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Flow diagramm for combined water and space heating
By mbeland, Section Solar Posted on Thu Apr 10th, 2008 at 01:57:39 PM MST
using Gary's homemade wooden storage tanks

Hi all,
In a previous post about absorber plates in flat plate collectors, Gary pointed to a link on his site
http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/Tank/Tank.htm
for a homemade wooden storage tank that seems very easy and cheap. Do I understand it right that the fact that the tanks are not pressurized has the consequence that water in the tank is not to be used in any piping but just for storing heat within the tank? That would mean that if I want to use such a tank for solar water and space heating (in NB, Canada), there would need to be three heat exchanger coils in the tank: one for circulating heat transfer fluid to and from the solar collectors, one to circulate water to and from the radiant floor tubing and one to circulate water to and from the back up electric water heater.
I suspect that the third heat exchanger coil would need to be pretty large because it would be used less continuously and would need to store some heat inside the tubing. Would this coil need a pump to circulate to the electric back up water tank or be used
Is this the way you think this type of tank could be used?
Anyone having a flow diagram using such a tank, could you please post it?
Thanks for your help
Martin
Eau, soleil, le vent

Flow diagramm for combined water and space heating | 19 comments (19 topical, 0 editorial)

Re: Flow diagramm for combined water and space hea (3.00 / 0) (#1)
by zap (bell47g5a at comcast dot net) on Thu Apr 10th, 2008 at 09:23:19 AM MST
(User Info)

Hey Martin... Gary's system has no heat exchangers in the tank, not that it couldn't be done. His collectors are a drain back system and the same water that's in the collectors is the same water stored in the tank and the same water that's pumped to the house. Have a look at his site for a simple flow diagram of the system here.

Using Gary's system, you would need to add a heat exchanger for DHW(domestic hot water) but the heat exchanger wouldn't need to be in the tank, it could be at point of use. You might be better off with a separate pump for DHW but with automated valves I would think one pump could handle both DHW and house heating under most circumstances.

zap

Re: Flow diagramm for combined water and space hea (3.00 / 0) (#2)
by mbeland on Thu Apr 10th, 2008 at 10:37:04 AM MST
(User Info)

Hi Zap,

Thanks for pointing me to Gary's flow diagram.

I already have the hydronic radiant heating system in place with a circulating pump and valves controlling flow to different (6) zones in the house. Thus I thought that for space heating I would just need to put a heat exchanger coil in series with existing pump.

If I understand you well, for water heating I could simply do as if the water heating loop would be an additional zone in the house thermostatted with a different set point. That seems interesting: saving on additional pump and having just one heat exchanger coil. The problems I see are the following:

- right now the plumber has installed a pressure limiter so that pressure in the radiant heat loops is lower than domestic water (that is probably required by code I assume to make it safer for leaks)
- right now, the water is not (or almost not) renewed in the radiant heating loops so there is no build up in the pipes (our water is hard). Tapping into the same circuit as domestic water would renew the water.

One idea for a progressive set up: Since the system would probably be sized for less than 100% heating and water heating, maybe I could concentrate on heating the house at first since it is the biggest part of my energy bill but with a set of valves, I could switch from heating the house to heating water during the summer or spring. Add additional coil and pump later so both could run all year long. Is it dumb or what?

What do you think?

Martin
Eau, soleil , le vent
Eau, soleil, le vent
[ Parent ]

Re: Flow diagramm for combined water and space hea (3.00 / 0) (#3)
by Gary D on Thu Apr 10th, 2008 at 11:00:15 AM MST
(User Info)

Tapping into the circuit through a heat exchanger for domestic hot water pre heat would be like an extra zone. It wouldn't matter what pressure you are running, not touching the household water supply (if this seems logical).

[ Parent ]

Re: Flow diagramm for combined water and space hea (3.00 / 0) (#4)
by mbeland on Thu Apr 10th, 2008 at 11:27:41 AM MST
(User Info)

Hi Gary,

I don't quite understand. Do you mean I could have heating zones running at a lower pressure than water heating loop without mixing the two? Do you mean by putting a heat exchanger coil in the domestic hot water tank? This what I would have liked to avoid and that your tanks I thought could allow to avoid.

Thanks or your help.

