Vitodens snowmelt & DHW question

Mike T., Swampeast MO

Why use the "External Demand" function?

Presuming you have some sort of control signal saying "I need snowmelt", just run it through a normally closed relay that tells the boiler to go into "External Disable" whenever the snowmelt <I>doesn't need</I> heat. MANY options for "External Disable" and I'm nearly positive that DHW production can still occur with the proper setting.

With appropriate manipulation of slope and shift (low or even zero slope with high shift) you [should] be able to produce a curve that requires 75C target in any weather where snowmelt is required.

Do though pay <I>very</I> careful attention to your flow primary vs secondary flow rates as you are CERTAIN to need the low-loss header in this application. Design for about 20% greater flow through the secondary and the Vitodens will exceed the "normal" 75C limit (up to the same maximum limit via an "external" call) to maintain target in the LLH. With higher secondary flow the boiler will actually limit its output as the required primary temperature will exceed its utter maximum output.

This all assumes that the snowmelt can <I>instantly</I> absorb the FULL output of the boiler <I>regardless of conditions</I>. While such is a near guarantee when you <I>start</I> supplying heat to the snowmelt, you have to consider what happens when the system heats... <B>Do not skimp on the tubes!</B>

Unknown

so, we just got the go ahead to do a commercial snowmelt project (approx 1900 sq. ft.) using a Vitodens 15-60 which is fine, however, my boss came up with the idea of using the Vito to generate DHW with a 79 gal. V-300 tank for a new commercial kitchen when not melting snow. it sounded like a great idea to me since it will be idle for probably over 90% of the time otherwise. but the Viessmann rep said that when in setpoint mode you can only have one temp. ie: 140°f for the snowmelt and the DHW would just have to take what it can get. this would work fine i suppose as we could set it up just as a DHW pre-heat but i would rather it be able to do various setpoints, anyone have any experience with this?

Unknown

DHW Kit

Zak,

Why not hook up the snowmelt on the heat side adn get a DHW kit for the vitodens? Set your curve so youi will get 145 at 10F and you are all set.

Hooking up this way will aslo prvent you from running ice water through your DHW tank.

Its all in parameters.

BTW - make sure to use a DHW tank, either the V300 or a Traiangle Tube unit wil LOW HEAD, HIGH GPM and HIGH TRANSFER rate or you will short cycle the 15-16.

wheels
this

ALH_4

setp & dhw

You can use the DHW production kit and set it up as any heating/DHW boiler, even when using the external demand. Set the OB8, 011, 027, OC5, and OA2 codes for the operation you want, and it should be fine.

edit: One nice thing about the big Vito is that you get to size the DHW circuit pump so head loss through the tank is not nearly as critical as it is with the small Vito, though there are certainly some tanks I would steer clear of, no matter what boiler I was using. I assume you are using the heating circuit for the snowmelt now with the external demand. It really doesn't make much difference which fittings are DHW and which are heating on the big Vito, because each has its own pump, and it is the same circuit.

Paul Pollets

DHW Priority

If the coding were set for DHW priority, it wouldn't take long for the big Vito to recover the tank. That's maybe 8-12 min.,depending upon tank size. The snowmelt system would be minimally affected for the short time it was in the "off" mode.

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ALH_4

what about...

One thing about having the tank on priority is that when the tank is satisfied and if the snowmelt is calling for heat, the boiler will go from DHW production temperature to the slab temperature immediately. That could easily be a 100-150°F change and seems like quite a shock. The coil-type heat exchanger in the Vitodens seems uniquely suited to this sort of thing, but it may stress the hx over time. It might be a scenario to run past Viessmann just to make sure.

Unknown

good ideas

i like the DHW priority option, sounds like that may be the ticket. but i will run it by Viessmann anyways. and yes we are using a Viessmann tank.

thanks a lot everyone!

Unknown

Viessmann sez...

i just talked to my rep and he said when the Vitodens is getting an external demand (in our case a Tekmar snowmelt control) the DHW will not work. again, he stressed that the DHW would only be able to go up to the snowmelt setpoint temp.
any thoughts on this?

ALH_4

hmm

What is his explanation for codes OC5:006 to 011 then? Tank heating is in normal operation with the external demand.

When the demand is present, you can disable the tank or leave it enabled. When the demand is gone, the boiler reverts to normal operation per note 3 at the bottom of page 56 of the Service Instructions. That way you could essentially prioritize either the tank or the snowmelt depending on the OC5 setting.

I would use the 4 connections on the boiler with one being the boiler pump for the snowmelt, the other being the DHW pump. You may be able to use the heat exchanger as your "low loss header".

