If your Glenview or Mount Prospect home has a boiler, there’s a good chance your domestic hot water comes from one of two aging arrangements: a tankless coil built into the boiler itself, or a separate gas water heater sitting next to it. Both work. Neither is what we’d install today.
The upgrade most boiler homes should be making is an indirect water heater — an insulated storage tank heated by the boiler through its own zone. It’s quieter, dramatically higher recovery, and usually cheaper to run. Here’s the full picture, including when it’s not the right call.
What You Probably Have Now
Tankless coil (internal coil in the boiler)
A copper coil inside the boiler heats water on demand. Common in homes built or re-piped between roughly 1950 and 1990.
- Problem 1: The boiler must stay hot year-round to make hot water. In July, you’re firing a 150,000 BTU boiler to run a shower. Summer standby losses are brutal.
- Problem 2: Flow rate is limited and temperature swings when someone flushes a toilet — the classic “shower goes cold then scalding.”
- Problem 3: Coils lime up in our hard water. Output drops year over year until it’s unusable.
Standalone gas water heater
Fine, familiar, and typically 8–12 years of life. Its downside in a boiler home is that you’re maintaining and venting two combustion appliances when one could do both jobs.
How an Indirect Water Heater Works
The boiler treats the water heater as another heating zone. When the tank calls, the boiler fires and circulates hot boiler water through a heat exchanger coil inside a heavily insulated storage tank. No burner, no flue, no combustion in the tank at all.
The results owners notice most:
- Recovery rates far above a standard gas heater. A 40-gallon indirect on a properly sized boiler can deliver first-hour ratings comparable to a much larger conventional tank — filling a soaking tub back-to-back with showers stops being a problem.
- Standby losses under 0.5°F per hour on quality tanks, because there’s no flue running up the middle of the tank.
- Long life. Stainless indirect tanks commonly carry lifetime or 25-year residential warranties; glass-lined versions typically 10–15 years. No burner means far less to fail.
- Boiler can go cold in summer — with a cold-start or condensing boiler, standby losses drop sharply versus a tankless coil setup.
Costs in Glenview & Mount Prospect (2026)
| Project | Installed Cost |
|---|---|
| Indirect water heater, 40–50 gal, glass-lined | $2,500 – $4,200 |
| Indirect water heater, 40–60 gal, stainless | $3,400 – $5,600 |
| Add-on: new zone valve, circulator, controls | Included above in most cases |
| Abandon tankless coil / install coil plate | $150 – $400 |
| Thermostatic mixing valve (recommended) | $250 – $500 |
| Expansion tank + new shutoffs | $200 – $450 |
| Boiler + indirect combined replacement | $10,500 – $22,000 |
For comparison: a standard 50-gallon gas water heater replacement runs roughly $1,800–$3,200 installed in this market, and a heat pump water heater $2,800–$5,200 before utility incentives. The indirect costs more up front than a basic tank, and typically lasts two to three times as long.
Operating Cost: Where the Savings Come From
The savings are not mainly in the energy used to heat the water — they’re in what you stop wasting.
- Versus a tankless coil: This is the big one. Keeping a boiler hot all summer for domestic hot water commonly wastes $200–$450 a year in Chicagoland, depending on boiler size and efficiency. An indirect lets the boiler cold-start.
- Versus a standalone gas tank: Savings are more modest — often $60–$180/year — coming from the indirect’s much lower standby loss and the boiler’s higher efficiency compared with a 0.58–0.62 UEF atmospheric tank.
- Maintenance: One appliance to service instead of two, and one fewer flue to worry about.
Realistic payback versus a tankless coil is often 5–8 years, plus the comfort improvement, which owners tend to value more than the arithmetic.
Sizing: Don’t Just Match Gallons
Indirect tanks are sized by first-hour delivery, and that depends on the boiler’s output, not just the tank volume. A 40-gallon indirect on a 120,000 BTU boiler outperforms a 50-gallon standalone gas heater comfortably.
| Household | Typical indirect size |
|---|---|
| 1–2 people, one bath | 30–40 gallons |
| 3–4 people, two baths | 40–50 gallons |
| 5+ people, or a soaking tub / body-spray shower | 50–80 gallons |
If you have a large jetted tub, tell us the tub’s capacity. That single fixture drives the sizing more than the number of bathrooms.
Installation Details That Matter
- Priority zoning: The indirect should be wired for domestic hot water priority, so the boiler devotes full output to the tank during a call rather than splitting it with heating zones. Recovery suffers badly without it.
- Mixing valve: Store hot (around 140°F) for capacity and to inhibit bacterial growth, deliver at a safe 120°F. A thermostatic mixing valve does both. We install one on essentially every indirect.
- Cold-start capability: Confirm your boiler and control can run cold-start. Older boilers with an aquastat maintaining a minimum temperature won’t capture the summer savings until the control is reconfigured or replaced.
- Expansion control: With a closed system and a pressure-reducing valve on the incoming service, you need a properly sized thermal expansion tank — otherwise you’ll be replacing T&P relief valves.
- Permits: Glenview and Mount Prospect both require a plumbing permit for a water heater change-out. It’s routine, and it’s worth having on record when you sell.
When an Indirect Is the Wrong Choice
We’d steer you elsewhere if:
- Your boiler is near end of life. Don’t tie new hot water to a 40-year-old block. Replace both together, or install a standalone heater now and revisit later.
- You’re planning to convert to forced air / heat pump heating. If the boiler is going away, don’t make your hot water depend on it. A heat pump water heater is usually the better path.
- There’s no space. An indirect needs floor space next to the boiler. Tight Mount Prospect mechanical closets sometimes can’t take one.
- Very low hot water demand. A one-person household may never recover the premium.
Frequently Asked Questions
Will my boiler run more with an indirect?
It will fire for domestic hot water calls, yes — but for short, efficient bursts instead of maintaining temperature 24/7. Total summer runtime usually drops significantly.
Can I use an indirect with a steam boiler?
Sometimes, with a proper heat exchanger arrangement or a tankless-coil-fed setup, but it’s more complicated than with a hot water boiler and not always the right answer. It needs an on-site evaluation.
How long does the install take?
Most straight indirect installations are one day. Add a half day if the boiler needs new controls or the piping requires reconfiguration.
Do indirects need maintenance?
Annually flush the tank and inspect the anode rod on glass-lined models (stainless tanks don’t use one). Have it done at the same visit as your boiler service.
Is a combi boiler a better option?
A combi boiler makes both heat and hot water in one wall-hung unit and saves space, but hot water output is flow-limited and it can struggle with simultaneous demand. In homes with two or more baths, we generally prefer boiler-plus-indirect.
Get a Recommendation Based on Your Actual Boiler
The right answer depends on your boiler’s age, output, and control type — which is why this is a 20-minute look in the basement, not a phone quote. MMDS handles boilers, water heaters, and full plumbing service across Glenview, Mount Prospect, and the north and northwest suburbs.
Call (847) 221-6280 or schedule an evaluation. Financing available through Sunbit and PowerPay.