DHW tank temp 140* but delivery temp low

Steelz

I have a Viessman Vitodens 200 boiler and a VitoCell 300-53g DHW tank. System is set with circulation pump. I have the system serviced regulary and the contractor isn't giving me any assistance. So, here I am.



Teperature is fine at locations where high flow is not required. If a shower or tub faucet is run the water goes luke warm in about a minute. The tank still reads 140* on the tank attached thermometer. If the high flow circuit is run continuously I will get variable temperature surges, but not very hot.



There's a valve connected to both hot and cold water close to the DHW tank. It's an ASSE 1017 mixing valve. That has shown signs of leaking in the past. Looks dry now.



Would that be the culprit?

meplumber

That would be where I would start.

The valve that you are referring to is a thermostatic mixing valve that takes the 140 deg water in the DHW tank and mixes it with cold water to lower the dhw supply temp to the fixtures to the allowable 120 deg F.  Under high flow conditions, sometimes the element in the mixing valve begins to malfunction. 



It is normal an easy change out for a pro.



Good Luck.

Fortunat

also consider the recirc line

I think you mentioned that the system is set up with a DHW recirc line.



If that line doesn't have a check valve (or has a sticky one) and the pump isn't always running, you can get cold water coming backwards up the recirc line and mix with the hot, thus giving you bad temperature control.



It should be easy to test by just isolating the recirc line and seeing if the problem goes away.



~Fortunat

www.revisionenergy.com

Mark Eatherton

Other mixers....

All new shower and tub filler faucets come with an internal mixer built into them. If you have and can keep good hot water to a lavatory faucet, It may not be the mixing valve at the tank that is bad. You may need to have the plumber adjust the mixer at the point of use.





ME

meplumber

I thought about that Mark.

But if it is at all the high flow fixtures and not just one, then I tend to lean toward the central mixing valve.

Mark Eatherton

Easy enough to check...

Get a strap on thermometer and put it on the outlet of the primary mixing/anti scald device, and if the temperature drops during shower/tub flows there, then it needs a rebuild kit, or a cold water choke...



ME

CMadatMe

Thermometer?

Vitodens 200 uses a DHW sensor. Have you ohm'd it out to make sure it's reading properly. Once that tank comes up to temp it holds it for a long long time and that is a very large coil. Tank should draw 90% before recovery so the hot water is there provided it's made.  Make sure the sensor is all the way up in the well. The well is rather long. Is this something that has always been a problem or something that just started? Is the indirect piped on the boiler side of the LLH? What size Vitodens 200? Is the Vitotronic control on the boiler controlling the re-circ pump? What size pump are you using for the tank?



Can you post a pic of diagram of the install?