Long story short I am making progress on my project here and wanted some opinions :

I am leaning towards having a packaged unit for my first floor in the basement
I am also planning to go with a split system for the second floor that would have the compressor in the basement as well combined with the fan powered box in the attic.

Would you recommend I get both with the desuperheater?

I am going to use my existing 50 gal electric water heater as a storage tank combined with a tankless water heater for our hot water needs.

a couple ways I was thinking :

1 - order both with the DSH and rotate through them periodically ( seasonally, annually or monthly )
2 - order one unit with the DSH and use it for the life of the unit.
3 - Order both units with DSH and have them both feed the storage tank

Any ideas / suggestions on the best way to accomplish my goals and have an efficient system?

Thanks again for all the help!
Dave

I would personally hook up both desuperheaters to the storage tank, during summer the upstairs system will run a lot more so you would want that connected for summer water and during the winter the downstairs system will run more so you will especially want all the hot water you can get during the winter.

I am starting a job like this coming up in a few months and that is how we are hooking up the system, package unit for downstairs, split for upstairs, two desuperheaters to one storage tank and a tankless to the storage thank.

Both units, and have them both feed into the tank.

Two desupers feeding just one 50 gallon buffer may be overkill

How many people live in home full time (current and forecast)?

What are the expected unit tonnages?

You can feed the desuper output from two units into a single tank. Just be sure to include check valves (swing checks work) to keep the active unit from backflowing warm water into the inactive unit during times when only one compressor is running

Thanks for all the replies - Good call on the check valves Curt!

There are currently 4 people in the house and the expected tonnages are 3 down and 2 up.

Looking into tankless water heaters they require a delta t of 30 degrees to startup - so if the DSH is running the tankless should not fire untill the storage tank is depleted - correct?

DON'T specify a tankless that requires a 30*f delta T to go with a DSH buffer...not all do. You need one that is compatible with warm - to hot inlet water. DSH is rare enough that water heater dealers (and most plumbers) will stare at you open-mouthed when you try to describe it. Solar hot water is more common, and imposes a similar requirement for flexibility on the part of a finishing heater, so you want a solar-ready tankless. (Actually, I dislike tankless altogether, but grudgingly admit they have their place in certain limited applications...saving money isn't one of their strong points unless the alternative is a center flue gas fired storage heater)

If you have one that is set for say 125 and it needs delta 30 to start, and the buffer feeds it 97 degree water, it won't run and showers will be tepid.

You could start with a 50 gal buffer and step up to an 80 when the 50 fails, but for 4 people, 80 gallons is the better choice.

Thanks Curt -

just to make sure I understand correctly - I need a water heater that will work with less of a delta difference to eliminate a lukewarm situation - correct?
Thanks

DJV, just confirming that you understand that the 2 desuperheaters should be plumbed in parallel (not series.) This is part of the check valve installation, but I still thought it best to mention to make sure your system is plumbed properly the first time.

A 50 gal tank will run hotter and respond quicker, 80 gal might never getup to temperature. Where are you located? What climate? 30 F delta is too much. Here is an example
http://welserver.com/WEL0478/

Instead of a tankless you may want to look at the AO Smith Vertex water heater, it is a 50 gallon tanked water heater but it is 96% efficient with a 164 gallon first hour recovery rating and its 100K BTU's. Its like a tankless in its recovery but like a tank in its storage capacity.

I agree that a smaller tank will be more responsive, but I advocate larger preheat tanks so as to best pre-position a day's worth of hot water upstream of the finishing heater so as to best make use of the fact that heating and cooling peaks occur at different times of day from peak hot water draws.

http://welserver.com/WEL0337/ 4 person household
http://welserver.com/WEL0603/ 2 person household


50 gal preheat, 50 gal finishing tank, buffer tank recovers within 4-6 hours after morning showers in 1 st stage, then DSH runs at idle, giving hottest water during day, larger buffer tank would never get up to temp. Dynamics change in heat dominated climate and units designed for aux heat when they run full time in first stage relatively early. No need then for a days full of hot water when the tank recovers fully after 4-6 hours.

In a cooling scenario, max production is late afternoon / early evening. If day's hot water draws are split between evening and morning, then I think a larger tank will work better.

The heating situation you describe makes sense to me as well.

Is one better off with 80 gallons of 100*F water vs 50@ 120*F ???- I think the answer would depend on the circumstances of the application.

If you are in a heat dominated climate, max hot production is during the night, so the buffer tank is hot and ready for the morning showers. But even in first stage for the day it recovers quickly. I took a shower this morning, and the DSH was running for 1 hour to bring the tank up to temp again. My buffer tank runs hotter than my seconds tank (140F), so it makes up for my stand by heat loss. My indirect water heater has not turned on since December, all the DHW is made via DSH. I have a wife and 3 teenage daughters....It took me a while to figure this out, and Curt is right, it depends on the application. But the WELs have helped me realize that the larger tank do not get up to temperature, so the second electric tank turns on the heat element more.

There are several more variables in play

1) system tonnage vs household size. More tons mean more recovery; more people mean more hot water demand. I've worked with as few as two tons serving 5 residents but also 5 tons serving two residents. (Guess which house has a 130 - 150*F preheat tank all summer)

2) System sizing vs load. Up north it is common to somewhat undersize systems relative to the heating load since aux strips easily make up the difference during cold snaps. That means the proportion of time spent in high stage is substantial. Down south, since there is no such thing as an aux cooling strip, enough tonnage has to be specified to meet ALL the cooling load. Basically that means that while making tonnage judgement calls, we round up when northerners round down. Thus, our systems run in low stage (heating and cooling) much more often. In low stage both saturation temperature and compressor discharge superheat are substantially reduced. Since in low stage all we can make is 100-110*F water, we might as well make a lot of it, so we install larger preheat tanks.

3) Entering domestic water temperatures - For argument's sake suppose the goal is to deliver 125*F hot water. Up north entering water temp is 50, so a preheat tank at 100 has provided 2/3 of the heating energy. Down south entering water temp is as high as 75, so a preheat tank at 100 has provided only half the required heating energy.

Curt

1) Sure.
2) I would actually argue that point. If you use larger equipment, it cycles more in lower stage, but also the DSH has more capacity. If you have ever seen the effect of a 10 ton heatpump on a 50 gal buffer tank, it does not has to run long, and that buffer tank is very hot, even if the heatpump cycles more. The HP might cycle more, but the total amount of BTUs are the same.
3) Low 40s right now in our area. Since the delta t the DSH has to make up (40F ->130F) is very large, it is important to make the tank not too large, otherwise it will not get up to temperature. Dealing with 75F entering domestic water would be a dream, a luxury we don't have.

Again I'm reminded there are so many ways to accomplish ones heating, cooling and hot water goals and so many regional variables.
I lean toward the 50 gallon buffer, but get Curt's strategy.

Posted By joe.ami on 02 Mar 2012 08:11 AM
Again I'm reminded there are so many ways to accomplish ones heating, cooling and hot water goals and so many regional variables.
I lean toward the 50 gallon buffer, but get Curt's strategy.

Dito