I'm getting a Bosch 4 ton system installed in my house in a few weeks. I was planning on using a buffer tank for my hot water...this made sense since the desuperheater only worked when the system was running and would therefore not heat the water to full temp, just "pre-heat it". I was then reading some info on the company who is installing my systems webpage: "When there is a call for hot water in the winter, half of the heat is diverted from heating the house for the first 5 to 20 minutes in order to bring the water up to temperature in the hot water storage tank. The temperature of the air exiting the registers will be cooler until the water temperature is satisfied because there is only half as much heat available. Once the hot water temperature is satisfied then all the geothermal system's capacity is dedicated to heating the home." I thought that the water was just warmed WHILE the heat pump ran (with some small proportion of the heat going to the hot water vs. home heating. However, this statement makes it sound like their systems heat the water to some temperature before switching all the heating potential to the house. Does this sound "right"...is this how all desuperheater systems are designed to run? What temperature do you set the buffer tank thermostat to in this scenario? Thanks for the help.

The desuperheater coil only uses a fraction of the total heat pump capacity. You would detect a slight difference in air temperature output during desuperheater operation, if any at all. It is true that it does take something away from delivering heat to the home but that is factored into the overall savings. You are still delivering BTUs that you are using and they are much less expensive than by fossil fuel sources.

What temperature do you set the buffer tank to...not the full temp that you would set your normal powered tank to, correct?

unless Bosch is different than other brands, you are right and the sales pitch is wrong. We will have to see if anyone here has more knowledge of how Bosch does a desuperheater.

Some desup pumps shut off when return water temp reaches 120F. This should be how Bosch works also.

Do you know the model number for the heat pump? Neither my spec sheet nor the installation instructions specify exactly how much heat my TA035 diverts from heating, but I know it warms the air up about 20 degrees in stage 1, which is up to specs, even though the desuper is running. I'd be VERY surprised if the sales pitch is correct.

Part of me hopes it works the way you stated though, as it might make it worth kicking the heat on for 20 minutes during the shoulder months to pre-heat my tank before the morning showers. Way cheaper to heat it with the desuper than the electric.

Check out this link

http://www.fhp-mfg.com/files/download/Catalog(Commercial).pdf

Page 46 has a chart listing BTU capacity for the different desuperheaters, which Bosch calls "Heat Recovery Packages". From what I can tell, assuming my 3 ton heat pump is the HR002, I should get just over 8K of BTU/hr. Stage 1 heating output shows 22K BTU/hr. Assuming (and I could be way off here) that 8K is being diverted, that would mean 1/3 of my heating output is being diverted. Not 50%, but much higher than I thought.

Someone please let me know if I'm reading that wrong.

I did a test overnight with my system. Ran the heat pump for 3 hours on stage 1 and it raised 40 gallons 30 degrees. According to my calculations (8.33 pounds per gallon X 30 degree rise x 40 gallons) I netted about 10,000 btu's. Averages out to around 3,300 btu/hour.

This would mean it is diverting about 15% of the btu in first stage and close to 10% in second stage, until the buffer tank reaches 120 degrees, at which point 100% of the btu's would go to house heating.

Disclaimer: I'm new to all this and just going off calculations I found online and some assumptions. I could be way off base, and if so I'm hoping someone corrects me.

Posted By khonderd on 06 Oct 2011 08:32 AM


This would mean it is diverting about 15% of the btu in first stage and close to 10% in second stage, until the buffer tank reaches 120 degrees, at which point 100% of the btu's would go to house heating.

 

this is what to expect from a  DSH

Posted By khonderd on 06 Oct 2011 08:32 AM

This would mean it is diverting about 15% of the btu in first stage and close to 10% in second stage, until the buffer tank reaches 120 degrees, at which point 100% of the btu's would go to house heating.

This seems correct to me as well. You have to remember there are 12K BTU's per ton, so at full performance you would get 36K BTU's not 30K BTU's. Second, if you are operating in first stage you should be getting 60-70% of the units capacity so 22K BTU's of capacity seems correct. Remember most charts and ratings are based on Enetering Water Temperatures of 50 degrees, in my climate we often see 40-45 degree temperatures so ratings and charts are not necessarily correct.

Posted By SkyHeating on 06 Oct 2011 04:33 PM

Posted By khonderd on 06 Oct 2011 08:32 AM

This would mean it is diverting about 15% of the btu in first stage and close to 10% in second stage, until the buffer tank reaches 120 degrees, at which point 100% of the btu's would go to house heating.

This seems correct to me as well. You have to remember there are 12K BTU's per ton, so at full performance you would get 36K BTU's not 30K BTU's. Second, if you are operating in first stage you should be getting 60-70% of the units capacity so 22K BTU's of capacity seems correct. Remember most charts and ratings are based on Enetering Water Temperatures of 50 degrees, in my climate we often see 40-45 degree temperatures so ratings and charts are not necessarily correct.

Geo units tonnage usually reflect the cooling capacity, the heating capacity is usually less. 3 ton units with closed loop are rated at 32F EWT, and usually put out a bit less than 30 KBTU/H. 50 degree EWT is used for open loops rating.

It's all going to depend on EWT. I thought maybe the OP was assuming 10k BTU per ton since a lot of my customers assume base 10 numbers for most things.

general concensus is about 10% of system capacity.
While this actually drives up heating cost of home, the savings to finished hot water cost is greater.
It is unlikely you will notice a significant air temperature difference without measuring equipment.
joe

Sorry Sky. Perhaps I should have specified that I was going off the heat pump specs. Before doing my research on heat pumps I didn't have a clue how many BTU's (hundreds, thousands, millions???) were typical for heating, let alone assuming it would have to be base 10.

Back to the subject at hand. I've seen a couple different experts recommend check valves between the pre-heat tank and finishing tank, and also between the cold input T and the desuperheater input. Is this something smb should be considering?

Absolutely, otherwise you might get reverse flow due to thermal siphoning.

I was looking at check valves last night and there are quite a few to choose from.  I'm assuming it needs to be metal to connect to my copper piping, but have little plumbing knowledge.  Does it matter what kind I get?  Any suggestions?

any that is approved for use with potable water is fine.
j

When looking into check valves I see I have the option of a swing check, or spring loaded check. One plumber mentioned that spring loaded is the one I would want because even if there isn't back-flow it will stop the heat transfer. Does this sound correct?

Edit: This is for a horizontal application between the preheat tank and the finishing tank.

We always put swing checks in. They will close too (gravity) if there is no flow.

Thank you, doc.  I hadn't gotten your message yet when I got to the hardware store, but after looking at them I came to the same conclusion you did.  Of course the fact that the swing valve cost half as much helped sway my decision too.