DHW BTUs

[dogwood]

In planning a wood gassifier system design approximately how many BTUs would you need to allot for recharging a standard 40 gallon DHW heater, and to keep it heated on average. There are only two people in our house, no children or teenagers, so the usage would be average I'd suspect. We do have a dishwasher and washing machine that seems to be always running. The DHW heater is an A.O. Smith propane fired model with R-8 insulation that I'll now be heating via a flat plate hx. Not sure how many plates I should get on that either. Any suggestions would be welcome. Thanks.

Mike

 

[jebatty]

Although not an exact answer, we too are a 2 person household, but use electric for DHW, using about 100 kwh per month, which breaks down to about 11,500 btu/day or about 500 btu per hour. IMO not worth considering use of a gasifier for DHW heating in this situation, except during the winter when you are using the gasifier for space heating. Keep in mind that 1 pound of wood produces about 6000 available btu's, give or take.

EDIT -- even during the heating season, the cost of a heat exchanger, plumbing, possibly an added circ, may make it unreasonable to use the gasifier for DWH heating. Our average electric cost for DHW is around $5/month. At that rate, it might take years to recover the cost to setup DHW through the gasifier.

 

[altheating]

$5.00 per month? What is your electric cost per KwH? We are paying 18.6 cents per KWH here in New York. I have customers who are claiming the reduction in DHW bills as much as $50 per month. If my monthly cost for DHW was $5 I wouldn't be running my gasser all summer. Dogwood, I have several customers on farms that use sidearms and plate HE's for making hot water in their milk houses, they are telling me that they are saving upwards of $600 a month on propane just for hot water needs in their milk houses. Electric here in New York (National Greed) is just not cost effective for the farmers. I have not seen anyone who is not happy with putting in a sidearm or plate HE for DHW.

 

[dogwood]

Thanks Jebatty. Maybe its not worth the trouble of hooking the DHW up into the system I'm working up figures for now. It seems from reading all the many posts on this site that hooking in your DHW to your system was a standard way to go. People go to a lot of time and expense to do just that. I'll have to think this over carefully as it is a lot of work too for maybe not enough gain. Any thoughts from those of you who have gone to the time, work, and expense. I am paying a flat rate of about $365.00 per month for propane for all the needs of my house now including heat, DHW, clothes drying, and cooking. I figured the DHW would be a larger portion of the total bill than just $5.00. My electric bill comes in at roughly $65.00 for everything else needing power. Maybe propane is way more expensive than electricity here. I'm not sure I'd even notice an additional $5.00 on the monthly electric bill. Maybe I should switch over to electric DHW. Food for thought. Thanks again.

Mike

 

[SolarAndWood]

We are paying 18.6 cents per KWH here in New York.

of which 13.6 cents is taxes

We go through 3 grill tanks of propane a month for DHW for a family of 4. That has been about $40 lately. While the investment in the DHW loop will take several years to pay off, it is a nice long-term hedge against inevitably higher energy prices.

Jim's point is well taken. If you aren't sure you will be in the house to get the return, you are probably better off with a good water heater. From my experience, people put little to no value on the expensive energy upgrades when they buy a house. For some reason, people would rather have a large poorly built inefficient house on a postage stamp lot in the right subdivision.

 

[pybyr]

Depends, too, on the age of the existing water heater- if it's old (and thus likely to spring a leak and need outlay of $$$ to replace) then suddenly the investment in the parts and work for wood-fueled DHW starts to look different and better...

 

[dogwood]

Altheating, we are paying 11 cents a kwh here based in my bill of $50.00 for 456kwh last month if I did the math right. On the other hand my propane rate is just being increased to $390.00 a month, I found out moments ago, reading through my bills to find out the kilowatt hour rate. I am heating the DHW with propane which is
$2.95 a gallon. I don't know what that would translate to for heating the DHW. Appreciate your input.

Mike

 

[SolarAndWood]

You are likely going to run into problems with your propane company when your boiler goes in and you stop consuming that volume of propane for space heating. As a result, I resorted to grill tanks until my boiler goes in.

 

[DaveBP]

I am heating the DHW with propane which is $2.95 a gallon

I just paid $4.42/gallon for bulk delivery propane this morning in S. Maine.

 

[Medman]

I have kept track of the difference in electricity usage when heating DHW with the sidearm and without. I have a new 38 gallon elec. HW tank, four people in house, average of three showers per day, and laundry. I estimate that I save about $40-50 per month when heating from the boiler.
Given the cost to add storage to my system (about $1000) and the cost of wood (which is not free for me) it would take about two years to pay off the investment at the current electricity rate. However, I expect a big increase in the cost of electricity when smart meters are installed later this year. Storage, even just to provide heat for DHW, is a big priority for me and will hopefully be installed by the fall.

