fool Munchkin into higher-output DHW mode?

R. Swatton

How about trying this.
If you pipe system with both pumps and wire them acordingly this will work with a 2 stage thermostat
Rich

John Ketterman

OK, so my Munchkin T80 is a bit undersized. Last night it went down to -6F and the boiler ran all night unmodulated, never reaching the target water temperature and never quite satisfying the stat.

One problem is that based on the gas meter reading, the boiler is drawing 75,000 BTU/hr rather than 80,0000. I think this derating happened when it was set up with a combustion analyzer. But even 80,000 BTU/hr will not be enough since the record low here is -26F.

So here's my plan. I'm going to wire to the appropriate terminals for a DHW call (I don't have DHW). I already have the circs wired so that when the boiler loop circ runs, the radiation loop circ runs also.

So when I connect the two terminals to simulate a DHW call, the circs will run and also the Munchkin will run at 110,000BTU (special output rate for DHW) for up to 30 min each hour. I'll only do this in exceptionally cold weather.

I know this will void the warranty. But other than that, anything wrong with this plan?

R. Swatton

What if you used a 2 stage thermostat, first stage conect to gray tt, second stage to blue domestic tt.

Call HTP to get aproval

John Ketterman

Good idea! Does the plan sound OK? I am making no wiring or hardware changes except connecting to the DHW call. (The radiation circs are wired to go on whenever the primary circ does.)

I can't see HTP okaying it, though.

R. Swatton

I think the only problem will be that when the domestic calls with blue tt it activates a 110v line that normaly runs the domestic pump. I will ask HTP about this for you.
You can e mail me @ summitplumbing@comcast.net.

Rich

hr

Why not

replace the unit with the proper size. Sounds like you missed the load by quite a bit. Can't see running it that way forever.

You might be able to trade that to someone shopping for an 80.

hot rod

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[Deleted User]

Why not...

try conservations efforts first. I like making my mod con work up hill, but it sounds like yours is back sliding...

ME

Brad White_34

On a similar note

instead of replacing it, supplement it with another Munchkin in parallel. If a single M140 is warranted, maybe spring for another M80 or even a 50 if you dare. That way you will have a spare engine and a great milder-weather boiler. Twin them with staging controls. I would think that this would be less cost and hassle than ripping out a good boiler that has only the misfortune of being too small for the job. But it sounds like the one you have can carry you for more than half the winter heating hours. You just need a good boost sometimes.

And as others have said, check your loads and reduce them where you can.

Steve_35

You don't design for record low.

Or your unit will be oversized 100% of the time almost every year. You only hit extremely low temps for a few hours a year, usually at night. The record low in our area is between -20 to -30F. We design for -10. Manual J says we could/should use 2F .

Plumb Bob

Yeah, but it activates the primary pump 110v as well (the primary pump has to be on whether it is for heat or DHW), and I have that wired to a relay so the secondary circulator runs whenever the primary does. So there should be no problem, the DHW pump power isn't connected to anything.

I will e-mail you in a few days.

Plumb Bob

The reason you can get away with that is that Manual J has quite a bit of cushion. When you use Manual J, you are oversizing. So even if you design for a design day rather than a record day, you have enough capacity for a record day. (Which is a good thing compared to undersizing, as the OP has done.)

Plumbob

Nope, I'm wrong, the primary pump does NOT run when the DHW pump runs. This is where the install manual is wrong, the Vision manual is right.

So I'll have to run the DHW pump power into my relay. More work.

Steve_35

We don't find that to be the case.

We have homes we've switched from electric BB to HWBB. We design to -10 and discuss this with our customers. Inevitably when it hits -20 we get THE call that the t-stat is set for 70 but it's only 64 or whatever.

I remind them of the design process and why we designed it that way - to save them money all the rest of the time. Since the new system typically costs them 1/3 to operate of their old that's usally the end of it. I ask them to wait a few hours and see what happens as the outside temperature comes up.

In any case our experience has been Right-J is pretty darned accurate.

R. Swatton

If you pipe sytem with both pumps, Central Htg and domestic using gray wires for central Htg and blue wires for domestic.
Use 2 stage thermostat and this will work.
Wiring for central Htg pump will be brown and orange from munchkin. Wiring for domestic will be pink and purple. Blue, pink, purple wires come with the Vision I package.

R. Swatton

If you pipe sytem with both pumps, Central Htg and domestic using gray wires for central Htg and blue wires for domestic.
Use 2 stage thermostat and this will work.
Wiring for central Htg pump will be brown and orange from munchkin. Wiring for domestic will be pink and purple. Blue, pink, purple wires come with the Vision I package.

R. Swatton

Try this with a 2 stage thermostat

R. Swatton

Authorized Heat Tranfer Products Sevice Agent

R. Swatton

Try this.

Guy_6

Careful

Remember that when you use a product outside of it's designed purpose, you are out there on your own. The liability is yours, as is the warranty. You would be best off to get the correct sized boiler and do it correctly.

Plumbob

Why would I need to add a new DHW circulator and plumbing? (I don't have DHW now.) What is the benefit? A far as I can see, all I have to do is connect the DHW circ power and cntl htg circ power both to the cntl htg circ, so that the two circs in your diagram are actually the same circ...

John Ketterman

It costs them 1/3 to operate because they switched from electric heat. The savings is not from excessively undersizing the boiler. A slightly bigger boiler would have had the same efficiency, and happier customers. Oversizing is a bad thing, but it makes only a few percent difference to efficiency.

Steve_35

Of course switching to another fuel is the reason

for the bulk of the savings. Another reason they save is because they have a correctly sized boiler with the proper amount of emitters.

We save them perhaps another 5% on their bill EVERY year with the consequence of possibly having a day every few years where they can't maintain 70F for a few hours a year. Remeber in our area Manual J says to design to 2.5F. We've already opted to increase our design dT by 20%. Just how far should we go? To the record low as the OP suggests? Why not a few degrees below the record low, just in case?

John Ketterman

No, it is never necessary to go to the record low temperature. It may be good to go to the record low daily average temperature, though.

But if your customers are calling you to say their homes are cold, then the boiler is undersized. This is more grief for you, and not any significant savings for them.