/week. With respect to your main storage water temp, I think you are saying that you do not want to do a short burn in the stove, and the oil burner kicks in to make up for the low temp in the main storage tank by heating just the indirect storage tank. Did I get that right? If so, with the GARN I expect to fire it about once a week in shoulder seasons, but I hope to have enough solar collection to keep the storage tank in the 140-150 range during peak summer periods (late May to Sep).

I'm currently using oil as my backup heat source. If needed, a 12 minute oil boiler cycle can provide nearly a day worth of hot water stored in the indirect tank.

I am shooting for a complete removal of the oil fired side of the system, but utilizing the unit to avoid rebuilding my zone distribution and to keep my HW coil functional. I do have a propane backup system planned, but it is intended to be solely a backup to burning wood, not as a supplement to the system by design.

I'm replacing my oil boiler with a tankless propane hot water heater, which will provide backup heat for my house. It can also provide a backup source for DHW by heating my indirect tank if the solar panels can't keep up. I wouldn't want to have it heat my big storage enough to provide useful DHW from there.

I am with you 100% (per my above comment). I have a 150k Btu/hr propane heat exchanger that should be able to keep up with demand in an emergency. However, it sure would be wasteful, and costly, to burn that much propane to keep hot water around the DHW coil. Wood and/or solar will have that job.

You know, this is the great part of this process. Sharing the experiences of others is a real honor, and I appreciate your and Eric's input. I really have a much better appreciation for the system design approach, rather than just looking at things piece by piece. I will get my system designed and built as best I can, then I will evaluate and "adjust" it as I get real life feedback. I am just trying to get close to the green, because there is now way anyone can expect to drop the ball in the cup on the first shot (unless you are a lot luckier than I). BTW, I don't know why I used a gold metaphor, since I do not like nor play golf.

Thanks again.