BTW, Propane here in northern Ontario is running about $5.00 US per gallon on a minimum 500 gallon delivery contract.

 

[DaveBP]

Propane here in northern Ontario is running about $5.00 US per gallon on

Ouch, guess I'll have to lower the volume of my whining.

Whenever I've compared current prices here, I've found that propane is a lot higher than electricity. I use propane for cooking and DHW. Been using it that way since before central electricity came out our road. I like it because it works when the power is out and because a tankless, on demand water heater can make hot water all day long when my wife is washing wool. An electric water heater is cheaper to install and cheaper to run than propane but the lower duty cycle is a show stopper around here.

Beside the financial payback angle, I think there is an esthetic element to heating DHW with wood. Like when I ran my household on photovoltaics years ago; it was very expensive to install but it had a warm fuzzy aspect that didn't have anything to do with money. It just felt good to think about and to watch it working.

 

[Nofossil]

Propane DHW tanks often have a MUCH higher standby loss than electric tanks - harder to insulate well.

A good figure of merit is around 60,000 BTU/day for a family of four. I know ours is right in that range. Personal habits can make a big difference, though.

 

[jebatty]

Our hot water is off-peak (2 tanks, 50 + 80 gal capacity), heat from 11:00 pm to 7:00 am, just under 5 cents/kwh. We also have added 6" of fiberglass around the tanks, on top, raised them off the floor and insulated underneath, added heat traps, and insulated all hot water lines. Insulation is the cheapest energy saver there is.

 

[dogwood]

Thank you all, Jim, NoFossil, Altheating, DaveBP, Trevor, Medman, and Solar and Wood, for your replies. I think I will put in the DHW flat plate HX after all. If it is possible to save even $20.00 a month just for the two of us that will more than justify our goal of being more self sufficient. Since the demand for heat for DHW can't really be parsed out over the day as your home heating is, how big a flat plate HX would you recommend from your experience for a 40 Gallon DHW tank. I'm planning on putting circulators in to run the water both directions, one circulating the tanks water, the other the circulating the boilers water, through the flat plate. My goal would be to have the water temp in the tank recover as quickly as it does with the propane if that is feasible. I really don't want to go to the expense of getting the alternative of a Superstor or the Amtrol Boilermate indirect water heater, which is the other alternative I explored. I think I can size my circ pumps accordingly if I had this info in hand, and an approximate top figure of 60,000 BTUs per day in mind, as a guide in case we have guests. There should be plenty of hot water available either from storage or directly from the boiler, if it is burning, to run through the flat plate if that is a consideration. Thanks for your advice.

Mike

 

[Gooserider]

Thank you all, Jim, NoFossil, Altheating, DaveBP, Trevor, Medman, and Solar and Wood, for your replies. I think I will put in the DHW flat plate HX after all. If it is possible to save even $20.00 a month just for the two of us that will more than justify our goal of being more self sufficient. Since the demand for heat for DHW can't really be parsed out over the day as your home heating is, how big a flat plate HX would you recommend from your experience for a 40 Gallon DHW tank. I'm planning on putting circulators in to run the water both directions, one circulating the tanks water, the other the circulating the boilers water, through the flat plate. My goal would be to have the water temp in the tank recover as quickly as it does with the propane if that is feasible. I really don't want to go to the expense of getting the alternative of a Superstor or the Amtrol indirect water heater, which is the other alternative I explored. I think I can size my circ pumps accordingly if I had this info in hand, and an approximate top figure of 60,000 BTUs per day in mind, as a guide in case we have guests. There should be plenty of hot water available either from storage or directly from the boiler, if it is burning, to run through the flat plate if that is a consideration. Thanks for your advice.

Mike

I would suggest you look at NoFo's site for a description of how he does the DHW setup - seems to me like it would work with just about any plumbing setup, and his claim is that it essentially never runs out...

Gooserider

 

[dogwood]

Gooserider, last time I looked at his site Nofossil was using a Superstor Indirect water heater (and maybe utilizing solar too for preheating, if I recall). If my pockets were bottomless that's what I'd do. As it is, I can manage to go with a flat plate of unknown size as yet, rather than a sidearm. If anyone has practical experience with a flat plate hx for their domestic hot water I'd love to hear what size works or not for you. I've read everything from 30 to 70 plates are appropriate. That's quite a range in the amount of hx and cost. Thanks for your input.

