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Hot Water Storage Tank vs Indirect Water Heater
wklopf
Member Posts: 44
I need to replace an old gas-fired boiler and stand alone gas water heater. My reading tells me that using an indirect water heater with a condensing boiler is an efficient way to generate domestic hot water. However, since there are combination boilers available which have a tankless coil, I am wondering how using such a boiler with a hot water storage tank would compare. My understanding is that the storage tank would have an aquastat so that the temperature of the storage tank would be maintained. Any thoughts? Thanks.
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Comments
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Sorry, that post was in the wrong thread."If you can't explain it simply, you don't understand it well enough"
Albert Einstein0 -
The heat loss has been done, and boiler size has been determined. The boiler will be modulating. Tankless coils and indirect water heaters are for heating domestic hot water.0
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There are lots of good high efficiency options out there.
More info on your heat loads and hot water needs would be needed to make a recommendation."If you can't explain it simply, you don't understand it well enough"
Albert Einstein0 -
The calculations say that it requires 85,000BTUs for the central heating. There are two full baths plus dishwasher and automatic washer. It is a rental house, currently vacant, so water use is difficult to predict. The three proposals which I have gotten are for systems of 110,000, 140,000, and 150,000 BTUs. All of these would be using an indirect fired water heater. I am curious as to the performance of a boiler-indirect water heater compared to a boiler with tankless coil and hot water storage tank. Has anyone had any experience with this setup?0
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If you have the space, I would install a mod/con boiler with an indirect.
The combi boilers tend to be oversized to the heating load in order to cover instantaneous hot water. This increases maintenance and reduces overall efficiency. They work well for some applications and in cases where boiler room space is a premium."If you can't explain it simply, you don't understand it well enough"
Albert Einstein0 -
The best choice for your boilers would be to match it to the heating load, your smallest choice. The 85,000 is for only say one or two days a year and most likely still over sized. With the heat loss it is the max that the house requires to maintain the temperature. Most houses have some mass i.e. the house and stuff in it that keeps them from dropping temp to fast. Then figure out the size of the indirect tank needed with that boilers recovery btu's going into the tank, to satisfy the domestic hot water needs of the showers and baths. The indirect will have tempering valve so you could always store high temperature water if you where running out of hot water, but that comes at a cost because the mod con boiler wont condense at it highest levels with the elevated temperature supply the indirect coil.
My only concern when sizing a big indirect on a small output boiler is to makes sure it doesn't stay on supplying the indirect for more than 30 min. Because anything after that then the house might feel cold, with the heat off because it is supply the indirect tank. With all that said if you have a Whirlpool tub or car was shower then I might consider larger boiler.
Back to you question, a boiler with a instant hot water generation is most likely done with plate ex changer. I feel it has more chance to scale up vs the indirect because the higher temperatures that the boiler needs to supply that plate heat ex changer for supplying the hot water. I would also think that the indirect would have better more reliable water temperature supply, both in supply and maintenance and parts. Also no cold water sandwich with indirect vs plate ex changer?
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Thanks for the information. I understand the scale concern. Apparently, the higher the temperature you run the boiler, the larger the scale problem becomes. I will check on the hardness of our water supply. I was looking at the installation manual for the HTP stainless steel indirects and they say that their 45 gallon unit can use 141,000 BTU/hr with 180 degree water. I have purchased a HTP 140,000 BTU with the tankless exchanger. It modulates down to 28,000. When I look at the plumbing diagrams for the indirect heater using boiler water vs the storage tank being heated by the tankless heater, the second one looks a more simple. I have been told by HTP that this boiler can be used with an indirect heater also but they did not provide any more information than that.0
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Why would you buy a 140,000 BTU boiler for your heat load?Unless you have a fairly small DHW load, you will be oversized for the heating and under for the DHW.
The indirect that can handle 141 will also run great (more condensing) with 85.
