DHW pumps for a Viessmann & VITOCELL system.

Tonerlow

Question.
Will a grundfos 26-99 pump (33GPM) create more heat transfer to the DHW tank than the grundfos 15-58 pump (17GPM) that I currently have set up to it? Will it be noticeable enough to justify $500 for the bigger pump? Boiler is a B2HB 94 (94000btu) 

Larry Weingarten

Hi, It's not a direct answer to your question, but what pipe size are you dealing with? 33 gpm looks like you would want 2" copper pipe to keep velocity under four feet per second.

Yours, Larry

heathead

Look at the pump curve for that boiler it's in the manual. Most Vitocell's tanks have a very low head pressure. What is the current piping size and distance for the Vitocell. You don't want to exceed the GPM thought the boilers heat exchanger.

hot_rod

How long has the tank been in, if performance has dropped, the coils could be scaled.

Running the tank at a higher temperature and using a mix valve can add some additional drawdown capacity.

Tonerlow

This is piped with 1” copper. I already have the storage temp set to 60 c so the mixing valve idea won’t add any additional capacity. Right now just kind of wishing I had went for the 53 USG tank. The 42 USG was the only one my contractor could find during Covid supply chain issues when we did the changeover (DEC 2020)

GGross

At some point you will be limited by the boiler capacity, and would need to maintain max/min flow requirements on the boilers heat exchanger. I don't think the 26-99 would run over the max flow rate but I also doubt it would get you much more capacity. going from 13GPM to 22GPM given the boiler output temperature limit of 176f and the BTU limit of 94k, according to the viessmann charts that would get you an extra 4 gallons per hour. You should have a first hour rating of around 200 gallons (or a bit more depending on the exact scenario). I am wondering if there is a "DHW OFF" time set incorrectly that is causing you to run out, or if you are really using that much water.
The setup should produce a substantial amount of water, maybe something isn't piped correctly? Ideally the IDHW will be piped at the boiler loop, with a separate pump, the viessmann sensor, and priority settings enabled (the control scheme is correct by default)
sometimes I use this DHW sizing tool to get an idea of what my total requirements are, you can input fixtures and adjust the DHW peak load time, it is not perfect but may give you an idea of what your actual requirements are so that if you ever do want to make a change you have a point of reference to start with

select "private residence" for building type
https://www.vitoteam.com/dhw_sizer/

hot_rod

Viessmann has excellent documentation, the tank spec sheet may show tank performance at various input flow rates. Sometimes the show pump model numbers also. The tank can only exchange what the boiler can provide 

1” pipe should handle 8-10 gpm and easily transfer 100- 120,000 btu. What size boiler?

get a 5 gallon bucket and run a drawdown test

Alan (California Radiant) Forbes

As many have noted, Viessmann indirects have a very low pressure drop. Your 42 gal. unit will have a pressure drop of 1.0 feet at 9 gpm which is hardly anything. Add that to the pressure drop of the boiler (if pumping directly through the boiler) and piping for a final pump selection. I'm guessing that the 26-99 won't show any improvement in recovery.

Tonerlow

@hot_rod 94000 BTU I believe. 200-w B2HB 94

Tonerlow

@ggross removed the tank cover tonight. I discovered that I was able to drop the temp sensor in another 2” before hearing it hit the bottom of the “well”. I’m wondering if this was causing a delay in the boiler starting to make DHW. One day when the plumber was here last it seemed to take a long time running hot water before the boiler kicked in to recover it. I will do some investigating here with opening taps and measuring the time it takes to start DHW production. 

Tonerlow

if you want an idea of the guys who installed this. When it was first done I was looking at it and the Outdoor temp sensor said it was -6 Celsius. When I knew full well it was below -20 outside. So I went outside and discovered they had the temp sensor mounted under the deck about 2’ from the dryer vent. To make matters worse UNDER MY DECK IS SHEETED IN 

one of the 2 techs who installed this in Dec 2020 was here on a service call a couple weeks ago. he walked in to the boiler room and said “Nice boiler who put this in?” 

Anyone want to move to Canada and run a small town plumbing shop out of business, it wouldn’t be difficult 

hot_rod

1” copper with that 15-58 should move all that boiler has to give, for btus.

Main thing with sensors in a well is that they have good contact to the well. Transfer grease is important

I've cut open thermistor type sensors, the sensing is right near the tip. But if the sleeve is in tight contact they will sense properly even if not pushed tight to the end.

The wells themself can get scaled over and have an impact on the accuracy

Pull the sensor out, warm it carefully with a lighter, see how the control responds. That is one way to troubleshoot thermistors and controls

Alan (California Radiant) Forbes

@hot_rod said

Transfer grease is important

I've tried to put transfer grease in a long well. By the time the sensor reaches the end, all the grease has come off. I no longer try. Maybe there's a good YouTube video that shows how to do it?

You should be able to reduce the differential on that sensor in the Viessmann programming.


The service manual for your boiler is here.