Martin
Eau, soleil, le vent
[ Parent ]

Re: Flow diagramm for combined water and space hea (3.00 / 0) (#6)
by Ungrounded Lightning Rod on Thu Apr 10th, 2008 at 06:14:59 PM MST
(User Info)

You can put a heat exchanger OUTSIDE the domestic tank.

Doesn't have to be a coil: Vertical pipe-in-pipe works. You can build that with copper pipe, Ts, and reducers - then wrap it with insulation. Hook one section to the tank at top and at/near the bottom (i.e. T off the sediment drain tap for the bottom if your tank doesn't have a tap low on the side). Pump the hydronic water in the top and out the bottom of the other part of the exchanger, to create an efficient counter-current setup. (Convection will drive the tank water in the bottom and out the top.) The tank circuit will be a convection loop rather than pumped, so use the part of the exchanger with the least resistance to water flow. (You'll probably want to make that the inside pipe so you don't lose part of the heat you've transferred into the tank part of the circuit by conduction to the air from the outside of the outer pipe. Make it big to minimize friction.)

Alternatively you could pull the insulation off your water tank temporarily and solder a coil of copper tubing to and around it before restoring the insulation. Again pump the hydronic water in the top and out the bottom to create an efficient counter-current heat exchanger (since the hot water will rise to the top of the tank).

[ Parent ]

Re: Flow diagramm for combined water and space hea (3.00 / 0) (#7)
by Ungrounded Lightning Rod on Thu Apr 10th, 2008 at 06:20:30 PM MST
(User Info)

The nice thing about pipe-in-pipe is that the central pipe runs straight through the Ts, reducers, etc. So unless something rots a hole in it you can be confident the liquid in the two circuits won't mix. The inner pipe can handle a pressure difference about as great as the pressure it can handle as a hunk of bare pipe. (And if the hydronic system has a lower pressure than the house water you'll still be OK even if you get a pinhole leak, since the water will flow from house to hydronics.)

Meanwhile any flaws in the fittings will cause a leak of the outer circuit water to the surrounding area, rather than between the two circuits.

[ Parent ]

Re: Flow diagramm for combined water and space hea (3.00 / 0) (#8)
by Lumberjack on Fri Apr 11th, 2008 at 05:40:51 PM MST
(User Info)

pipe in pipe is not legal in many areas.... it does not count as double walled.

[ Parent ]

Re: Flow diagramm for combined water and space hea (3.00 / 0) (#10)
by Ungrounded Lightning Rod on Mon Apr 14th, 2008 at 01:37:06 PM MST
(User Info)

Good point.

OK. Then solder a coil of copper tubing to a hunk of copper pipe, wrap it in insulation, and you're all set. Hydronic water through the coil top-down, convection loop between bottom and top of water heater through the inner (fat) straight pipe.

[ Parent ]

Re: Flow diagramm for combined water and space hea (3.00 / 0) (#11)
by Lumberjack on Thu Apr 17th, 2008 at 04:52:25 PM MST
(User Info)

One of the better schemes is to remove the insulation from a water tank and wrap it with copper tubing after which you replace the insulation....

[ Parent ]

Re: Flow diagram for combined (3.00 / 0) (#5)
by Lumberjack on Thu Apr 10th, 2008 at 06:01:57 PM MST
(User Info)

Ummm, No i think. None of the system is pressurised. Water is pumped from the tank to the collectors and gravity returns it. All that is required is the pump intake must stay submerged at all times.

The only exchanger required is a double walled one for the domestic hot water. all the remaining water is used directly.

If you really intend to space heat as well as domestic then you want to consider some form of dump load for the summer. Pool heating is a good way to go if you have the option.

I have a diagram but no way to post it....

Two manifolds - supply and return with a valved bypass between them for servicing.

water from the collectors is pumped backwards through the system from the return to the supply side.

each circuit has a separate pump and check valve.

Re: Flow diagramm for combined water and space hea (3.00 / 0) (#9)
by GaryGary (gary@BuildItSolar.com) on Mon Apr 14th, 2008 at 08:42:27 AM MST
(User Info) http://www.BuildItSolar.com

Hi Marten,

One thing to check on is the minimum temperature that your current boiler is allowed to operate at. For a lot of boilers, this can be rather high -- 140F+. This is bad for trying to work solar in because the solar collectors don't work as efficiently at these high temps.

We like the radiant floor solar heat more than our "backup" forced air heat, and we are in the process of trying to work out a good way to add a boiler style heat source to the existing solar system (kind of what you want to do, but from the opposite direction).

