Solar DHW / tankless integration question

[DamonHD]

Hi,

(Admin: couldn't decide if this should be heat or controls, please move as needed.)

As I will be a man of leisure from the end of next week, my solar thermal (DHW) plans will probably be postponed until ~Feb of next year.

This gives me time to think harder about integrating the solar element with my existing mains natural gas tankless 'combi' heater, which although only ~80% efficient and unable to accept solar-preheated water, I'm not really willing to replace for a number of reasons.  Mainly I want to reduce gas use altogether and not just put in newer ways of doing it (for example, we added 20cm of loft insulation yesterday ready for winter).

In the summer I intend to let the tank get hot (~90C) and so will have a TMV regulator to avoid water over 55C getting to the taps.

But what I think I need to integrate this with my gas combi without using the combi to heat the tank contents (ie I want to stay tankless when using gas at all as I think that that remains the most efficient and usable route) is another sort of thermostatic valve, but I don't know what it would be called or if it even exists for normal domestic plumbing...  One of you here will be able to steer me I'm sure!

Basically I'd like something that clamps to the top of the tank, and if the tank is cold (eg <50C) diverts water to come from the gas combi rather than the tank.  It should do this entirely mechanically with no electricity required if possible.

So, does this exist?  What is it called if so?

Rgds

Damon

[fcfcfc]

Hi: That's an easy one... just use a 3 way diversion valve and a snap disk. The common out goes to the fixtures. One input is the solar heated tank, the other is the tankless. Put the snap disk on the out of the solar tank. When the solar tank is above your chosen point the valve pulls from the solar, below it is from the other direction and pulls from the tankless. Will work easily, just not real efficient for the solar....

.....Bill

[DamonHD]

Thanks!  I shall do some researches!

What I was concerned about was that without the water flowing all of it will be cold and thus the actuator would always switch to the combi side, so I really want to have it driven by tank temperature rather than the water in the pipe.

But I'll do some research on snap-dis[kc]s...

Rgds

Damon

PS. Yes, I realise this is not the most efficient solution, but others are looking horribly complex and/or expensive.

[Lumberjack]

Two pumps and a differential controller would work. Then again you could simply base which pump to run on the solar tank temp alone. IE (if solar > setpoint run pump 1 else run pump 2)  any reverse flow can be stopped with a check valve.

[Lumberjack]

Re-read the post... two electricly operated valves would work in lieu of the pumps.

[DamonHD]

Well, I'd like to do it without electricity (ie so nominally it'll work in a power cut, but also won't be a 'vampire' power consumer), and all at mains pressure (no pump in the DHW circuit).

Rgds

Damon

[fcfcfc]

Hi:

Yes, when I say on the out pipe, I mean right at where it goes into the tank, so it will represent tank temp. even at no flow.

Snaps as example:

http://www.grainger.com/Grainger/wwg/productIndex.shtml?from=Search&newSrch=yes&operator=key
wordSearch&search_type=keyword&action=Go%21&QueryString=snap+disk

.....Bill

[fcfcfc]

OK, that was confusing. Try again. Right where the out pipe comes out of the tank.. Its just easier sometimes to attach to the pipe than the surface of the tank body itself....

[DamonHD]

OK, we're on the same page, thanks!

Rgds

Damon

[Darren73]

Hi Damon,

Have a look at the "spirax sarco" range of thermal valve controls, totally self contained, no electric needed for operation.

I'm away from my desk until tomorrow but if you need some more detail I can get it tomorrow.

Regards

Darren

[DamonHD]

OK, just trying to get past the brochure-ware now!

Thanks

Damon

[panamman]

Great idea, but you can go all the way for just a little more $.

You can improve on that by using an automatic temperature shower controller set 5 deg below the max input temp of your demand water heater and place it in front of the demand water heater with the hot side coming from the solar tank and the cold side the original input to the demand water heater.

ALL demand water heaters have a relatively high input temp before shutdown. This system lets you PRE-MIX the input water to always be just below the max input which minimizes the delta T (temp rise) that must be provided by the demand water heater. This in turn minimizes fuel use.

1. When the solar tank > shower temp you pull from the solar tank.
   
2. When the solar tank < shower temp you pull from the demand water heater which uses the solar tank water to PRE MIX the demand water heaters supply water.
   
   

In most cases this will cut the energy use of the demand water heater by 5 to 10 times.

Some people (with high end demand water heaters) just use the pre-mix and skip the direct solar / demand switchover connection. I like both for off grid since pro-PAIN is a hassle to hand carry in.

Of course you can always just wait until the solar tank is at the right temp before you use hot water.

best part of all. the wife will never notice! that is unless you make her carry the propane bottles. LOL

.

[TomW]

This system lets you PRE-MIX the input water to always be just below the max input which minimizes the delta T (temp rise) that must be provided by the demand water heater. This in turn minimizes fuel use.



In my opinion, it also wastes energy [cooling your already hot water so you can reheat it]. Just seems silly wasteful.

I never used one so no experience but cooling already hot water so you can reheat it seems dumb.

I use a preheater into a common tank type heater. I KNOW I don't waste energy by cooling my water so it is acceptable to my heater. It works very well. Nobody in this area gets any life from the demand heaters due to lime buildup so even if I wanted to try one it would be a mistake here.

But then you apparently are an expert.

Tom

[DamonHD]

To be fair TomW, it would just be a mixer value mixing say 3:1 warm to cold so you get the heat fed in slowly to the boiler; it's not really 'cooling' the hot water, simply bleeding it it as fast that the boiler will allow.

But having been generally unimpressed with the responses to my queries for quotes I am leaning to my complex PV + AHSP solution!

Rgds

Damon