The Vitodens has a ton of control options, and it is easy to misunderstand the operation.

Unknown

USE THE LOW LOSS HEADER!!!

The boiler needs to keep it's flow on its own... DO NOT install this boiler without a low loss header.. EVER.

FYI

steve

ALH_4

Steve

When I mentioned using the heat exchanger as the low loss header, I meant an external flatplate heat exchanger for the snowmelt. The glycol concentration required for snowmelt is too high to pump it through the Vitodens heat exchanger. Sorry for being unclear.

Unknown

on the 15/60?

There is no plate exchanger on the 15-60 model. Why would you not be able to put the 40% glycol through the exchanger???

ALH_4

hx

I mean with a snowmelt system, a Flatplate heat exchanger must be used to separate the higher glycol concentration snowmelt fluid from the lower concentration boiler fluid. I believe somewhere around 40% is about Viessmann's cutoff for the Vitodens. If the system needs more than about 30-35%, I would use a Flatplate heat exchanger. And if that external heat exchanger is added, then that could possibly be used as a pseudo-low-loss-header.

ALH_4

WIWD

For what it is worth, this is what I would do:

Piping:

I would not use a low loss header. That viscous 50% glycol cannot be pumped through the Vitodens. The Flatplate HX becomes the hydraulic separator.

Controls:

I would probably use a tekmar 665 if automatic snowmelt is what the owner wants.

I would use the external demand because when the snowmelt calls you want to that 15-60 to put balls to the wall and melt that snow. You want control of your supply temp that is more exact than guessing at a heating curve.

Contrary to what the Viessmann rep says, the Vitodens can certainly be used with DHW and external demand. Even if you disable the DHW with the external demand via the OC5 code, it will resume normal operation once the snowmelt is satisfied.

---


Take it or leave it, but that will work. Please excuse my very crude sketch.

Unknown

your input is much appreciated! we're getting the system engineered by our in-floor tubing manufacturer so the tubes won't be skimped, and i believe they are also doing a piping layout(how much of it we use we'll see when we get the prints).

andrew: i looked over the manual and it does look like there should be no trouble coding it for DHW and setpiont demand just like you said, thanks!

Unknown

both ways will work

Andrew,

With the correct circulator and flow both ways will work. You obviously like to use a plate heat exchanger, I prefer a low loss header. The Vitodens doesn't care if you use water or a glycol solution. The SS will not be effected by either.

With the glycol you will lose heat transfer because Glycol of any percentasge will not carry as much heat as water. Simple physics. Wether that lack of transfer is in the exchanger of the Vitodens or at the plate heat exchanger it does not really matter.

Why add more expansion tanks and more fills and more circs to a system that does not need it. Just plan on the approx 15% less heat transfer and 15% more head during the sizing of the system components.

I personally really hate to add more stuff to a system that is not needed. A low loss header will give the end user a place to measure the temp, a place to seperate the flow rates, a place to get rid of air, a place to get rid of dirt, and is less expensive then a heat exchanger (an a lot lower chance of clogging).

Just my 2 cents.

skw

ALH_4

glycol

Steve,

I agree with you, except I remember being told by Viessmann that 40% glycol solution was the maximum they recommended in the boiler. As I remember, it is in the literature somewhere. I wish I could remember where. Maybe with the newer 300 "heat engine" in the big Vito they have revised that? I agree that heat exchangers should be avoided whenever possible.

Unknown

Only When the Circ is Supplied IN the Vitodens

Andrew,

In the two smaller sizes (6-24 & 8-32) the concern with higher percentages is the pump is sized to move water at the correct speed. The smaller ones don't like more then a 40% mix.

As you and I both know it is up to us (read the designer or contractor) to size the circulators and supply them correctly for the 11-44 & 15-60 models.

It is not in the literature at all about the glycol.

Viessmann's concern on the larger sizes is the transfer of heat. Plan on the lower transfer efficiency etc etc and all things are good.

Steve

ALH_4

glycol

Steve,

I stand corrected wrt Viessmann's recommendation. 50% is stated as the maximum in the Technical Data Manual on page 49. I do not see where it even says anything about a lower percentage for the 24/32. As I understood, it was an issue of viscosity and passage size in the Vitodens heat exchanger. Viessmann needs to spell this out in the literature, because as you stated, an 8-32 would certainly have trouble with 50%.

As a side note, viscosity is notably absent from the pressure drop curves in the TDM. It would be nice to have curves for 30% and 50% propylene glycol solutions also.