Water heater as buffer tank

Snowmelt

Starting a new thread here…… I’m thinking it would work from what I asked last time.
I had 4 questions……
1 I was going to use the tapping for the pressure relief valve and the drain valve for the primary loop?
2 hot and cold for the heating loop?
3 small heat load of 70,000 btu ( probably smaller) so was just going to increase the 3/4 to 1 inch with a bushing , nipple and bushing ?
4 The original water heater pressure relieve vale is set for domestic pressure, I was going to take it out, should I put one that matches boiler or leave it out ?

Jamie Hall

Make sure there is a pressure relief valve somewhere on the system which matches the pressure rating of the boiler -- typically 20 psi.

Snowmelt

I was ust going to get the numbers off the boilers pressure relief valve. Technically owes it need one because of the one on the boiler,it’s not a flat plate hex.

Amie what about the other questions

hot_rod

Common tank type water heaters are built to 300 psi , rated and relief valves at 150 psi, 210 F

a 30 psi valve should be plenty adequate

I always left the 150 psi T&P valve in place, added a 30 psi when using water heaters as radiant heat source . So it still had temperature runaway protection

Maybe a drawing of what you are proposing?

Snowmelt

Here hot rod a simple drawing keyword buffer tank, 2 air separator 1 make up water

hot_rod

I’d recommend a 2 pipe buffer

Either use the two 1” element holes 

Or increase the drain port to 1”
On top connect the two 3/4 connections into 1”
be sure to remove the dip tube

Air purger on the boiler supply pipe

Expansion tank and fill valve connects at the bottom 1” header

Add a spring check on the supply from boiler to tank

A drawing with both options

Snowmelt

Thanks hot rod, I’m following the diagram but not understanding how the water is mixing in the tanks, I wanted to keep it simplifies. I guess the other question is would the diagram I gave work. It looked to me like it would mix with the hole tank .

hot_rod

Any idea of the flow rate on the boiler and heating side. I assume the boiler has a circulator inside? Probably moving more flow into the tank than the heating side, gpm?

So with a two pipe as I show, you get direct to load flow. If it leaves the boiler at 120F, 120F goes directly to the load side.

In your drawing, the tank is always involved and you will get a blended temperature, so you may need to run the boiler hotter, maybe much hotter to get desired supply to the heating loop. If you knew the flow rates on both boiler and distribution you can calculate that mixed temperature. My suggested piping eliminates the mixing.

The dip tube on the C top port is probably 1/2" ID, so depending on flow rates you may get some high turbulence in the tank.

Be sure it is not a "hydro-jet type dip tube with slots, shown on the attachment. Those slotted tubes are intended to stir up the tank to keep minerals from settling out. But you certainly don't want that in a hydronic buffer.

It is a very subtle piping difference between what I propose and you have drawn, but it does make a big difference in how the tank stores energy for you. Since you are using a buffer, may as well maximize the tank. Piping should be no more complicated or expensive? Same fittings in a different location

The more you can "stack" or stratify the tank, the more useful that volume of water becomes, the longer you can run without firing the boiler.
If that is the intent?

Once the heat call ends the boiler recovers and stacks the tank for the next heat call.

Hydro-Jet dip tube example attached
Examples of tank stratification attached

I would prefer to see the expansion tank closer to the boiler if that pump is pumping into the boiler? Same with the air purger, the hottest point will be at the boiler, best air removal opportunity is there.

JakeCK

Any recommendations on an electric water heater that you have found that works particularly well for this? 

Snowmelt

I’m going to redo your 1st diagram so we’re both on same page , I also like what you sent me fig. 3-5/

hot_rod

JakeCK said:

Any recommendations on an electric water heater that you have found that works particularly well for this? 

I have used all different brands. I found that some of the Lochinvars come with two extra side taps. Maybe model specific and that is what the supplier I dealt with kept in stock. They catered to commercial accounts.
I bought one scratch and dent from a box store that had side taps, Richmond brand maybe?

Stubby tanks seem to have side connections in addition to top. Tall would be a bit better for stratification however.

You might research models online. Also compare to the Boiler Buddy or HTP buffer tanks that are built specifically for that purpose.

GroundUp

@hot_rod thank you for that explanation of the 2 pipe system- I'd always struggled to understand it very well. I do have some questions, however- how is the temp controlled? Does one use a sensor in the tank to tell the boiler when to come on or use the normal zone thermostat and just dump unused BTU into the tank during heat calls? If the latter, what purpose does the tank really serve aside from a heat sink to prevent short cycling? If the former, where does one set the sensor? If we want 120* SWT going to the load and set the boiler output at 120* to work with this 2 pipe buffer, we will never achieve 120* in the tank which would never give us 120* output to the load. Could you please touch on this?

hot_rod

The boiler only looks at the tank as its load, the boiler operating sensors goes about mid point on the tank. It doesn't know or care what the distribution is doing.
Its only job is to maintain tank temperature. It is hydraulically separated from the distribution loop also. So no pumping conflicts.

To maximize the tank, you would want to run it up above the required SWT. the hotter the tank, the more useful energy it stores. The longer the boiler off cycle.

Think of OWF, they have large water content to store and lessen on off cycles. I look at them as a buffer tank that happens to have a fire below or within

With a condensing boiler it becomes more of a trade off. Unless you have tiny micro loads, a fraction of their lowest turndown, rarely would you need to buffer a mod con. And if you run the tank hot, you lose condensing efficiency.

