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Combi units

First, I'd like to define a Combi unit, and I'm sure many heating contractors in here will agree.

A Combi unit is a Boiler with an H-stamp (ASME Stamp) on it's heat exchanger, that can do both hydronic heating, and domestic hot water. Typically the domestic hot water is done through a built in plate exchanger (i.e. Viessmann Vitodens 200 WB2A combi) or through a small tank type heat exchanger of some sort (Prestige Excellence) which adds some minor buffer capacity.

I like the idea of a combi unit personally, however right now the combi units avaible are a true compromise due to the restrictins on boilers on modulation rate (5 to 1). For example, a combi that may better match the heat loss of a certain size house as far as reducing short cycling, may not have the hot water output. And if you had a larger boiler, it may be suitable for most domestic water output, but may short cycle during shoulder season type heating loads which is not optimal. Also worth considering is how much buffer capability is built into the heat exchanger. If it's just a plate type exchanger, you could see some short cycling to keep the combi unit hot, if it's got some buffer capacity,in theory less short cycling. There are some larger shell & tube type heat exchangers out there that may be suitable if they had a tapping and some insulation around them, at working with a boiler for an external or add on type  combi type of system. The IBC 15-150 comes to mind as a boiler that is probably the closest at having the neccessary minimum output (15,000 BTUH) to reduce short cycling for a smaller house, with a decent max output (150,000 BTUH) to produce at least a shower and a half worth of domestic hot water, however this boiler is an anomoloy with a 10 to 1 firing rate.

Viessmann is apparently working on a Vitodens 100 combi add on which is apparently a heat exchanger that you just add to the boiler for domestic hot water output. They also apparently have something similar to what they offer in Europe in a Vitodens 200 with a built in buffer capacity type heat exchanger.  The Vitodens 100 I'm told isn't too far away, the 200 is a bit farther away. I suspect the Vitodens 100 will be tuned for their 37 to 118 boiler, and I suspect the 200 will be tuned to work with the 31 to 124 200 version.

I'm curious if anyone can point out anyone else who may be offering a combi type of system, or at least an add on, or if you do them yourself with a heat exchanger and what your success rate is.

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  • zacmobile
    zacmobile Member Posts: 211

    I was just checking out Rinnai's new combi boiler, It's quite small and has a built in outdoor reset control, circulator, expansion tank & low loss header module. I have a bad taste in my mouth from following ignorant contractors around and servicing their Rinnai water heater based heating systems, however this looks like a proper heating boiler and is quite (relatively) inexpensive.
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