Thoughts

kpc_19

add a storage tank w/ a bronze circulator. it would be simpler control wise...kpc

Bob Forand

Modulatiing and ODS

Just thinking out loud here for a few moments and I am looking for your thoughts and or opinions.

Why use an outdoor sensor on a modulating gas valve ? Here is why and what I am thinking....

I have a Baxi HT330 boiler and I use the outdoor sensor with it. I oversized my radiation to heat at 160 degrees at -20 degree day. So in the average winter my water temps are low. Well now I have don the unthinkable. I have added a large tub to my home and want to use change to an indirect tank to recover the water needs. Well I have looked for the answer on how can I make the indirect a heat zone and use the ODS. I thought about using a relay to cut out the ODS when that zone calls but the Baxi uses the heating dial for it slope adjustment on the heating curve. That won't work... So now I am going to have to disconnect the ODS and move on.

But as I am thinking about this situation, I began to wonder what are the real benefits of using the ODS if the gas valve modulates on the return water temps anyway. The gas valve will operate on low when the temps rise outside because the radiation will only be able to give off so much of it's heat. Besides the initial heating of the water, why would the boiler use any more fuel than with a lower water temp ? There is very little water content to the system for the boiler to heat anyway.

I just don't see it...Am I missing something ?

hvacfreak

\" the heating dial \"...

...I don't know this product...but just wondering what that " varriable " is ( 0 -10 vdc , 4- 20 ma ? ). Does the manufacturer have a DHW interface for this ? Or any suggestions for a controller ? I'm thinking " making " a relay that switches to a sensor on the DHW return ( and possibly cycling on boiler high limit ). The manufacturer should have a solution for this.

Brad White_185

What you are describing

in your sequence of operation is "Domestic Hot Water Priority". The boiler would normally fire to track the outdoor temperature then when DHW calls, over-ride that to make HW for the relatively short time it takes. Then it goes back to monitoring outside air temperature reset (ODR). During DWH production the boiler fires to a limit, often 180F or 167 on certain German boilers.

I am not sure how that is instrumented on a Baxi, I will leave that to the more hands-on techs.

Now, there is one point we may need to clear up a bit. The modulating gas valve is a tool, mostly to achieve minimal cycling. This is secondary (but a very useful and efficient!) means of achieveing outdoor reset. In other works, you can do ODR with on-off fire (and a wider differential one hopes) but this is never as good as true modulating fire.

I might also add that your design in this configuration may benefit from a decoupling of the heating circuit. A 3-way valve for example to prevent the high DWH-generated temperatures from "spiking" the heating loop. Overall this may not pose much of a problem though, the time and water volumes are low.

To your last question, the boiler water content may be low but it is the entire system and the rate of water flow through it that has to be heated ahead of it's loss if that makes sense. By monitoring the return water temperature, the boiler can tell indirectly how much work it has done and down or up-fire accordingly. ODR will save money. It takes less energy to heat water to lower temperatures, simple as that. With constant circulation the comfort is also superior to on-off cycling with a constant temperature, but you knew that.

Maine Ken

That unit is the combo unit. I believe you need to add an indirect kit to it and your hot water needs will fire to full output. Goes around the ods barn so to speak.

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Bob Forand

Ken

Maine Ken, I thought that would be the case as well, but it wont work. The "sensing" of flow is different between the HT133 and the HT330. I even checked to see if there were minimal parts to change to have it work with the indirect kit.

As for the content of my system there is less than 300' of 3/4 tubing and the boiler water content. But I have also added a hydroair system that will require hotter water temps anyway.

I do recognize that I am trying to beat the system with a product that is not designed for this, so it is my bad.

But now that I have invested so much time in trying to find the way, I am questioning the ODR on my system. With so little water content, and the modulating gas valve, I wonder if I am really saving anything. It is like the other day, I had the heat off during the day because it was mid 60's. When I turned it back on in the evening, it took ver 3 hours to bring it from 63 to 68. That is because the boiler/ODS was doing its job well. Reducing water temps. Yes, I now know I should have let the system do it's thing, but this was a test for me. I needed to see. Now, how much fuel was consumed during that time, whether it was reheating or even if I had left it to do it's thing. How much would I have used if I did not have the ODS ? I believe I would have used less, the radiation would be hotter, and give off more heat. The boiler would have only spiked to high for a short time due to the water content, and then just modulated along until the heat call ended. So that is what I am questioning....

Bob Forand

Brad

Sorry Brad, I forgot, it is decoupled. The boiler works on a three way diverter valve for DHW. When there is flow it changes the valve for priority. But what I am trying to do is find a way around that flat plate HX.

That, Ken, is why the DHW kit won't work. The DHW kit would change the position of the diverter valve and flow would go through the plate exchanger.

Kevin, are you talking about piping it up the way System 2000 does it ? I went to a training seminar yesterday and I seen the way that they pipe up their tank. The only problem that I have with that setup is that the in the Baxi case, whenever there is a call for hot water the boiler goes to high fire and stays there until the call has ended, because of flow across the flow switch. The problem there, is that the boiler would fire during the whole time I am filling the tub, it would fire every time someone washes their hands, etc. That would cost me more in this application to run the boiler for DHW.

kpc_19

Bob,..

don't pipe it that way. Pipe it the same way you would pipe in a storage tank in a tankless water heater set up....try this link.
http://www.noritz.com/u/(3a1)N931M-DHW_ST.pdf