Combi Boilers with a Small Reserve DHW Tank

November 6, 2023 6:43 AM

We are looking to install a combi boiler (heat and domestic hot water (DHW)) and are having trouble identifying which makers/models have small indirect tanks for DHW. To be clear we do NOT want a separate indirect hot water tank connected to a boiler, and we have reasons for not considering indirect tanks.

So far, the only make and model I can find that advertises it having a small tank built-in is the Laars FT model which advertises having a 1-gallon reserve tank built-in to provide on-demand instant hot water [The brochure highlights this mini-tank feature]. This is important to us for a few reasons including that where we can mount the unit is on the opposite side of our (small) house from where all of the taps are, and our current tankless hot water heater takes a while to get hot water to the taps. We need to replace the 40+ year old boiler for a few reasons so we are using this opportunity to do both -- and use the current tankless hot water heater's existing hole in wall to vent the new combi unit.

So please accept that we are getting a combi and that we are looking for options in addition to the Laars to provide residential heat and DHW that also have a small storage tank to provide some amount of hot water on-demand. Thanks!

posted by terrapin to Home & Garden (4 answers total)

I'll tell you right now that the most important feature of a residential boiler is that you have a local maintenance company that knows the brand and keeps parts in stock. We had something go wrong with our boiler a few winters ago and didn't have hot water in the house, and everyone was quoting us six weeks to get parts from Spain. This was before the pandemic!

If you live someplace where residential combi boilers are common this may be less of an issue. But where I live it's a pretty niche choice, only a handful of companies service them at all, and if I were looking for a specific feature on a boiler I would absolutely just call my heating guys and ask them to recommend something they'd feel comfortable installing and repairing.

All that said, I have a Bosch Greenstar 151 and it doesn't have an onboard storage tank. So you can cross that one off your list.
posted by potrzebie at 9:10 AM on November 6, 2023 [2 favorites]

You say that the unit is on the opposite side of the house from where your taps are. I suspect that the dominant factor in determining how long it takes to get hot water at the taps will be the pipe length, not how long it takes the heater to ramp up. The laars will indeed provide instant hot water _at the heater._ Adding a recirculation line/pump would go much further to solving the problem than swapping out your tankless. (Though doing both would be the best solution)
posted by Larry David Syndrome at 11:25 AM on November 6, 2023 [1 favorite]

This is important to us for a few reasons including that where we can mount the unit is on the opposite side of our (small) house from where all of the taps are, and our current tankless hot water heater takes a while to get hot water to the taps.

Part of that delay is the volume of water in the pipes between the boiler and the tap, which won't appreciably change with whatever type of combi boiler you're installing. I've always solved that with a small electrical boiler right under the sink or the wash basin.
posted by Stoneshop at 11:27 AM on November 6, 2023 [2 favorites]

Yeah I was going to suggest a recirc pump too but I assumed that was a solution they considered and rejected since it seems pretty standard. Agree that if what you want is instant hot water, it doesn't really matter that much if you have a tankless or a tank if your fixtures are far from your boiler. What you want is to move the hot water closer to the fixture, either by constantly circulating hot water through your heating system or by adding a boiler or tank someplace closer to your fixture.
posted by potrzebie at 1:49 PM on November 6, 2023 [1 favorite]