Combi's

Ron W.

I have heard that the combi is not the best idea for baseboard heating systems as running the temps in the boiler over condensing temperature compromises the savings the customer expected and paid for. Please enlighten me.

EBEBRATT-Ed

Domestic hot water loads are usually large but for a shorter duration than heating loads which are more moderate and steady.

With a standard boiler or a mod con boiler with an indirect hot water tank domestic water is stored so that when there is a call for domestic water some of the stored water is used and the boiler then reheats the tank. A small boiler sized for the heating load can do this

With a combi there is no domestic how water stored so the boiler must be larger than needed to heat this instant load.

HVACNUT

Because a combi needs to be grossly oversized to meet domestic hot water needs, I'm not a fan, unless there's serious space constrictions for the boiler location. A mod con with a high turn down ratio like 10:1, outdoor reset and an indirect water heater, having fin tube baseboard shouldn't be an issue. Worst case it will need a buffer tank. And dont forget about the heat loss calculation.

hot_rod

The Combis have the same turndown as the boiler only versions. 80K is about the smallest mod con I have seen with 10-1 turn down. 80K will turn down to 8,000. There are smaller mod cons but not with 10-1. a 110K is about as small as you might go with a combiner's for adequate one or two faucet draws.

Ramp delay and anti cycling settings will also help limit short cycles on either type.

The combi will handle small loads exactly the same as a mod con, boiler only.

A buffer can be added to either if you have frequent micro loads.

The key to to do an accurate heat load, especially on replacement boilers.

There certainly are tradeoff with any combination product. To me the bigger question for the HO is if they can live with the DHW production they actually offer, not the cycling so much.

If they need or want a 199K for large amounts of DHW and have a 20K, multi zoned heat load, that may be too much of a mismatch without a buffer. A separate boiler and tank would be a better choice.

LuckyDog

Hope I am not hijacking this thread....

My home is a 50k load. All low temp radiant heat. Radiant Floor and a couple of panels. Three zones

My boiler installer REALLY wants me to put in a combi. A Bosch Greenstar combi 100 p or Lochinvar NKC110L.

What happens, say, after the wife draws her tub but there is a call for heat? Does that high temperature water then go into the heating system?

Does the boiler stay at a high temperature like the oil burner did when I was growing up? (180 day or night 24/7/365)

A plumber friend told me he hates "tankless hot water" because of the "cold sandwich" effect. He told me the water heater would purge, and when he got into the shower after his wife; there would be hot water still in the pipe, a shot of cold water due to the heater being cold and just starting, then all the hot water he needed.

If this might happen to my wife, then I don't want a Combi.

ETA: corrected model of the Bosch.

JohnNY

LuckyDog said:

A plumber friend told me he hates "tankless hot water" because of the "cold sandwich" effect. He told me the water heater would purge, and when he got into the shower after his wife; there would be hot water still in the pipe, a shot of cold water due to the heater being cold and just starting, then all the hot water he needed.

If this might happen to my wife, then I don't want a Combi.

We install small 10-gallon electric water heater buffer-type tanks on the outlets of tankless water heaters to resolve this. It works every time but adds a bit to the costs of installation and operation, obviously.

DZoro

Lochinvar Noble has a setting to prevent the cold sandwich, and it works great. Can also set the max btu's for heating as low as 10,000 or 15,000, or 19,000 depending on which model you have.
D

Le John

I have a Lochinvar Noble NKC 199. It provides endless hot water and can say that it maintains the set hot water temperature very very well. I have never noticed a fluctuation in hot water temperature. Great boiler.

Henry

We stopped installing combi units in 2005. Our water at the end of January is 33F through the end of February. The combi units cannot keep up with the demand no matter what the size.

hot_rod

The basic BTU formula will indicate what it takes

500 X 3gpm X [33-123] = 135,000 btu

A 155k would cover that

It always comes down to expectations and crunching the numbers

In_New_England

Le John said:

I have a Lochinvar Noble NKC 199. It provides endless hot water and can say that it maintains the set hot water temperature very very well. I have never noticed a fluctuation in hot water temperature. Great boiler.

Hi, I'm considering getting a Noble 199 for a 1400 sq ft house with 87ft of baseboard. I worry that it is oversized. Could you mention your home size, baseboard length etc. My estimate is that my minimum baseboard load sort of matches the lowest output, but I might be more comfortable with 150. Thanks

pedmec

lochinvar noble combis have rate limiting which will limit the amount of btus for the heating system but allow for maximum btu output for the hot water demand. it also uses a fire tube hx which has 1.4 gallons to 2.7 gallons depending on the boiler size in the hx and the boiler will maintain a minimum temperature in the hx for immediate hot water needs and it prevents the cold water sandwich as there is always hot water on the boiler side of the flat plate heat exchanger.

hot_rod

In_New_England said:

Le John said:

I have a Lochinvar Noble NKC 199. It provides endless hot water and can say that it maintains the set hot water temperature very very well. I have never noticed a fluctuation in hot water temperature. Great boiler.

Hi, I'm considering getting a Noble 199 for a 1400 sq ft house with 87ft of baseboard. I worry that it is oversized. Could you mention your home size, baseboard length etc. My estimate is that my minimum baseboard load sort of matches the lowest output, but I might be more comfortable with 150. Thanks

How much hot water do you want? I had a Nobel 110 at my last place, it will get you 2 gpm or more DHW with a 77 degree rise. 110 should be plenty for your heat load?  If you want a bit more DHW, 3-4 gpm, go with a 150

In_New_England

@hot_rod thanks! Yes, I convinced the contractor to go with a 150. They initially wanted a 199 and pointed out it would modulate down to 20, but I did some heat calcs and pointed out that the 150s low fire was better matched to my shoulder month heat loss AND since I only had one bathroom, the 150 would be adequate.

I hear you about the 110, but I was a bit wary of running into issues with the hot water. Shrug.