How many heat zones do you have?"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
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with a rental it would be best to have an indirect storage, wouldnt run out of hot water this way and keep the tenants happy"The bitter taste of a poor install lasts far longer than the JOY of the lowest price"1
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The indirect is your best plan. You could do the "tankless coil" w a storage tank but you would need to use a bronze/ stainless steel circulator to move the water though the coil... That's not cheap. Furthermore some of the plate heat exchangers have a pretty steep pressure drop. That means you need a high head version SS/ bronze circ....that down right $$$.
Keep it simple.
Get the properly sized Boiler and a decent sized indirect.
OR you could put in an electric water heater for each unit and have them pay for their own hot water....
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If your heat load is 85k that boiler will be way over sized. Can't wrap my head around why you would do all the calculations and then ignore the numbers. Sizing the way you are is equivalent to guessing.wklopf said:I have purchased a HTP 140,000 BTU with the tankless exchanger.
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If heating was the only concern, then I would have bought a smaller boiler. Adding on the DHW function changed my thinking. A stand alone tankless water heater would be in the range of 140,000 and up. Combining the two functions simplifies the system. I had had some heating contractors look at the job. They took so long getting back to me with their proposals that I assumed that I was not going to hear from them. I bought the boiler. Then I got the three proposals. As I said previously, one for a 110,000 system, one at 140K (actually using the same boiler which I had just bought) and the final one at 150K. So, if I am guessing, so are the pros who looked at my needs and came to the same conclusion that I did.KC_Jones said:
If your heat load is 85k that boiler will be way over sized. Can't wrap my head around why you would do all the calculations and then ignore the numbers. Sizing the way you are is equivalent to guessing.wklopf said:I have purchased a HTP 140,000 BTU with the tankless exchanger.
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That's not how you size it.
You are comparing apples to oranges. For an indirect like you are doing a better comparison would be to a tank water heater which runs in the 40-50k range, and keeps up just fine. The 140k instant water heaters you are comparing to, function completely different from what you are doing so they need those high inputs.
You should have sized to the heat load. And yes those contractors most likely were guessing or using rules of thumb like many of the hacks out there.
Not trying to beat you up, but this is a public forum and if anyone comes across this post during research they should know what you did was incorrect no matter how you slice it.0 -
Are you installing this system yourself now?0
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I should have added to the above that when I contacted an installer who I got through the store where I bought the boiler, he told me that I needed AT LEAST 200K BTUs for my application, and would not install the smaller unit.1
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^ just for ballpark estimating purposes- what is the square footage of the heated area of the house?
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You are going to be hard pressed to find someone qualified that will install homeowner purchased equipment.
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I don't think that I'm doing apples to oranges. All three of those systems were for boiler-indirect systems. The first was a Triangle Tube unit which combined the boiler with a 14 gallon indirect in the same unit. The other two used HTP boilers and indirect units, so all three were making use of indirect water heaters. The house is about 2000 sq ft. It is about 110 years old, recently insulated and I'm currently working on tightening up windows and doors.KC_Jones said:That's not how you size it.
You are comparing apples to oranges. For an indirect like you are doing a better comparison would be to a tank water heater which runs in the 40-50k range, and keeps up just fine. The 140k instant water heaters you are comparing to, function completely different from what you are doing so they need those high inputs.
You should have sized to the heat load. And yes those contractors most likely were guessing or using rules of thumb like many of the hacks out there.
Not trying to beat you up, but this is a public forum and if anyone comes across this post during research they should know what you did was incorrect no matter how you slice it.0 -
At 2,000 sq/ft you may have severely oversized your boiler. Unlike old cast iron boilers, oversizing a mod-con has consequences including poor performance, high gas usage and can lead to early equipment failures that will not be covered under warranty. Have you heard of the term "short cycling"?
The good thing is your boiler can modulate down to 28K BTU's but that's still a fairly high output especially during shoulder seasons with and ODR sensor where you only need 110F heating water.
There may be possible solutions to help your boiler fit in better, but more info will be required from you If you're interested...
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A tankless is a completely different animal from what you are installing that is the apples to oranges comparison I am referring to.wklopf said:A stand alone tankless water heater would be in the range of 140,000 and up.
I fear you are working from the false assumption that the quotes you got were from competent people. High likelihood they are not.
