reverse indirect/buffer tank sizing question

Jamie_5

I am a DIYer attempting to design a combined space heating and DHW system. I would like someone to tell me if my assumptions are reasonable or off base.

I have done a room by room heat loss calculations and come up with an anticipated design loss of 35,000 BTUH. Peak hot water usage is in the morning, when people take showers. (We are a family of four.) This morning, for instance, my shower used about 23 gallons of water total, (hot and cold combined), and our 14 year old daughter used about the same amount. (I include her age because teenagers are reputed to be more profligate water users.) I didn't time her shower, but mine was between 8 and 9 minutes. So water usage for a shower is around 2.5 gpm.
The first assumption is that about 60% of that flow is hot water. If so, make the second assumption, that peak usage would be two showers running simultaneously for about 10 minutes, thus using about 30 gallons of hot water during that time.

The third assumption is that the hot water has to undergo a 90 deg. F temperature rise from cold to hot (from 50 F to 140 F). If so, it would take 22,500 BTU to heat that water, correct? (30 gallons at 8.33 punds per gallon times the 90 deg. delta T, no?)

Fourth assumption: one is using a reverse indirect buffer tank like an ErgoMax with a 48 gallon capacity storing water at 200 deg. F high limit and 160 deg. F low limit. Fifth assumption: the boiler is capable of producing around 40,000 BTUH, has a cast iron heat exchanger, and should be protected from return temperatures lower than 140 deg. F. Sixth assumption: the design water temperature for the space heating is just under 160 deg. F at design conditions and the space heating system is using constant circulation with variable speed injection connected to outdoor reset for the variable speed injection.

Is it wrong to believe that there are nearly 24,000 BTU available in the buffer tank/reverse indirect to heat hot water, since a 60 deg. delta T in the tank (200 deg. F down to 140 deg. F) times the number of gallons in the tank times the number of pounds per gallon equals that? Am I correct in believing that the domestic hot water capacity of the buffer tank/reverse indirect is sufficient such that I would (1) not run out of domestic hot water during a period of peak usage, (2) not drop the buffer tank temperature low enough to cause condensation on the boiler heat exchanger, and (3) would essentially thereby prioritize domestic hot water production over space heating for the period where the buffer tank temperature drops below the temperature required to handle the design space heating load?

Please tell me if my reasoning is faulty or correct. Thanks.

PJO_5

Maybe I can help a little...

I am not a pro, but have an Ergomax for a different use...and am pretty familiar with them.

A "48 gallon" storing capacity is for the bolier-side water, and not your domestic hot water. The Ergo has three or four sets of coils inside this boiler water. It removes the issues of storing water/legionella/etc. that are typical with tank-type HWH's and indirects.

It will also help your boiler if it tends to short-cycle often, because you just added 23 gallons to what the boiler sees flowing through it.

I believe an E23 would probably work for you, but maybe an E24 due to the teenager :-) (it has the fourth set of coils). It will hold 23 gallons of the boiler water...and you probably won't need anything larger as long as the boiler can handle the load.

Speaking of the boiler, you stated a 22,500 BTU load for domestic...but in ten mintues. That equates to a 135,000 BTUH load for a boiler...because it's over the course of an hour. I believe (an indirect or) an Ergo is great here because you have, say, 36 gallons of hot boiler water (counting your boiler and piping + the Ergo) at 180F and if it goes to 140F you are already about 12,000 btu's "ahead" without any heat input from the boiler. If you had it set to 200F, then you are another 6,000 btu's "ahead".

For domestic, you could use the Ergomax and put a tank after it if you are worried about supply. If you only have a 40,000 BTUH boiler that might get a little cool if the dayghter decides to double her shower time :-)

Also consider a GFX heat exchanger. They hook to the shower drain and preheat the incoming water to the domestic. I don't have one, but they look pretty cool, and the Wallies that have them like it.

If all your space heating is 160F at design and the boiler up to 200F, you need to think about mixing down the temps. especially because you won't see 160F except maybe 10% of the season. This is where you night consider a larger indirect or Ergo, and keep the boiler at 160F on the high limit.

Just some thoughts...hope this helps. PJO

Jamie_5

Thanks for the response. I was aware that the water stored in the Ergo is boiler water, except for the minimal amount in DHW coils. I also understand that the 22,500 BTU in ten minutes would equal a 135,000 BTUH load.

Your suggestion about being 18,000 BTUs "ahead" is basically what I was trying to talk about, in my own simple, humble, faltering way. Basically, I'm wondering if the energy stored in the form of 200 degree water in the Ergo as a buffer is sufficient to handle what I've calculated as a peak DHW load without adding a standard indirect as well and without massively oversizing the boiler or reducing the return temps to boiler too low.

By the way, under this scenario, I am planning to have the space heating as a separate loop connected to a small primary loop on the Ergo/buffer, with a variable speed injection pump hooked to a reset controller to modulate the water temperature to the heat emitters. The buffer tank/Ergo would serve as the heat source, and hence would have to maintain at least 160 deg. F to handle design conditions.

Does this make sense to you?