When do indirect water heaters make sense?

Pulse

Planned Boiler: Lochinvar Knight 85k

It makes sense that an indirect would be efficient during the heating months because the boiler is already in use, however, in the non-heating months the boiler will only be firing for the indirect. This will cause short cycling and the boiler may not even condense, only achieving ~86% efficiency.

A tankless water heater on the other hand will always achieve high 90s efficiency because of cold ground water temperatures. What I do not understand is how a tankless prevents short cycling if you are only turning on the kitchen sink for a bit.

I do know not if my train of thought is correct, please educate me.

EDIT: I should have posted this in domestic hot water, sorry.

Alan (California Radiant) Forbes

I agree with your logic about efficiency.

A good, stainless steel indirect will last the rest of your life. A tankless will last 15 years, at least in my part of the country. So, it depends how long you plan to stay in your house.

A tankless can't prevent short-cycling.

STEVEusaPA

Most efficiency numbers aren't accurate or true. If you're making hot water at 86% for your indirect, you're doing quite well.
I personally don't like on-demands. The usual complaint is the warm-cold-then hot water effect. Not able to fill a tub, etc. I'd always recommend an indirect, or even an EK.

Usually recharging an indirect (in my case specifically) takes about 8 minutes in the summer, after a shower.

Technically, whenever you are recharging an indirect you're probably not condensing as most controls will reset the target temperature for 160° and up.

Hopefully you are sizing your boiler for heat loss only and not oversizing for domestic hot water, unless you have an extreme use case and need faster recovery, in which case maybe a larger storage tank.

Pulse

Alan (California Radiant) Forbes said:

A good, stainless steel indirect will last the rest of your life. A tankless will last 15 years, at least in my part of the country. So, it depends how long you plan to stay in your house.

Yeah I saw, most of these SS indirects have a lifetime warranty. ..but will I be pushing added burden onto my more expensive boiler?

STEVEusaPA said:

Most efficiency numbers aren't accurate or true. If you're making hot water at 86% for your indirect, you're doing quite well.
I personally don't like on-demands. The usual complaint is the warm-cold-then hot water effect. Not able to fill a tub, etc. I'd always recommend an indirect, or even an EK.

Hopefully you are sizing your boiler for heat loss only and not oversizing for domestic hot water, unless you have an extreme use case and need faster recovery, in which case maybe a larger storage tank.

I was reading about that sandwich effect. A 1-2 gal buffer type tank can prevent that, but it seems only navien has a tankless with that buffer tank.

I did size for my heatloss. I could go down to a ~50k BTU boiler, but there is no point since the turn down ratios are all around ~8k anyway(just like this 85k). These indirects call for at least 100k BTU on all the specs I have seen, which also doesnt make sense to me..haha

..but my main concern is am I harming my boiler in the summer months with an indirect?

Alan (California Radiant) Forbes

I don't think that 100K citation is a specification, just an example to show recovery rate if you had that size boiler. Your recovery rate with a 50K boiler will be longer.

I have a 50K Munchkin boiler with a 50 gallon Viessmann indirect in my basement and it's been fine for 13 years. Two people in the house most of the time, three when the kids, in turn came to live with us for months. No problems.

Munchkins kick ****, even when their heating contractor owner neglects them. Your Lochinvar will be fine.

Tim Potter

My Turbo-max Reverse indirect does a good job of stratifying the water in the tank, so there is plenty of cold water at the bottom of the tank to be drawn in to the boiler. so you could condense most of the recharge.

Tim

Zman

I think the tankless efficiency numbers really depend on the application. They are likely only accurate during a fairly long call for water. Hand washing, cleaning, and cooking will cause short cycling. This greatly reduces net efficiency.

The controls on most mod/cons will allow you to adjust the parameters to get any cycle length you desire. By adjusting on/off differential and boiler firing rate you can make them pretty efficient.
Another great trick with a mod/con indirect setup is to select an indirect model with a large heat exchanger. This allows you to run the boiler in condensing mode for much of the cycle.

I often hear people say that the indirect needs to "match" the boiler output. This is pure nonsense, a big indirect + small mod/con = high efficiency.

Zman

Tim Potter said:

My Turbo-max Reverse indirect does a good job of stratifying the water in the tank, so there is plenty of cold water at the bottom of the tank to be drawn in to the boiler. so you could condense most of the recharge.

Tim

I have never been able to get my head around using a reverse indirect with a mod/con. You need to generate and store very hot water for the system to work. How can that be efficient?

Alan (California Radiant) Forbes

My last customer, Gerry had a Viessmann dual coil indirect. He was planning on using the second coil for solar, but didn't go that route. When we installed his new boiler, we piped the second coil in series with the first and return water temperatures from the coils dropped dramatically.

Pulse

Tim Potter said:

My Turbo-max Reverse indirect does a good job of stratifying the water in the tank, so there is plenty of cold water at the bottom of the tank to be drawn in to the boiler. so you could condense most of the recharge.

I did briefly look at them.. what is the cost and can an individual even buy one?

