Can you add DHW to a boiler currently not setup for it?

Bob Sweet · December 2006

a nice choice. If it were me I would pipe the indirect in para llel rather than off of a secondary loop, and put my heating side off a secondary loop.

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Al Roethlisberger · December 2006

Can you add DHW to a boiler currently not setup for it?

So, while I am contemplating next Spring's rehab of my current boiler setup, I realized today that our current electric DHW tank isn't going to "cut it" for the 5 of us long term. It just takes too long to recover, but all my past gas fired water heaters seemed to recover much more quickly.

I am going to research plain gas-fired water heaters, tankless/instant heaters, etc(and I welcome any thoughts on the best in these categories by the way)... but also thought that since I'll probably be buying a Tekmar 260 reset that can also manage DHW, I should probably check if I can retrofit DHW generation through my current boiler.

So that's my question: What would it take, or is it even feasible/efficient, to create and manage DHW via my existing Dunkirk gas fired boiler?

As a background/reminder, I have an old overhead gravity system, converted to a closed circulator system in 1991. It is currently not optimally controlled or plumbed, so I'll be doing some reengineering next Spring. So I am trying to cover all bases before I tackle the job. Hence questions along these lines. And no, I have no "pros" within about 100 miles that seem to know much of anything, so it is up to me to figure out =P

Thanks again.
Al

Timco · December 2006

It would take some re-piping and new controls to give the DHW priority, but could certainly be done!

Tim

Al Roethlisberger · December 2006

Thanks, thinking of Tekmar 260 and pulling off of pri/sec piping

Thanks!

I thought as much.

I am thinking of repiping this into a primary/secondary setup, and would pull a DHW zone off of that via a Tekmar 260 and its built in DHW control.

Al

D107 · December 2006

Hopefully retrofit cost is much less than half cost of new boile

HO here; I too hope to do this soon. I am intent on not throwing good money after bad. My old stand alone HW heater is on its last legs and I've still got re-piping and insulation to do before I get new boiler in a few years. Since my boiler is about 3x too large, pro told me we could do without a DHW priority control, that the HW would just become a 4th zone piped off the 'other' unused side of the main zone flow control valve. So I'll do the minimum possible. Since water main has no backflow preventer, I can probably also avoid an expansion tank for the indirect.

In your case if your retrofit cost approaches 50% of install of new boiler with indirect HWH and control, then a new more efficient boiler might be worth contemplating.

Good luck,

David

john@TR · December 2006

Do without priority control? I don't know if you want to do that. The point of making the domestic have priority is to keep the temp up in the shower when you have a high demand. You're going to have a cold shower on a really cold morning, I think. I didn't get to see the set-up so I could be wrong but you might want a second opinion on that.

Al Roethlisberger · December 2006

Does the Tekmar 260 integrate priority control for DHW?

I know the 260 will control firing the boiler as well running the pump for DHW, but am not sure if it manages priority between house heating call and DHW.

I'll go back and do some more spec reading, but anyone know off hand if the 260 manages priority between the two aquastats? Or do I need another control?

Al

Bob Sweet · December 2006

Yes

the 260 provides domestic priority.

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Al Roethlisberger · December 2006

Why pipe DHW in parallel versus off of secondary loop?

Just curious, as I saw this suggested in one of Dan's books that the DHW come off the secondary?

Also, when you say pipe in "parallel" to the primary loop, what do you really mean physically speaking in pipe? I was trying to imagine it, but could only really visualize it in series to the DHW tank/loop, which I'm sure you didn't mean.

Also, if in parallel to the primary loop, wouldn't the DHW loop now need another expansion tank of its own, or would the exansion tank on the primary loop be sufficient?

Thanks
Al

brucewo1b · December 2006

Al

If you pipe out of a boiler to a tee to the HWM then go to the primary loop the HWM gets the hotest water and any excess hot water goes to the loop to be taken up by the heat loop and at the return the warm return water goes back to the boiler being mixed with the loop water. If the HWM comes off of the primary loop it grabs the hot water first the cooler water mixes with the loop water before going to the heating zone so going parallel your excess water that reahes the heat loop is hotter and will usually be so that you do not need to use priority.

Timco · December 2006

The taco controls give you priority as well, and as a rule, I like to install new controls with a re-pipe anyway. The 260 will need a controller of some sort anyway. As for the parallel question, in the attached photo, you will see the circ heading up that is not connected yet...this is for the DHW piped in parallel tot he primary loop. The black T on the bottom will be the return for the DHW.

Tim

Bob Sweet · December 2006

Another thing to consider

with the 260 you will have a powered relay to operate the boiler pump and an isolated relay to control the indirect pump.

If you were to pipe the indirect off of a secondary loop (Ive done it both ways they both will work) you will have to think about your control stategy in addition to powering the indirect pump you will have to add an additional relay to power the pri. pump (if your utilizing priority). If you pipe the indirect in parallel the 260 can power the indirect pump when in priority and shut down the boiler pump.

When in heating mode the 260 will power the boiler pump and you can use either the isolated relay off of your zvc controller or your sr controller depending on whether your using zone valves or zone pumps to power the system pump. And shut down the indirect pump.

Edit: If your going to pipe the indirect off of a sec. loop I would strongly suggest using priority due to the mixing of waters coming back from your space heating.

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Al Roethlisberger · December 2006

Very helpful, thank you. What is that \"extra\" bypass though?

So, based upon my layman's analysis of the photo, it appears that the rest of the primary loop runs behind the old water heater? If so, what is the bypass running vertically right beside the diaphram expansion tank for then?(it is the one with the half closed ball valve).

Thanks again. Great photo.
Al

Timco · December 2006

That bypass leads to a grey 3-way valve, a TV or Thermic valve. It opens @ 140* and closes @ less than 140*. It closes the primary until the return gets to 140*, then allows flow. The half closed valve is just a 'circuit setter' to ensure flow to the TV and the primary loop. The pri loop just continues behind someone else's WH...

Tim

Al Roethlisberger · December 2006

Does the valve shunt the water, or just cut it off?

Hi Tim,

Thanks, this is very interesting, what with all the options... much like a tinker toy, or computer networks, which is what I design.

I hope you don't mind answering some of these questions, as I am just trying to cover the essentials so I understand what I'll need.

First, is something like this TV valve and bypass required in a primary loop? Or is it just one more level of insurance against thermal shock to the boiler and/or just helping heating it up faster for efficiency?

Also, so if I understand your description correct:

1) The system calls for heat, and the boiler fires and kicks off the primary loop circulator to get the boiler up to spec heat.

2) Normally the water in the primary loop would circulate freely from boiler supply to return in a loop in the primary(ignoring the secondary loop takeoff and return for now).

3) However, this TV valve is initially closed until the primary loop water temp is 140 degrees.

Question: When this three-way valve is closed, does is shunt the water coming around the primary loop from the boiler back "up" that bypass with the ball valve? If so, wouldn't this dead-head the pump or create a low pressure in the boiler since there is no water coming back through the boiler to the inlet of the primary circulator pump? I suspect I am not understanding how the TV valve operates here.

4) Once the water reaches 140 degrees in the primary loop, this valve opens(does it close the bypass at this point?) and water circulates freely in the primary from boiler output to return.

Is this correct?

Thanks! This has been very helpful. It seems very interesting. Maybe I should study-up on this and get licensed since no one here seems to know anything about this =P It actually seems very similar to the business I am in today in some ways.

Al

Can you add DHW to a boiler currently not setup for it?

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