DHW - pre-heat, or just zone

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Vande

Vande

Member
Apr 4, 2009
38
Northern NH
Hey Guys,

Been on this forum for some time and have tried to digest everything available. I am working towards installing a gasser this year. One question that I am trying to answer is would it be beneficial to install a HX in a un-pressurized storage for preheating dom H20, or since my system is already basically hot water based and I have a indirect water heater, why not save the costs of the HX and just use the zone as designed. When the wood boiler is running, it is just another zone calling. When the boiler is not running, it is just another zone to be satisfied from the storage. I understand the one difference is the pump on the zone running, but cannot find any other differences. For the price of installing copper exchangers in the tank, I can buy quite a bit of electricity. Thanks for the feedback, I am sure I will have more questions as I make my way towards the light. And yes, I already have wood css and I should have more coming after Road Bands are lifted
 
I specify indirect for wood and gas boilers daily. They don't seem to be popular with the solid fuel crowd, but an indirect water heater is the perfect solution to domestic hot water. Super-insulated, commonly available to 120 gallon and as you say, just another zone. You can make DHW once a day when the boiler is hot and use the DHW all day long until the next batch feed. 30 year life, super-insulated, no-brainer.
 
What if your wood boiler fire goes out or you just don't want to burn it for a while in the summer - can you get indirects with electric elements in them too?
 
Thanks for the replies. I should probably explain a little more. I currently have a oil burner with a indirect hot water heater, works good. I want to install a wood boiler later this year and I am getting started on designs, boiler selection, piping, controls etc. I have seen many installs on this site, and many who have unpressurized storage pipe their incoming domestic water through a heat exchanger in the storage tank before it goes into the hot water heater. If the storage is hot enough, the hot water heater aquastat would not call for heat and the water would pass through to the point of use. What I am curious about would be, not installing the pre heat domestic HX in the tank and just using the current domestic hot water zone for the heat. I understand, not preheating the domestic water would require the zone pump to run when there is a call from the aquastat. We currently do not run out of domestic hot water, the tank is 70 gallons, so the oil boiler obviously keeps up. The question I guess I have is, does anyone pipe this way without the pre-heat, and does it work consistently, or on high demands, does the fossil fuel kick in. Thanks for the help. Sorry if I am confusing, I am not sure I have all the vernacular correct.
 
Vande said:
Thanks for the replies. I should probably explain a little more. I currently have a oil burner with a indirect hot water heater, works good. I want to install a wood boiler later this year and I am getting started on designs, boiler selection, piping, controls etc. I have seen many installs on this site, and many who have unpressurized storage pipe their incoming domestic water through a heat exchanger in the storage tank before it goes into the hot water heater. If the storage is hot enough, the hot water heater aquastat would not call for heat and the water would pass through to the point of use. What I am curious about would be, not installing the pre heat domestic HX in the tank and just using the current domestic hot water zone for the heat. I understand, not preheating the domestic water would require the zone pump to run when there is a call from the aquastat. We currently do not run out of domestic hot water, the tank is 70 gallons, so the oil boiler obviously keeps up. The question I guess I have is, does anyone pipe this way without the pre-heat, and does it work consistently, or on high demands, does the fossil fuel kick in. Thanks for the help. Sorry if I am confusing, I am not sure I have all the vernacular correct.
Click to expand...

I kept my entire oil system intact. 80 gal indirect and all. I have pressurized storage, and let the indirect zone heat the DHW just like it always did. If the top of storage temp falls below 135.. any zone calling for heat will trigger the oil boiler. Above 135.. the circ from storage to my injection loop comes on instead.

Seamless switching between wood and oil.

JP
 
I always build in redundancy and auto changed over, it's just to simple to do in most cases why not.. I know when I have been lazy because I hear my furnace come on. Think it's currently set at about 125F water temp
 
Thanks JP and BB. I think I kind of get it. If it works and is not broken, leave it alone. Along with, if you are repiping everything, which i will be, putting in redundancy makes sense. I guess for me, since I am planning on non-pressurized storage, building/buying another HX will costly. If I have no power, pre-heat will not make a difference to taking a shower since I have a well and need to provide power to it. If I had muni water, that may make a difference. I would need to get power up and running to have hot water and just water. So for budgeting, I guess I will not plan on paying for HX for dom H2O. Thanks. Van De
 
I use a Geyser heat pump to heat my 350g Heat Bank tank in the summer.
We originally had it tied to an 80 gallon foam insulated stainless steel Amtrol tank.
The electric use has been the same per month. About $20. (at 18 cents/kwhr)
I do like having 350g of 120F hot water in the summertime and the heat pump keeps the basement dry.
Since this is a small house (1100 sq.ft.) and the basement walls are foam insulated, it tends to cool down the first floor.
This is a plus in the non-space heating months.

In the winter, my DHW hx draws directly from the tank. Since the house is R-65 in most places, my dhw load is more than space heating.
 
can the geyser unit handle a 500 gallon storage tank in the summer for dhw?
 
Answering original question: Indirect with preheat is best of both worlds. Need at least one tempering valve since preheat might be too hot. I use two mixing valves. Here's how mine is set up:

hotwater_diagram.gif


I also 'superheat' the DHW tank when burning wood - I run it up to 160. I have a writeup on the evolution of my DHW solution here.
 
