Open loop system: how is the solar heated water controlled into the furnace

Reinier · September 2012

While waiting for the replacement of my broken differential controller, I am trying to get a better understanding of my system. I have not yet seen the system working properly, so what I am writing here are educated guesses. See this thread for some pictures and discussion.

This is an open loop system , used to heat the tapwater in the house. When hot water is requested in the house, it should come from the solar heated hot water tank first, if its contents are warm enough. Looking at the different pipes that go in and out of the furnace and the solar system, I can not understand how this process of drawing hot water from the solar tank into the furnace is regulated. I do not see any thermostats or other components that might be responsible for this.

Does anybody have any experience with this kind of system? Which part of the system is responsible for the hot water flow from the solar tank to the furnace? My guess is that this happens somewhere inside the furnace.

I have made a schematic drawing to illustrate the situation.
Thanks in advance,
Reinier

Attachment not found.

peakbagger · September 2012

Re: Open loop system: how is the solar heated water controlled into the furnace

Looking at the schematic, the round circle is a thermostatic mixing valve, it mixes a small amount of cold water into the hot to keep the temp below 140 F (or whatever setpoint you want). if you open the hot water in the house, the majority of the cold water is sent through the SHW tank, then the furnace prior to going to the mixing valve. I am not a fan of this type of piping arrangement during the summer as SHW should be able to supply all the hot water needs and running hot water through the boiler heats up the boiler (and cols the hot water)which causes heat to vent up through the boiler stack. I have a bypass around the boiler for late Spring summer and fall. In the winter I have it piped like your design. I expect with some logic it could be automated to switch in shoulder seasonsbut I have a PV powered system and wouldnt want to add the phamtom load for a controller. I dont have my backup electric element wired on my SHW tank but given the price difference between heating oil and electricity, it would be cheaper to use electricity as backup (expect that in the winter I burn wood in a wood boiler and I have it set up to heat my hot water.

Reinier · September 2012

Re: Open loop system: how is the solar heated water controlled into the furnace

Thanks peakbagger, that answers many of my questions. One thing I do not understand though:

if you open the hot water in the house, the majority of the cold water is sent through the SHW tank, then the furnace prior to going to the mixing valve.

Looking at my picture, the horizontal line at the top right annotated with "cold from well" provides the cold water. To me, it looks more likely that the water flows straight into the furnace than in a 90 degree turn towards the SHW tank. Could you explain what mechanism makes the majority of the cold water go through the SHW tank as opposed to straight into the furnace?
Reinier

BB. · September 2012

Re: Open loop system: how is the solar heated water controlled into the furnace

I am guessing--But your current design, more or less, is a pre-heat for the (oil?) furnance hot water boiler)?

If so--Then it requires your boiler to be "fired" / operational 12 months of the year.

If you install a boiler bypass (direct to solar hot water tank), then you could (in theory) shut down the boiler for ~6 months of the year and still have domestic hot water.

Do you need the boiler fired 12 months of the year?

Have you tried "manual" operation of the pump yet (say turn on at 10am and turn off at 3pm during a sunny day) and see how much hot water/temperature rise you can get in your storage tank yet?

Have you reviewed how to shutdown/drain the system when a hard freeze is due (I am sure you know more about this than I)?

Also, there may be a valve somewhere outside that turns on and dumps a little water flow through the exterior pipes/solar thermal collectors to prevent freezing in moderately cold weather. Watch that the drain water does not go somewhere bad (basement, erosion, etc.). And that it does not start pulling a lot of hot water from your boiler.

-Bill

solar_dave · September 2012

Re: Open loop system: how is the solar heated water controlled into the furnace

If the solar water is substainly higher temp than the boiler set point the boiler should not fire, at least that is how my gas hot water heater works. When I get to the point in the year where I don't get enough hot water I turn up the set point on the gas fired. Usually here that only in mid Dec to maybe The first week of Feb.

Reinier · September 2012

Re: Open loop system: how is the solar heated water controlled into the furnace

Hello Bill,

Thanks for your explanations. You might have read in a different thread that my system is actually a drain back system. A few of your questions I can answer by now:

BB. wrote: »

I am guessing--But your current design, more or less, is a pre-heat for the (oil?) furnance hot water boiler)?

You are correct. The furnace has two inlets, on for cold water directly from the well, and one for the pre-heated water.

Have you tried "manual" operation of the pump yet (say turn on at 10am and turn off at 3pm during a sunny day) and see how much hot water/temperature rise you can get in your storage tank yet?

Yes I have, the temperature rises slowly. I have seen it go up to to 80F/27C but did not want to let it run because I did not want to run the risk of overheating and I could not stay close to the tank all the time. Waiting for a new differential controller to arrive later this week.

Have you reviewed how to shutdown/drain the system when a hard freeze is due (I am sure you know more about this than I)?

I am relieved that this is a drain back system so this is not an issue -- right?

Reinier

BB. · September 2012

Re: Open loop system: how is the solar heated water controlled into the furnace

I replied in the other thread, but here is a link to a page with detailed drawings of their standard system designs:

http://www.solarroofs.com/tanks.html

Is this an automatic or manual drain back system... And does it only drain the panels and related plumbing--or are you draining the whole storage tank too (i.e., for winter shutdown/going away where the tank could freeze)?

-Bill

Open loop system: how is the solar heated water controlled into the furnace

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