Right now, as Zap mentioned, my system circulates water directly from the storage tank to the collector loop and the floor loops -- no heat exchangers. I think that I may have to add a heat exchanger on the floor side to keep the boiler happy -- still not sure yet. I'd rather not, since the heat exchangers reduce efficiency and cost dollars, but may have to.

On the controls side, I talked to a local HVAC guy who is doing a solar/boiler system of his own, and the system he is thinking about works like this: at any given moment it would use EITHER the solar heat tank water OR the boiler heat source. Which heat source is used is controlled by an electrically actuated valve that basically connects the floor loops to the appropriate heat source. When there is enough heat in the solar storage tank it uses that source, if not, it uses the boiler.
I like his idea in that it allows you to use the solar heated water at lower temps and keeps the solar collectors in an area where they operate more efficiently. I think that under cold conditions where the solar heated water is not hot enough to keep up with the house heat loss, it could even switch between the two heat sources every once in a while to allow you to keep up with the house heat loss by using the boiler once in a while to "catch up".
Anyone see any downside to this approach?

Gary
Gary gary@BuildItSolar.com www.BuildItSolar.com

Re: Flow diagramm for combined water and space hea (3.00 / 0) (#12)
by mbeland on Sat May 3rd, 2008 at 08:39:38 PM MST
(User Info)

Hi Gary,

I'll check about the minimum temperature for the boiler. I would not think it would be a problem since it is made to heat th efloors at just enough above the set point to maintain the temperature in the house. My boiler has a sensor under the soffit that tells the boiler to fire up one two or three elements depending on the outside temperature.

I don't see the point in adding controls to have the system chose between the storage tank or the boiler. Why not simply run the piping in series, if there is not enough heat in the storage tank, the boiler turns on?

Martin
Eau, soleil, le vent
[ Parent ]

Re: Flow diagramm for combined water and space hea (3.00 / 0) (#13)
by GaryGary (gary@BuildItSolar.com) on Sun May 4th, 2008 at 09:05:17 PM MST
(User Info) http://www.BuildItSolar.com

Hi,
I'm still trying to work through this myself -- so if the following seems wrong, I'd like to hear about it.

lets say that you have a pipe coil heat exchanger in your solar storage tank with the output side running to the boiler inlet, then the boiler outlet goes to the floor loop, then the water goes through the floor loop and returns to the other side of the heat exchanger coil in the solar tank -- i.e. a series connection.

Suppose the solar tank is at 100F, and suppose that its cold enough outside to require 130F water running through the floor to heat the house. The water would come out of the solar storage tank at 100F, the boiler would boost it to 130F, the water will probably only lose about 10F going through the floor loop, so it will be returned to the solar tank at 120F. It seems to me that you will end adding heat to the solar tank rather than getting heat out of it. Probably not what you want to do.

This is fundamentally different than a solar water heating system, where all the water comes in at 50 or 60F and needs to get up to 120F -- then its fine to let the solar provide what it can, and boost up the rest of the way with a boiler or tankless heater. But, with a floor heating system, if you need 120F water in the floor to meet the heat load, its going to come back from the floor at around 110F -- this return water from the floor may already be hotter than the water in the solar storage tank.

There may also be limits imposed by the boiler manufacturer. Are they OK with circulating water through the boiler without the boiler adding any heat? It would be good to ask your boiler builder what they think if this. Tankless water heaters will handle this OK, but not sure about boilers.

If you do the other system where the floor loop feed is from either the boiler or the solar tank, then you can heat from solar whenever there is enough heat there to do it, and, if not, heat from the boiler without having the two interfere with each other?
I think you could even alternate between the two to make use of solar when its not hot enough to take the full heat load.

Or, I may be totally missing the boat??

Gary
Gary gary@BuildItSolar.com www.BuildItSolar.com
[ Parent ]

Re: Flow diagramm for combined water and space hea (3.00 / 0) (#14)
by mbeland on Mon May 5th, 2008 at 11:36:25 AM MST
(User Info)

Hi Gary,

Very interesting point. Everything seems right with your reasoning. The thoughts that arise to me are the following:

- If there is no limit imposed by the manufacturer, then you could simplify the valve configuration and simply have two valves controlling a bypass through the storage tank. Otherwise, you need 4 valves don't you??
- Your scheme would require some kind of optimization method to chose when to alternate between the two heat sources. Complicates things... I need to check the specifications and ask the manufacturer what are their proposed solutions.
- I need to check if my circulating pump is steel or brass because if made of steel it could be affected by corrosion due to oxygen renewal in the non pressurized tank.