Zman said:

Another great trick with a mod/con indirect setup is to select an indirect model with a large heat exchanger. This allows you to run the boiler in condensing mode for much of the cycle

Any suggestions for tanks with large heat exchangers? I havent been able to find anyone to supply Viessmann in my area, so that dual coil one is out.

Zman

The manufactures will publish the surface area. Large tanks generally have larger ones. Amtrols are generally smaller. HTP also makes a dual coil commercial model.
We are only talking a few percent savings in DHW production. Pay attention to the sizing, but don't spend $1K extra to save $20 a year.

Pulse

Zman said:

The manufactures will publish the surface area. Large tanks generally have larger ones. Amtrols are generally smaller. HTP also makes a dual coil commercial model.
We are only talking a few percent savings in DHW production. Pay attention to the sizing, but don't spend $1K extra to save $20 a year.

Alright, scratch that.

I still dont understand how I can stop my boiler from short cycling with an indirect..lol

Zman

1 btu is the amount of energy required to raise 1 lb of water 1-degree Fahrenheit. 40 (gallon water heater) x 8.34 (weight of 1 gallon) x 10 (on/off differential) = 3,336 btu's to to maintain temp under no load. A boiler with an output of 75,000 btu/hr = 1250 btu/min. So, just under 3 minutes is your worst-case scenario for cycle length. In reality, most cyles will be much longer than that because you will also be heating the water you are using at the time.

75,000 / 500(constant)/ 70 (degree delta) = 2.14 gpm. Anytime you are using more than 2.14 gpm, the boiler will run continuously.

If you set this up and you don't like the cycle lengths, you can change the on/off differential and the boiler output during a domestic call and get whatever cycle length you like.
I doubt you will get such an offer from the instantaneous heater .

EdTheHeaterMan

@Zman math is good... with the combustion efficiency factor of say 86% since you are only recharging an already hot water tank by say 10°... so your will be more like 4 minutes.

My Riello burner (in a Buderus cast iron) actually runs for 6 minutes if the tank is just recharging the 12° difference in the winter when the boiler is already heated from a heating cycle. In the summer the cycle is more like 9 minutes if the boiler needs to cold start. If I don't use any hot water for 21 hours, the boiler stays off until the tank drops 12° again. Thermodynamics is not instant. So the cycles are short when the demand is short. Long when the demand is higher.

Modulating gas appliance are still going to be better even with indirects. If you are going to go with a tankless for DHW, Remember to factor in the cost of maintenance on that heat exchanger periodically.

TAG

My original Buderus 112 Btu CI boiler w/ eccomatic -- the original heavy thermoglase indirect would come up to temp faster VS the new Buderus SS tank ..same 40 gal. I think the mass of the old tank and the larger coil actually transferred heat to the boiler for the first few -- then the boiler ran until the tank was satisfied. With the new tank -- the boiler hits 180 and turns off before the tank has reheated ... so, the coil is not as efficient. The process is longer now ... sometimes the boiler fires again ..other times the boiler has enough heat to satisfy the tank.

The eccomatic keeps the indirect off after a set time in the evening .... recharges in the morning. It eliminates and cycling at night. In any event -- with NG the cost to heat my water -- dryer and stove in the summer before the pool heater kicks on is $22. How much can you save ?

CMadatMe

Umm. Generally we take low temp showers during the non heating season months, than we do during the heating season months. You can always limit the modulation rate of the boiler for the indirect during the off heating season.

Pulse

Zman said:

40 (gallon water heater) x 8.34 (weight of 1 gallon) x 10 (on/off differential) = 3,336 btu's to to maintain temp under no load. A boiler with an output of 75,000 btu/hr = 1250 btu/min.

75,000 / 500(constant)/ 70 (degree delta) = 2.14 gpm. Anytime you are using more than 2.14 gpm, the boiler will run continuously.

Couple questions:
1) What is the "500(constant)"?
2) Is there a calculation for how many GPM an indirect can supply?
3) What happens to my indirect if during the heating season my SWT drops down to 130, 120, 110, ect?

mattmia2

1. the 500 converts the units from btu/hr and Fahrenheit degrees, using the specific heat of water to gallons per minute
2. it depends on the heat exchanger area, boiler temp, and to a lesser extent flow. It isn't all that useful in itself because you have reserve in the tank. There is a pamphlet on calculation dhw sizing I'll post if i can find it.
3. Most Mod Con boilers have a dhw call input that uses a separate setpoint. Do more sophisticated Mod Cons allow you to set a curve based on rate of tank temp drop and if the tank still isn't rising at a given supply temp?

Pulse

mattmia2 said:

2. it depends on the heat exchanger area, boiler temp, and to a lesser extent flow. It isn't all that useful in itself because you have reserve in the tank. There is a pamphlet on calculation dhw sizing I'll post if i can find it.

I was hoping to figure out how long I could run 1 shower before I run out of hot water. ..well, 1 shower may never run out, but what if I add another shower into the mix

Alan (California Radiant) Forbes

Good discussion, but there is a point in time where the heat loss from anxiety exceeds a short-cycling boiler.

How about a standard atmospheric water heater sized for your DHW load?