Nofossil - ya mean really "hot" water don't you. Not superheated. Sorry couldn't help myself.
 
I've got dual 119 gallon indirect water heaters. Only been using one in the winter, but I can flip a switch and charge both up to 160-170 for summer time. I can easily go 5-7 days off the two tanks. But standby losses become an issue so we are not shy about using copious amounts of hot water.
 
The Geyser seems to maintain big tanks well.
I think the functional issue here is that the tank starts off with a very low heat loss.
Our tanks are 4.5" of foam on the sidewalls and 4" on the top and bottom.
Beyond our own testing, we seem to keep getting good reports about low heat loss when people other than us are assembling them.

That being said, we are only heating to the Geyser "sweet spot" in terms of output temps which is 120F.
We can deliver hot water via a hx all day at that temp.

We have had folks who have done this and are reporting my experience as well.
The Geyser is merely topping off the usage on a daily basis, so it is not starting with a cold tank.

It is only delivering 8,000 btus/hr, but again if it is only restoring DHW used, it seems to fit well.
Bigger output units are coming. I would not use them or feel they are necessary in this case.
 
I'm debating doing what Tom's talking about, in fact I've been bugging him for a little while about it.

My plan would be to use the Geyser to heat my 820 gallon heat bank. I use a 35 gallon indirect currently and the great experiment would be to see how well the two played with each other. Adding a DWH to my 820 gallon tank as NoFossil suggests would be the best of both worlds. If I over ran what the Geyser could handle then the oil boiler would kick on and make up the difference.

maple1: my plan was to use dip tubes in my 820 gallon tank for the Geyser.

As far as the indirect, it works well when I'm running on wood. Depending on how much hot water you are using it can put a decent load on the boiler/storage though.

I need to hit the lottery so I could do all these things to save money. :)

K
 
I've been very interested in the Geyser since I first read about them on here while researching my boiler project a couple of years ago, and still very much am. I think if I had one, and it could keep my storage warm enough to handle all my off-season DHW needs, I wouldn't even bother burning wood during those times. The rest of the house would benefit from cooler temps & lower humidity levels during the summer, and it should (from what I've read) cost less to operate than using my electric hot water tank during that time (which I think would run me around $30/month @ about 0.16/kwh). Plus save some on wood use. I'm pretty sure I could plumb one into my system now without a great deal of hassle, if it's not a bad idea to plumb directly to the system/boiler water - I would 'just' need to justify the cost of the Geyser itself. I'd feel a bit more comfortable about considering that situation though if the Geyser could output just another 10° more or so - around 130°.
 
The Geyser can top out at 130 deg and come preset at 125 deg. At least the documentation I have states that.

Not sure what the trade off is in power usage for those extra 5 to 10 degrees is though.

K
 
Those of you with open storage ought to be able to add a solar panel or two pretty easily and keep your storage warm. Not sure how the cost would compare to a geyser.
 
mikefrommaine said:
Those of you with open storage ought to be able to add a solar panel or two pretty easily and keep your storage warm. Not sure how the cost would compare to a geyser.
Click to expand...

Yeah, that's another thought I had. I think you could build and install your own panels for about the same cost as a Geyser.

The trade off is the Geyser would be almost plug and play (easy install) and you get the benefit of dehumidification. You do have ongoing electric costs.

Solar would be fun but a lot more work to install.

They both have good points for sure. I'm still going back and forth on what I want to do.

K
 
Sorry I have not checked up on this.
The Geyser can circulate unpressurized tank water directly. The unit has a circulator built in.
I just put two lines into the tank, one high and one low and set up the plumbing so it can be purged of air.
It is very simple.
Granted, the unit costs about $1000, but it dehumidifies the space it is in. So, if you are running a dehumidifier in the summertime, the DHW is free.
 
There are no issues tying to a pressurized system. It is made to hook to a DHW tank. Same thing, different water.
 
Thanks guys. Took me some time to digest all, and figure out what a geyser is. NoFossil, thanks, checked out the link and I think I understand your DHW configuration, took me some time to understand the two mixers, but I think I get it. But I still come back to the original question, why spend money on a HX in a storage tank when the zone can satisfy the demand. I get,'best of both worlds' philosphy, but I only have limited funds in one world. I also understand the super heating during boiler running, and I think this control is within my reach. But from a preheat point of view, all things being equal, the BTU 'cost' is the same whether the dom H2O goes through the tank, or the zone needs heat from the tank, if transfer efficiency is the same from either HX. What am I missing in my simple brain?
 
Vande said:
What am I missing in my simple brain
Click to expand...

You're absolutely right - a BTU is a BTU. Here's my thinking: In the summer, I heat storage with solar panels, and the top foot or so of storage gets pretty hot - the rest doesn't. The preheat coil allows me to heat 55::F well water with the cool-to-lukewarm water in the bottom 2/3 of storage, without messing up stratification. I'll actually get the bottom of storage down to 70::F or so, while the top is at 150.

During the summer, I almost never have the circulator come on.

If I have to turn on the circulator and heat DHW using the indirect coil, the return water (from the DHW to the bottom of storage) will be WAY hotter than 70::F, even though I run the circulator at slow speed to maximize delta T in both directions. That has the effect of warming the bottom of storage and cooling the top - not what I want.
 
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