Is it efficient to have combi boiler supplying hot water only during warmer months?

[Nanker Phelge]

I have been searching for info on combi boiler operation during warmer months without much luck. During colder, winter months, when the boiler will ideally be operating more regularly for long enough durations to take advantage of its efficiency I can see how providing hot water in addition would also make efficient sense.

But in warmer months when the boiler would otherwise be idle with no space heating loads, wouldn't short durations to supply hot water almost mimic short cycle operation that undesirably derails efficiency and puts unnecessary wear and tear on the unit

I am sure I have likely overlooked something obvious in my search for info, but if not, is there a better way, more efficient and cost effective, to provide hydronic space heating during winter months in addition to supplying hot water all year long?

 

[Leon82]

There are applications where a tank water heater heats the DHW and the heating system thru a heat exchanger. I think mostly lower temp applications.

 

[Sponsor]

 

[BadgerBoilerMN]

No. A condensing boiler with an indirect water heater is one of the most efficient and high performance appliances on the planet. 95% combustion efficiency is the same, winter and summer.

 

[Jadnashua]

Modern condensing boilers are designed to cold start. They only run when there is a call for heat then shut off. And, when they do it, as said, they are typically quite efficient. A combi needs to be up to temperature all of the time, constantly wasting heat into the room. Most modern indirect water heater tanks are quite well insulated - at least as good as a typical electric WH, and probably better. Plus, some of them have a lifetime warranty. They do cost more, but usually, you come out ahead because of the faster recover, and longer life and higher efficiency.

 

[Dana]

A low mass wall hung combi boiler isn't maintained at temperature during idle, and isn't any worse serving DHW-only than a condensing tankless.

The high EF test numbers of a condensing tankless are overstated due to the differences between how the test is run vs. how people actually use hot water. Yes, short draws are EXACTLY the same as short-cycling, and the nameplate EF 0.98 of a condensing tankless is really going to deliver between 75-85% net thermal efficiency in real-world use, which is comparable to a condensing boiler married to an indirect tank or a condensing tank type HW heater.

A 98% combustion efficiency condensing tank hot water heater has standby losses that add up to about the same real-world efficiency as a tankless, but unlike the tankless those losses WOULD show up in an EF test. They get around that by setting up the burners to always be bigger than 75,000 BTU/hr, at which point they're not required to run an EF test, but can simply market it on it's steady-state efficiency. (If you ever wondered why the burner on a Vertex or Phoenix Light Duty is 76,000 BTU/hr and not 75,000 BTU/hr, this is why.)

A combi boiler isn't any less efficient than other options for heating hot water, but they're not always ideal for space heating. The most common error is to tie a combi boiler to a heating system with insufficient radiation to not short-cycle at condensing temps during the heating season. Most combi-boilers can't modulate low enough to not cycle on/off at condensing temps when serving 50' or less of baseboard per zone, but there are a few mod-con boilers out there that can serve a 30' zone at condensing temperatures without cycling. So when looking at the options, make sure to look at the MINIMUM firing rate output, and compare it to your radiation's output at 125F average water temp on each zone. If it's low-mass stuff like fin-tube baseboard the ratio of min-fire output to baseboard length in feet needs to be under 250 BTU/ft-hr.

 

[BadgerBoilerMN]

A 95% efficient condensing boiler coupled to a properly sized indirect water heater never operates below 86% combustion efficiency and never short cycles on any call for domestic hot water. All but the Amtrol BoilerMate lose less than 1/2 degree Fahrenheit per hour.

As Dana suggests, a properly sizing a condensing combi boiler/water heater is not easy since the incidence of heat load and domestic load matching the typical combi boiler is rare--only made easier by a bit of storage as with the Viessmann 222.

 

[Nanker Phelge]

So it seems then that to try and "make" a combi work in a house with enough room to hook an indirect water heater would be foolhearty?

The combi is best used, it seems where space is tight, likely the case in many European homes, where the combi is quite common.

Would it be fair then, to think of an indirect tank, hooked up to a condensing boiler, as simply another load calling upon the boiler for hot water, like whatever style heat emitters are deployed in a hydronic heated system?

 

[Leon82]

Yes.

you could control the dhw settings in the boiler control for nighttime setback and temperature.
It will usually have priority and stop the space heating pumps to run the tank pump.
once the tank is at its setpoint space heating will resume.

 

[BadgerBoilerMN]

Nanker is correct. And an indirect can be sized to satisfy nearly any high demand or multi-demand load such as a large tub or multiple showers. You will still have the "never-run-out-of-water" performance that most "tankless" fans demand but with the added benefit of sizing your main high efficiency burner to the main energy load of the house i.e. space heating...about 75% in cold climates.

 

[Jadnashua]

Most of the time, an indirect is setup as a priority zone...when it needs heat, it gets ALL of the heat available from the boiler. I've never noticed a comfort issue doing that in my home. some of the best indirects list less than 1/4-degree/hour losses in standby...pretty impressive. THrow in that some of them come with a lifetime warranty (often because they use stainless steel), and done properly, you should never have to worry about it again. The boiler may wear out first.

 

[Zl700]

"The high EF test numbers of a condensing tankless are overstated due to the differences between how the test is run vs. how people actually use hot water. Yes, short draws are EXACTLY the same as short-cycling, and the nameplate EF 0.98 of a condensing tankless is really going to deliver between 75-85% net thermal efficiency in real-world use, which is comparable to a condensing boiler married to an indirect tank or a condensing tank type HW heater. "

That's not all true
I've witnessed the new UEF tests (universal energy factor)
This will be mandated as soon as DOE gets it together, perhaps 4th Q this year.

Turns out the condensing tankless drops only 2 points but the condensing storage claiming 95% thermal efficiency drops into the low 80's for a UEF.

This new procedure uses adjusted realistic water draws. The tanks lose on elevated temps not condensing and real standby loss that includes piping.

Obviously tank manufacturers are not happy and causing the stall that was supposed to be announced 1stQ 2016

 

[Dana]

Can you describe the UEF test particulars?

Both in-situ testing and apples to apples bench testing different use profiles, volumes and draw patterns done in California 8-10 years ag for utilities to be able to determine rational subsidy levels showed a much bigger hit than 2% on a condensing tankless.

The 92% steady-state tankless bench tested (Heater #6) took at least a 5% hit in performance even at very high use profiles, and a 17-18% hit at low use profiles, dropping deep into the 70s for as-used efficiency. That would imply that a high 90s tankless might hit the low-mid 80s at modest use levels, low 90s best-case.

The condensing Vertex tank with the on/off 76K burner (Heater # 4) hit around 80% under high use profiles, but was abyssmal at very low use.

The stratified tank with the modulating burner (heater #5- I think it was the early version of the HTP Phoenix, possibly a Versa Flame or Hydro) with a 94.8% steady state combustion efficiency hits in the low 90s at high use, but no lower than ~87% under any use profile.

If done well a condensing mod-con with an indirect can do about as well as that, but if done poorly can take as much as a 30% hit when only serving hot water.

 

[Zl700]

A lot of reading:
Part C is the draws-Low, medium and high
http://energy.gov/sites/prod/files/2014/06/f17/rwh_tp_final_rule.pdf