Advice choosing a standard or combi unit in multi-family home.

Mictrik · April 10

I have to make decision,

My friend who will install the unit likes the combi units from Burnhum. He believes I should see 15 years with a Combi before it needs to be replaced. However he did say that a standard boiler would last longer with less reliance on maintenance . My thoughts are go with the a standard boiler but the Combi is attractive in that I could add another down the line and tenants would pay their own heat. Of course this is if we can find the separate hot water lines and that they split in the basement and not up higher.

At this time the house is oil fired with separate hot water heater directly fired (Burner for each) There are three showers / baths two full size kitchens and 1 small.

This is two family (with potenial third apt) house that will be rented so I am concerned about that too... Anyone with ideas, thoughts or experience I would appreciate your input?

One plus for the Combi is not having to line chimney.

Thanks all / Mick

Jamie Hall · April 10

The primary downside that I see with combis is that it is hard to match the domestic hot water peak demand heat requirement with the heating load. If you can manage a reasonable match between the two, they do have the advantage of being only one unit instead of two.

The advantage of a simple cast iron boiler — and the separate oil fired hot water heater — is that the resulting maintenance is much simpler, and they do tend to last…

EBEBRATT-Ed · April 10

Combi's are troublesome IMHO I would use a boiler mod con or CI boiler and either keep the sep. HW tank or use an indirect with the new boiler.

LRCCBJ · April 10

You wish to replace an oil fired system with a combi?

Do you currently have NG at that location?

If not, this install is going to cost you far more than you anticipate.

Additionally, the combi will never keep up with the demand that you have offered (3 showers/baths and three kitchens). You must maintain a tank (even 50g is marginal if two showers run simultaneously) A good argument can be made for 80g.

Hot_water_fan · April 10

I see no reason to use a combi. You can get two separate water tanks. They could be direct or indirect fired. The only benefit of a combi is space savings. If that’s not a constraint, skip it.

GroundUp · April 11

I'd never have a combi in my own home, or any tankless WH for that matter, but a lot of people seem to like them since they're cheaper upfront than the alternatives. As has been mentioned above, a combi is most likely going to fall short if you have everybody showering at once and will still probably be oversized for the space heating load. You can utilize a heat-only modulating/condensing boiler which vents the same way as a combi and gains some efficiency in the process, maybe even add an indirect WH if you want to spend the coin, but personally I'd keep separate standard WH tanks for each unit and let the tenants pay their own utilities.

GGross · April 11

Multiple tanks, one for each unit, make it easy on yourself. It tends to be less money upfront, and you can source and replace each tank the same day if something happens. You get every tenant deciding to use hot water at the same time with a combi and you will be getting complaints, a tenant that doesn't understand how a combi works and you will get complaints, you want to be fielding phone calls from ticked off tenants about hot water or do you want happy tenants that you only need to hear from when they pay the bill?

Mictrik · April 11

I very much appreciate the input from everyone!! Thank you. I have a gas line at the house now. The oil fired system presently running handles both apartments with believe it or not a Superstor 30 Gal. That Superstor could heat water almost like a tank-less but it is now leaking after 15 years of service. Not sure if the recovery rate of most gas units can match that superstor?

mattmia2 · April 11

unless the syuperstore is kept at a higher setpoint and has a thernmostatic valve on the outlet that mixes it down, its temp will drop significantly before the temp drops enough to generate a call and the boiler heats enough to be adding heat to the indirect.

Jamie Hall · April 11

You have a good — and underappreciated point — about the oil fired Superstor hot water heater. The recovery rate of oil fired water heaters is usually at least as fast as gas, if not faster.

worldclasshvac · April 12

Stick with CI and you could throw electric water heaters in for tenants if need be. Combis are throw away boilers nowadays I have 2 Bosch leakers I cannot get parts for that are only a few years old....waiting on supply chain. Every guy in town has what he needs to fix a traditional boiler right in the truck. Can't beat that with a stick

Kaos · April 12

Replace the indirect if resistance tanks, one for each unit running off each unit's meter.

When the oil burner dies or the tank springs a leak, replace the boiler with two small modcons, one for each unit on its own gas meter.

Jamie Hall · April 12

Um… guys… the OP has two independent oil fired hot water heaters now, and is not muttering about them. I'd say to keep them, or if he goes all the way to gas heating, replace them with gas fired units. Yes, I know, oil fired hot water heaters get overlooked, for some reason — but I have a total of three to play with. and they work just fine.

Kaos · April 12

https://forum.heatinghelp.com/discussion/comment/1852006#Comment_1852006

I'm now confused, were is the superstore that is leaking?

Jamie Hall · April 12

Got to admit, @Kaos , I'm a bit confused too. I THINK we are talking about some kind of replacement for one (or is it two?) oil fired boilers for heating, and one (or is it two?) oil fired hot water heaters…

Advice choosing a standard or combi unit in multi-family home.

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