It depends on what you expect from a buffer?

It prevents or lessens short cycling most importantly. Piped properly it becomes a hydraulic separator. It a good place for multiple inputs to end up, wood fired, gas fired, maybe solar thermal or any waste heat from a generator perhaps.
Air and dirt removal to some extent.

Snowmelt

Ok here is what i think your doing.
Tank is heated up by boiler to what ever temp when call for heat.
The zone takes water fro the tank to the return off the drain valve area back to the boiler , boiler will fire to satisfied.
On the supply the water will be drawn from the tank because of lease head pressure

Snowmelt

Yikes I forgot this is a condensing boiler I think I need closely spaced tees at the boiler

GGross

The buffer should act as a hydraulic separator and give you primary secondary piping

JakeCK

@Snowmelt I do not believe you would need the closely spaced T's if using a buffer tank. The tank provides the hydraulic separation. That said why do you want to use a buffer with a modcon boiler? 

@hot_rod I did find this one but it is a mobile home tank. I doubt that would matter, however, in this case. It has 6 taps on the side for the elements, hot and cold, t&p valve and drain. And one on top for the anode. Can the dip tubes be removed from side port water heaters? It mentions in the manual about a spiral jet system to prevent sediment build up too. Would the hot and cold ports even be useful in this case?

https://www.menards.com/main/plumbing/water-heaters/electric-water-heaters/richmond-reg-essential-reg-6-year-mobile-home-electric-water-heater/t2v40-d/p-1444452213419-c-8690.htm

Snowmelt

I posted the last comment without thinking.
Only reason to do this is because it’s existing, customer is claiming of short cycling

hot_rod

The tank is maintained hot, always, not just on a heat call. Because if the tank is hot and a heat call comes on, the boiler doesn't fire until the tank temperature drops. It can drop to the lowest possible useable temperature. So the tank could run a 20- 30 even higher delta. Load it to 180F or whatever, pull it down to 150, 120, whatever temperature covers the load. Make sense?

Even better yet would be to pull the loads off via an outdoor reset control. So then the "exact" temperature is pulled from the tank for the load.

With a mod con, if you know what the smallest load possible is, and the lowest turndown on the boiler, 17,000 I imagine? Plug it into this formula. Buffer tanks for modulating boilers are much smaller due to the turndown of the boiler.

If your smallest load isn't below 17K, maybe even below 8,000 btu/hr. I see no reason to even add a buffer tank?? If it short cycles at a 17K output it could be the control is not dialed in?

Look for a ramp delay function. It locks the boiler at a fixed modulation for the time you determine.
My Lochinvar has 6 or 7 steps. I have a 15 minute increment set. So it stays at 8k for 15 minutes, which on mild days covers the load. Every 15 minutes it bumps up another 10- 15% output, In an hour it is at 100% if the load has not been covered.

I also have boost function. If the boiler sees temperature dropping, falling behind the load at it current firing rate, it understands it needs to bump up.

As mentioned above the tank IS the hydraulic separator. And that is regardless of a 2, 3 or 4 pipe setup.

hot_rod

JakeCK said:

@Snowmelt I do not believe you would need the closely spaced T's if using a buffer tank. The tank provides the hydraulic separation. That said why do you want to use a buffer with a modcon boiler? 

@hot_rod I did find this one but it is a mobile home tank. I doubt that would matter, however, in this case. It has 6 taps on the side for the elements, hot and cold, t&p valve and drain. And one on top for the anode. Can the dip tubes be removed from side port water heaters? It mentions in the manual about a spiral jet system to prevent sediment build up too. Would the hot and cold ports even be useful in this case?

https://www.menards.com/main/plumbing/water-heaters/electric-water-heaters/richmond-reg-essential-reg-6-year-mobile-home-electric-water-heater/t2v40-d/p-1444452213419-c-8690.htm

A great tank for buffer. Usually the dip tube is molded into those nipples you see and the nipple could be removed and a standard nipple installed. i think a company in Tennessee makes most of the water heater nipples and dip tubes. The side nipples usually have a gooseneck plastic tube inside to direct flow. If it has the slotted cuts in the side of the dip tube it stirs the incoming water.

Snowmelt

Hot rod thanks, the last diagram that I sent, is that pretty much what you draw, only other thing was, I think you mentioned a check valve on the supply ?

hot_rod

Snowmelt said:

Hot rod thanks, the last diagram that I sent, is that pretty much what you draw, only other thing was, I think you mentioned a check valve on the supply ?

All the ups and downs are making me dizzy, but you have the concept

I assume this bottom right hand pipe goes behind the tank, not into the bottom right side of the tank?

Just be sure you aren't designing a solution for a non existent problem or symptom.

Is this a 175K boiler or tankless DHW heater?

I'd sit alongside that boiler for an hour or so on a mild or shoulder season load. Or even a single small zone load and see what the cycle looks like.

I'm feeling more like a control reprogram may be an adequate fix?
The industry seems to agree on a 10 minute minimum run time when a boiler fires.

Snowmelt

Your right the pipe goes around the tank , I’m not a 100% sure but I think it’s a 175,000 Bosch combo , I think minimum fire is 18,000. It’s an existing system that has issues of runaway heat, to many check valves, primary/ secondary isn’t used properly. Transformers all over the place, extra zone valves, then on top of that there is an electric water heater used as a buffer.