Best Design for System with DHW and HX ?

[rickh1001]

I need advice on how best to design the plumbing for my new EKO 60 with 1000 gal of storage that I am getting ready to start the plumbing for. Here is the layout. I am going to heat two homes - one a 70 ft long converted barn, with two small hot air furnaces near each end and two gas HW heaters as well, both near the furnaces. The second home is smaller with a single furnace and HW heater, and will be supplied by a buried PEX line of about 50 ft in length. For now at least, I plan on installing 3 HX's, one in each oil burner plenum. I will design a primary/secondary layout, with 3 separate zones and circulating pumps. Eventually, I would like to install radiant baseboards and radiant floor heat where I can, but that is not in the cards this year. The trick is how to handle the DHW heaters.

The easiest way seems to be to install a side-arm HX for each hot water heater. I would run the supply through that first, then onto the larger plenum HX. With a by-pass valve and piping, I could shut off the oil burner plenum HX in the warm weather, yet keep the HW going. If I do this however, I will need to run the circulators continuously for the HW. If I do that, then in the winter, I would also need to circulate through each plenum HX continuously as well, and have the furnace fan (but not the burner) come on when the thermostat calls for heat. However, it seems I would lose a lot of heat since the HX would be continuously radiating 24/7. I suppose the heat would migrate up through the ductwork and not really be lost.

What is the best approach for a system like this? I could just make each HW heater its own zone, separating it out from the furnace HX's. Then I would have to set up 6 zones and pumps. The three serving the hot water heaters could be small, and run continuously. The furnace HX's could be used only in the winter. Would one normally have the thermostat start up a circulating pump only when each furnace called for heat, or would you normally just circulate continuously, then have the furnace fan come on when heat is needed?

Sorry for the long post, but without prior experience, I am not sure of the pitfalls of each approach.

 

[WoodNotOil]

Each DHW tank side arm should have it's own circ and be it's own zone. No circ should run continuously. Thermostat would control both the circ and the fan. Thermostat in DHW would control it's circ. Some of this could be done with less circs and zone valves. However circs aren't very expensive. Run a feed and return line to the other house and run two zones off of it (plenum hx & DHW). That would minimize the amount of pipe needed to be buried. Be sure you size the boiler large enough for both houses. I am no pro, but these are my thoughts on it. I hope it helps.

 

[Redox]

I'd have a big variable speed pump in there and put a 2 way zone valve at every HX, including the DHW. Have the pump wired so that a zone has to be calling for heat before the pump can start. They have zone valves with end switches for this or a relay across the valve will do the same thing. Taco was supposed to be coming out with a variable circulator, but you could buy a small 3 phase pump and a cheap speed drive and pull it off. I think you want to stay away from continuous circulation through a HW coil as the losses can be significant.

I sell/service speed drives, so I am biased...

Chris

 

[rickh1001]

Thanks WoodNotOil and Redox for the replies and info. I have question though. If we are using LP Gas HW heaters, how can I wire a circ up to its thermostat? It is just one of those little built-in ones at its base.

Also, if I create a separate zone for the house, which in turn will need to have either 2 circs or zone valves of some type for the HW and HX, would I have the circ feeding the underground PEX supply/return line on continuously, or have it only come on when there is a call for heat from either the HW or HX in the second house, and then simultaneously have one of the circs in that house come on as well? If so, I will need to lay suitable wire(s) in the trench when I lay the PEX lines to the house. It seems there would be significant lag in such a system? Also, the stagnant water in the PEX lines would become cold if there was no call for heat for long period, and that slug of cold water would have to flow through the HX's and back to the system. Is this the normal way to do this, or do OWB's with underground PEX lines run their main supply loof continuously?

Although not a pro by any means (not even an amatuer yet!), I used heat loss calculation software, the general chart that EKO supplies, as well as calculations based on last year's oil usage. They all converged on similar overall BTU requirements. I am getting an EKO 60 along with 1000 gal of storage. I am also going on an insulation kick and a general tightening up of the building, so I expect my heat demand to be significantly less than in previous years. The boiler is located in the refinished/insulated cellar of the barn, so any waste heat will be used to heat the cellar and migrate upstairs. It should be a cozy setup. I considered an OWB, or putting the EKO in a separate shed, but the thought of trudging out through deep snow in -20 deg weather late at night didn't appeal to me as much as sitting in front of the EKO on a cold winter night reading a magazine! Plus, being a barn, I have room for 7-8 cords of wood in the cellar right near the EKO.

 

[Bob Rohr]

Fluid in a typical hydronic system , properly designed, moves at 3-4 FPS (feet per second) flushing the cooler water on start up is not an issue to worry about. In winter months the system will be running quite often. On design day, a properly sized system would run non stop.

Constant circ is used in some "Euro" design systems that use TRVs on every emmiter or radiant floor systems. Real modern ones use pumps like the Wilo ECO or Grundfos Alphas that are both ecm motors and delta P (pressure) function. They change speed automatically and run down to a low 7w consumption at shut down mode.

Constant circ provides near perfect comfort with miminal pump power consumption.

in your case a bang-bang control, via end switched would be fine. You never want to run a typical wet rotor pump against a no flow condition (dead headed) No relation to Jerry Garcia

hr

 

[Redox]

I think you are going to want to replace those LP heaters with electric just because the standby losses on a gas HWH are so high. The thermostatic gas valve can't control an electrical circuit, but it would be a simple matter to cut a hole in the jacket and put a contact thermostat up against the tank for control. If you keep the LP heater, you could disconnect it from the gas and plug up the center flue with some insulation to cut your losses.

If you are going to the trouble of trenching up your yard for the PEX you are going to want to consider laying a piece of PVC conduit for controls. I think you would only need a single circuit in parallel with all the zone valves, but with conduit, it's easy to pull a new wire if you decided you need it later. 3/4 electrical PVC is fairly cheap stuff.

Yes, that slug of cold water can be a problem, but really only to the boiler. Primary/secondary piping minimizes this. I will defer to others here on the specifics...

I wish I had a barn...

Chris