Lochinvar boiler isn’t keeping up

your indirect tank is too small for your dhw load. you could get somewhat more capacity buy keeping the tank at a higher temp and using a thermostatic mixing valve but to make for example 2.5 gpm of hot water with a 65 f rise you need 81,000 btu/hr. (i used about 35 f incoming water temp and 100 f final water temp). once you have mixed enough cold water in that indirect tank to male the water noticeably colder, you aren't replacing as fast as you are using it. your boiler is 80,000 btu/hr in and around 70,000 btu/hr out. there uis also some lag between when you start using water and the aquastat in the tank sees the tank is cooling, then some more lag for the boiler to pre-purge, light, and get hotter than the tank and start adding heat to the tank.

the tables in the manual for the indirect are for conditions of incoming cold water that is much warmer and a boiler that is much bigger than in typical northern climate installations.

DHW will always take priority over space heating if it's properly connected and piped. Are you using a sensor or aquastat in the tank? What is the tank setpoint? What is the boiler setpoint for DHW? Turn that time back to 30 minutes and raise the tank temp along with possibly the boiler temp as well so it'll store more hot water, then you shouldn't need a bigger tank or boiler.

if the input is 80,000, you have more like 69,000 btu/ hr to cover the dhw, so once the dump from the tank is gone filling the tub your recovery will be with 69,000, more like a 2 gpm rate depending on incoming water, possibly keeping up with a shower.

The tub dump load may not be a good match for a 40 gallon indirect, if you want a shower at the same time or after a tub fill

what size tub?

Something feels off. Triplex with a 50gal power vented tank with a 40k burner and keeps up. How many GPM is your shower and tub filler (quick test, time how long it takes to fill a 5 gallon bucket).

Pictures of the setup first, I have feeling something is either not plumbed properly or the controls are incorrect.

If you do need more volume, you don't need a bigger indirect. You can add a regular electric tank downstream of the indirect. If the electric tank thermostat is set bellow the indirect output, the elements will only run to make up for standby losses or after larger draws that empty out both tanks. If you have hot water recirc you don't even need to plug in the tank as the recirc can be set to re-fill it off the indirect.

i have a similar setup to theirs but with a 20,000 btu/hr larger boiler and i can run it out of hot water with a long, adequate shower all by myself. if they want to fill a big tub or use 2 or more showers at once their capacity is way too small. they would need a huge electric to do it with electric because of its essentially nonexistent recovery rate compared to their consumption. that boiler can recover an 80 gallon tank in less than half an hour. electric would take about 130 minutes to recover 80 gallons.

it is usually better to size the boiler to the heating load and get the needed dhw capacity with bigger indirect tank(s). if you get the capacity through boiler sizing then the boiler won't have sufficient turndown to match the heating load when it isn't at design conditions which will be most of the time.

usually it is the other way around. it immediately switches to the dhw zone on a dhw call and turns off the other zones but there is a timer that if the dhw call lasts more than x minutes it resumes the domestic heating zones in addition to the dhw zone.

Yikes, those 3.5 gpm shower heads and the 6 gpm tub spout should be plugged into your math equation (time x flow volume) when figuring your dhw needs in a given use-scenario. Your boiler (and tank) is too small. Maybe…change shower heads? Co-ordinate showering and bathing habits? Ah, "first world" problems.

While the 40 may not be enough I don't believe the controls are set properly. If you set the controls right and keep the tank at 135-140 and mix it down and be a little more selective in your water, use you might get by.

Not sure from their post if that is the case. They say "It seems like the boiler can keep up if it starts heating to 100% right away." but it isn't going to start until the dhw tank cools enough to drop the aquastat/tank sensor below the cutin point so it isn't clear if it isn't doing dhw priority or if the OP just doesn't understand how it works. There are probably some icons on the Lochinvar that show which calls it has and their state if you look at the control panel pages of the manual. Usually they will do something like light the dhw icon when there is a dhw call and flash the dh icon when there is a dh call but servicing that call is paused because of dhw priority. It is also possible that the draw has gone on long enough that the dhw priority timer has expired when they look at the boiler.

But basically they have a tank and boiler combination that is sized for one bath if they are careful and they have 3 baths connected to it.

what temp is your incoming water? i can run my 40 gallon superstore with a 100,000 btu/hr in boiler out with a single shower if i'm not careful. some times of year the incoming water temp is about 35 f here.

All this is partly a matter of usage and perception. If your squad comes off maneuvers and takes a Marine shower one by one on the way to mess, yes you can get by with a 40 gallon water heater.

If you wear your hair long and want to get it really clean, probably not.

If you like to soak in a nice hot bath, certainly not.

I have never quite figured out why people resist putting in a big enough hot water heater. I can see it in some situations — space constraints, sometimes availability constraints. But if you can put in a big enough unit, why not do so?

I'd still prefer to know how a system is controlled and piped prior to changing it out for a bigger tank I think.

@GGross Yet boiler guys keep installing indirect that cost more (bom and install), have about the same operating cost and don't really get more hot water.

My experience with my coil based inidirect is that it worked well for a couple of years but once scale built up the heat exchanger, recover dropped. I eventually converted it to plate HX to feed the tank which did work much better in the long run.

Agree with @Ironman If your running out that quick with 80k input and 40 gallons of storage your wasting water.

I also agree with @hot_rod DHW use varies widely.

I know people that get by with a boiler with a tankless heater. You learn to live with what you have.

If you want to pay for the horsepower to run the dishwasher and the washing machine and 3 showers and a tub you certainly can do that.

I think the ops system may be undersized for its present use btu I also think there are control issues.

A couple of questions.

What is your altitude? Higher altitudes decrease boiler BTU rated output.

What is the flow rate to the heat exchanger on the T Tank? A I" T Tank heat exchanger can accommodate a higher flow. Heat exchange transfer relies on two factors, water temperature and flow rate. Increasing the T Tank storage temp, as stated, increases the tank heat storage capacity and with a mixing valve keeps the delivery temp at about 120-125 degs..

Is the input and return to and from the boiler to the tank piped correctly?

This may or may not apply. Do you have a re-circulation circuit on the T Tank? If so not piped correctly, colder water could be mixing with the hot output at the faucet. Measure the temp of the hot water at the faucet when the boiler is only putting out hot water to the faucet. Does that match the hot water output at the T Tank?

Some thoughts.