If you needed more hot water, upsizing the indirect would have been a much better choice in the long run.
2000 sqft and 140k boiler. With upgraded insulation I would suggest in all likelihood the 85k was high. An old house with zero insulation would probably come in around 40 per sq ft, with upgrades it can drop dramatically. It's going to short cycle itself to death.
You will for sure need a buffer tank to have any hopes of getting run times out of that boiler. Has anyone suggested that?0 -
I am in climate zone 8 and have never seen a heat loss over 30 btu/sq.ft. I think your heat loss is off.
There is a relationship between DHW instantaneous heating capacity and storage. The less storage you have to buffer the spikes in demand, the more boiler capacity you will need. I would guess that your 2 rental units would need over 200,000 BTU/hrs to satisfy a truely instantaneous system. Depending on the size of tubs and high use items, you could probably do just fine with 85k of boiler and 40-60 gallons of storage.
Your 140k boiler is kind off the worst of both worlds. Not enough for instantaneous but grossly oversized to the heating load.
The triangle tube excellence is kind of somewhere in between (think cantaloupe in the apples to oranges analogy). If you have low flow shower heads, and no large tubs, it may well work just fine.
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein1 -
This guy is an idiot. Why not put in a 400k unit, just to be sure?wklopf said:I should have added to the above that when I contacted an installer who I got through the store where I bought the boiler, he told me that I needed AT LEAST 200K BTUs for my application, and would not install the smaller unit.
I like to drive a tractor trailer to the supermarket just in case something is on sale..."If you can't explain it simply, you don't understand it well enough"
Albert Einstein
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KC_Jones said:
Part of my problem here is that the bid which included the 150K boiler was prepared by a person who works for the area distributor of a major brand of space heating and water heating equipment. I am not immune to persuasion. I have long known that the boiler should be sized so that on the coldest day it is just able to keep up, running constantly. What I might not understand is what is the difference with a 140K boiler modulated down to 85K and an 85K boiler putting out 85K? Any insight would be appreciated.wklopf said:.
I fear you are working from the false assumption that the quotes you got were from competent people. High likelihood they are not.0 -
OK, you have convinced me. I will be returning that boiler and getting something smaller. What I don't know what to do about is finding someone in my area who has some experience with these systems. I am especially surprised that a representative of a distributor didn't know better. Thanks for the input.0
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You still need to determine the output for the replacement boiler...
Give the Slant/fin app a try... it will give you a fairly decent ballpark idea of your heatloss figure. Post the results once compete.
http://www.slantfin.com/slantfin-heat-loss-calculator/0 -
Where are you located?0
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Better served to understand a 140K BTU boiler that can modulate down to 28K BTU's and why everyone's stating that it's oversized.wklopf said:
What I might not understand is what is the difference with a 140K boiler modulated down to 85K and an 85K boiler putting out 85K? Any insight would be appreciated.
Heatloss is directly related to outside temperature, and there's only maybe one or two days of the year you'll need to match your Design Day heatloss.... the other 363 days you'll need less.
The Outdoor Reset Sensor will adjust the modulating boiler's output down to match the heatloss related to the outdoor temperature.
Even if you home was on the way-high heatloss side, say 30BTU/Sq/ft you'd come in at 60K BTU's at your DD temp. Your boiler will have no problems keeping up when it's 15 F outside. Now consider a mild day at maybe 45F outdoors... where the heatloss is maybe only 20K BTU's. Your 140K mod-con with a minimum output of 28K BTU's can't fire down that low so it heats the heating loop water till it reaches the high setpoint (maybe 135F? for 45F outdoor temp) then it stops firing but continues circulating the water till it reaches the low setpoint then the boiler fires up again till it reaches high setpoint then stops firing again but continues to circulate the heating loop water. This bouncing off the high and low setpoint cycle continues on and on, it's called "short cycling" and will quickly shorten your boilers lifespan.
If you went with maybe a 80K mod con with a 10:1 turndown that modulates down to 8K BTU... it would do fine even during shoulder seasons and warmer days.