Mike

 

[Gooserider]

Gooserider, last time I looked at his site Nofossil was using a Superstor Indirect water heater (and maybe utilizing solar too for preheating, if I recall). If my pockets were bottomless that's what I'd do. As it is, I can manage to go with a flat plate of unknown size as yet, rather than a sidearm. If anyone has practical experience with a flat plate hx for their domestic hot water I'd love to hear what size works or not for you. I've read everything from 30 to 70 plates are appropriate. That's quite a range in the amount of hx and cost. Thanks for your input.

Mike

I think you are correct, but that part of his approach is mostly irrelevant - you as I understand it, you can accomplish everything the Superstore does with a salvaged electric WH tank and some additional plumbing, albeit with a bit less elegance. What I thought was more interesting is NoFo's mixer setup where he uses two mix valves in sequence to give a more stable output temperature and maximize the amount of useable hot water seen at the tap from a tank of preheated water... IOW, I think the plumbing around the tank is more important than the nature of the tank itself.

Gooserider

 

[deerefanatic]

I for one am thinking of going the flat plate route. Right now I have a sidearm that I'm not happy with and going to be putting a pump on it. But, Anything over 30 plates for a flat plate I think is overkill. A 70 plate heat exchanger can heat 65 degree tap water to 125 degrees in one pass...... While using about 200,000 btus to do it! I think for use heating a tank, a 20 plate is absolutely perfect..... And much cheaper than a 70. Trust me, I know!

 

[dogwood]

Matt, thanks for the sizing information. With a 20, or even a thirty plate flat plate hx, do you, or anybody else, have an idea how long it would take to reheat an existing 40 gallon DHW tank, if the hot DHW was depleted after showers, dishwashing etc.; and the boiler heated water in storage is reasonably hot? I would like to avoid wifely dissatisfaction if the A.O. Smith 40 gallon DHW heater we have now in doesn't reheat in a reasonable amount of time.

Gooserider, I will carefully examine that two mixing valve arrangement. I was trying to dope it out last night when I reexamined Nofossil's site at your suggestion, but it got too late at night, and nothing was making much sense other than sleep. I'll look at it again today. Thanks

Mike

 

[solarguy]

BTU's per hour = gallonsX 8.33X the temperature degree rise(normally 90 - 100)

40 gallons @ 90 degree rise = 29,988 btu's per hr.

Bob

 

[dogwood]

Thank you Bob. If I was zoning the DHW line with a circ pump and also pumping through water through the flat plate HX from the DHW heater, it would make sense both these circuits would utilze the same size circ pumps. Is that a correct assumption? What size circ pumps would anyone recommend to circulate water through a thirty plate flat plate HX to heat the forty gallons of DHW in a reasonably short time, hopefully commensurate with the reheating time we get now using propane to fire the DHW. Thanks all for your help.

Mike

 

[solarguy]

The size of the circulators would be based upon your flat plate heat exchanger selection.
In all likelyhood you could get away w/ 3-4 gpm depending but I'd suggest obtaining that information based upon the heat exchanger specs.

Bob

 

[jebatty]

What size circ pumps would anyone recommend to circulate water through a thirty plate flat plate HX to heat the forty gallons of DHW in a reasonably short time, hopefully commensurate with the reheating time we get now using propane to fire the DHW.

The question you state is simple but the answer will take more effort on your part. You need to determine your domestic water supply temp, your desired domestic water temp rise, your boiler supply temp, and your boiler return temp. Typical numbers might be: 50F - 140F; 180F - 150F. You then need to determine the gpm that you want to supply to reheat your hot water tank. Based on this information, you need to look at the HX selection table for your desired HX to determine the gpm needed on the boiler side and the pressure drop (convert to head) on each side of the HX at the determined gpm. Then select your circs on both sides to provide the desired gpm at the determined pump head. Be sure to include pump head from the plumbing on both sides of the HX.

As stated by the prior post, a target gpm to reheat the hot water tank might be 3-4 gpm. If 3 gpm, and if the typical numbers apply, a 40 gal tank would reheat in 14 minutes. This is based on a big IF, though. Will your boiler always be maintained at 180F, for example? During the summer and other periods that you are not also space heating, maintaining a gasser at 180F to supply a small amount of DHW may involve lots of idling, even to the point of the fire going out, and a lot of inefficiency. Gassers operate best when they burn full-out until the wood load is consumed.

Think through your plan carefully. The concepts are fairly straight forward, but the devil is in the details.

 

[dogwood]

Thank you Bob and Jim. I guess more figuring is in order again. I will utilize the info you both provided and reapproach this problem. Planning this whole wood gassification project is really more difficult than I'd ever anticipated, probably because it is all new to me. I will persevere however until I keel over. Thanks again.

Mike