Thanks for your contribution to my education about solar heaters.

Martin
Eau, soleil, le vent
[ Parent ]

Re: Flow diagramm for combined water and space hea (3.00 / 0) (#15)
by GaryGary (gary@BuildItSolar.com) on Tue May 6th, 2008 at 12:41:39 PM MST
(User Info) http://www.BuildItSolar.com

Hi Martin,

I put the sketch that our local HVAC guy Eric drew up for me here:

http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/AddBoiler.htm

It seems like a pretty simple way to go, and he does a lot of radiant floor systems (but no solar ones so far)-- so I think he knows the area well. This is what he is thinking about for his home when he adds solar.

Have not thought a lot about the control, but maybe a two stage thermostat could select solar for the first stage, and then switch to the boiler when the 2nd stage is needed.
Heat pumps often have two stage thermostats, so its not uncommon.
You would want to have some way to go right to stage two when the solar tank is too cool to provide any heat.

Right on the pump, you should use stainless or bronze. I use a Taco bronze pump -- they offer most of their pumps with a choice of case metal.

Gary
Gary gary@BuildItSolar.com www.BuildItSolar.com
[ Parent ]

Re: Flow diagramm for combined water and space hea (3.00 / 0) (#16)
by mbeland on Wed May 7th, 2008 at 06:54:41 AM MST
(User Info)

I don't quite understand. In this scheme, everything seems to be in series.

Martin
Eau, soleil, le vent
[ Parent ]

Re: Flow diagramm for combined water and space hea (3.00 / 0) (#17)
by GaryGary (gary@BuildItSolar.com) on Wed May 7th, 2008 at 08:44:25 AM MST
(User Info) http://www.BuildItSolar.com

Hi,
After looking at more closely, I think the label I added to part (1) to remind myself what it did is wrong -- its a pump, not valve.

Number (1) is just the regular pump in the boiler that circulates water, not an electrically controlled valve as the label says.

So, when you want the boiler to supply heat: energize the boiler pump(1), deenergize the solar tank pump(3). Boiler heated water flows from the boiler, through the floor loops (on left of diagram), and back to the boiler.

When you want heat from the solar tank: energize the solar pump (3), deengergize the boiler pump (1). Water then flows from the solar tank, through pump 3, out through the floor loops, and back to the solar tank via the top line.

Check valve (2) prevents the solar pump from pushing water through the boiler in the wrong direction when in boiler mode. The check built into pump (3) prevents the boiler from pushing water back through pump (3) and into the solar tank in the wrong direction when in solar mode.

Maybe its confusing that the floor loops are not completely shown? The four floor loop lines on the bottom left of the diagram go around and connect to the 4 lines coming into the top left of the diagram.

Gary
Gary gary@BuildItSolar.com www.BuildItSolar.com
[ Parent ]

Re: Flow diagramm for combined water and space hea (3.00 / 0) (#18)
by GaryGary (gary@BuildItSolar.com) on Wed May 7th, 2008 at 07:16:12 PM MST
(User Info) http://www.BuildItSolar.com

Oops!

This paragraph:
Check valve (2) prevents the solar pump from pushing water through the boiler in the wrong direction when in boiler mode. The check built into pump (3) prevents the boiler from pushing water back through pump (3) and into the solar tank in the wrong direction when in solar mode.

Should read like this:
Check valve (2) prevents the solar pump from pushing water through the boiler in the wrong direction when in SOLAR mode. The check built into pump (3) prevents the boiler from pushing water back through pump (3) and into the solar tank in the wrong direction when in BOILER mode.

Gary
Gary gary@BuildItSolar.com www.BuildItSolar.com
[ Parent ]

Re: Flow diagramm for combined water and space hea (3.00 / 0) (#19)
by mbeland on Fri May 9th, 2008 at 11:15:10 AM MST
(User Info)

I think I get it. It could be a nice way to do without valves. GREAT!

Thanks
Martin
Eau, soleil, le vent
[ Parent ]

Flow diagramm for combined water and space heating | 19 comments (19 topical, 0 editorial)

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