You still have to consider the length of your zones, and condensing temps to see if a mod con will be a good fit for your home. Keep in mind that traditional hot water heating systems were designed to heat the house with 180f water which puts out 510 BTU's per ft of fin-tube. Mod cons only start condensing with 130F return water- that means 140F average water which only puts out 320 BTU's per ft of fin-tube. If your old system was say based off 80K BTU's (157ft of fin tube) output for radiation it will now only out out 50K BTU's. Can you heat your house with 40% less BTUs?
You need to do a proper heatloss analysis and also measure your baseboard (if you're using fin-tube) before deciding on a mod con.
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^^^^^ This, we may know someone in your area that knows what they are doing.kcopp said:Where are you located?
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I am heating 2400 square feet in Alaska ( in the warmer part, however) and my heat loss came in at 32,000 btu. My house is also very well insulated. I am running a 65 gallon indirect for my hot water needs. By comparison to what you are getting for information, I had one job where the other bidder wanted to put in a 240, 000 btu boiler in an older six plex apartment, and not very well insulated. I did the math and went with a 175,000 btu boiler and a 120 gallon indirect for the hot water. It has been running complaint free for 15 years. I will bet I could have gone a little smaller even.
So yeah, don't go that high of a boiler btu.
The 85,000 btu Lochinvar boilers I have been putting in most of the jobs I have done will modulate down to 8,500 btu, which is still too high for most jobs. I think the 50,000 btu boiler they have is now capable of going down to 5,000 btu. I will have to check it out.
Rick1 -
Get the Triangle Tube Tri Max 110. Get a 40 or 50 Gal. indirect heater. All done.0
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Does Triangle Tube still use the bladder design?
Yuk!0 -
I have done a heat loss calculation using the Slant Fin program. It worked out to just under 58,000 BTUs. My earlier figure of 85,000 had come from a website which asked the age, size and a couple of questions about condition. The Slant Fin is much more detailed. At 50 degrees, the loss is 16,500. I had replaced most of the old radiators on the first floor with the equivalent about of cast iron baseboards, so, with two exceptions, the whole system is cast iron. There was not enough wall space in the kitchen for just baseboards, so I added a toe-kick heater. In one of the bathrooms, I installed a hydronic towel heater-radiator, in addition to some cast iron baseboard. The whole system is capable of 94,500 BTUs with 180 degree water and 27,800 BTUs with 120 degree water. So, it seems to me that I can take advantage of a condensing boiler. I have only one heating zone. The radiators are on a two pipe system. Each room, except the one with the thermostat, has a thermostatic valve to control room temperatures. There is an adjustable valve in the room with the thermostat to ensure that sufficient heat gets to the other rooms. These seem to be widely used in Europe. OK, let me hear what all I have done wrong here. The system seemed to work OK last winter with the old boiler. I'm looking for a boiler of under 80,000 with a 10 to 1 turn down ratio.0
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I would suggest the Lochinvar WHN 86 with a 40 gallon indirect."If you can't explain it simply, you don't understand it well enough"
Albert Einstein0 -
Based on Zman's recommendation, I found the area distributor for Lochinvar and eventually located a local contractor who installs Lochinvar. When he came, I showed him the area where the boiler will go and I told him that the radiators could deliver 94,500 BTUs with 180 degree water. I also told him that the heat loss calculation indicated 57,500 BTUs. He then told me that he sizes his boiler to the available radiator out-put. That way he knows that there is enough heat whatever the weather brings. He also told me that since this is a two story house it needs a very large expansion tank, so big that it sits on the floor. Any thoughts?Zman said:I would suggest the Lochinvar WHN 86 with a 40 gallon indirect.
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My thought, keep looking for a contractor. He is essentially proposing a boiler that is roughly double what you need.
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Sounds like he bought a huge expansion tank by mistake for a previous job and is trying to offload it on his next unsuspecting customerwklopf said:...He also told me that since this is a two story house it needs a very large expansion tank, so big that it sits on the floor. Any thoughts?
You'll probably need a 4.4 gallon Exp Tank....
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