Residential heating & air conditioning contractor in Merrimack, NH
GSMS is proud to serve the residents of Merrimack, NH, with top-quality heating and air conditioning services. As a family-owned business with over three decades of experience, we are dedicated to providing reliable, efficient, and professional HVAC solutions tailored to your home’s specific needs.
GSMS is proud to be your Merrimack Lennox Premier Dealer!
Whether it's an emergency repair, a total replacement, or routine maintenance, we have you covered. Call 603-424-8258 today!
Air conditioner repair & replacement
- AC Repair: We handle cooling problems big and small with speed and attention to detail. Our technicians are trained to troubleshoot and fix refrigerant leaks, thermostat issues, electrical failures, compressor problems, and more.
- AC Replacement and New Installation: Time for a cooling upgrade? We offer high-quality systems from Lennox and other leading manufacturers to improve efficiency, enhance comfort, and reduce homeowners' carbon footprint.
Furnace repair & replacement
- Furnace Repair: Dealing with a broken furnace? GSMS technicians work efficiently to track down even the smallest of problems and repair them in no time. Common furnace issues we repair include pilot light problems, thermostat malfunctions, ignition system failures, and blower motor problems.
- Furnace Replacement and New Installation: Upgrading your furnace takes your home's winter warmth to the next level. Our energy-efficient systems provide consistent heating, reduced maintenance, and enhanced safety.
Boiler repair & replacement
- Boiler Repair: Common boiler problems with pressure, thermostats, pilot lights, and leaks, can be a homeowner's nightmare. GSMS technicians work quickly to repair your boiler and get your home warm again in no time.
- Boiler Replacement and New Installation: Whether you are considering switching to a boiler system or it's time to replace your existing system, GSMS is here to help you increase efficiency and reliability with advanced units.
Mini-split heat pumps
GSMS provides comprehensive services for mini-split heat pumps, including installation, maintenance, and repair. We specialize in Mitsubishi ductless units, known for their reliability and efficiency. Whether you’re looking to install a new system or need service for your existing mini-split heat pump, our technicians are here to ensure optimal performance and comfort in your home.
Ductwork cleaning & installation
- Duct Cleaning: Regular duct cleaning helps improve air quality, increase HVAC efficiency, extend system lifespan, and eliminate odors. We use an advanced Roto Brush system to reach every corner of the ductwork and keep your home clean and comfortable.
- Duct Installation: Proper ductwork installation is crucial for airflow, efficiency, comfort, and durability. Choosing GSMS to install your ductwork gives you peace of mind that your new air ducts are safe and secure.
Merrimack plumbing services
GSMS is proud to provide comprehensive plumbing services to homeowners in Merrimack, NH. Our experienced and professional team is dedicated to delivering reliable and efficient plumbing solutions tailored to your specific needs.
Quick plumbing repairs, complete installations, and preventative maintenance are what we do best. Call 603-424-8258 today!
Water heater plumbers
- Repair: Our skilled technicians can quickly diagnose and repair common water heater issues, including inconsistent temperature, leaks, and strange noises.
- Replacement: When it’s time to replace your old water heater, we offer professional installation of both traditional tank models and modern tankless systems.
- Maintenance: Regular maintenance is essential to keep your water heater running efficiently. Our comprehensive maintenance services include inspections, cleaning, and tune-ups.
- Tankless Water Heaters: Upgrade to a tankless water heater for endless hot water, improved energy efficiency, and space savings. We provide expert installation and maintenance for these advanced systems.
Kitchen plumber services
- Faucets: From repairing leaks to installing new faucets, we ensure your kitchen faucets are functioning perfectly.
- Sinks: We handle all sink-related services, including installation, repair, and replacement, to keep your kitchen fully operational.
- Garbage Disposals: Our team can install and repair garbage disposals, ensuring efficient waste management in your kitchen.
- Dishwashers: We provide installation and repair services for dishwashers, making sure they are properly connected and running smoothly.
- Water Lines: Whether you need new water lines installed or existing ones repaired, GSMS ensures a reliable water supply for your kitchen appliances.
Bathroom plumber services
- Toilets: From fixing leaks and unclogging blockages to installing new toilets, we provide comprehensive toilet services.
- Showers and Tubs: We offer repair, replacement, and installation services for showers and bathtubs to keep your bathroom comfortable and functional.
- Sinks and Vanity Plumbing: Our team can install and repair bathroom sinks and vanity plumbing, ensuring everything operates seamlessly.
- Pipe Repairs and Replacements: Whether it’s a minor leak or a major pipe replacement, GSMS provides expert pipe repair and replacement services to prevent water damage and maintain optimal plumbing performance.
Why choose GSMS in Merrimack?
- Proven Track Record: With a long-standing presence in Merrimack, NH, GSMS has earned the trust of countless homeowners. Our dedication to excellence and reliability has made us a preferred choice for HVAC and plumbing services in the community. We are proud of our reputation and continually strive to uphold the high standards that have set us apart.
- Local Expertise and Dedication: GSMS has been serving the Merrimack, NH community for over three decades, building a reputation for excellence and reliability. As a family-owned business, we are deeply committed to our local community and understand the unique needs of Merrimack homeowners. Our local expertise allows us to provide tailored solutions that keep your home comfortable and efficient year-round.
- Comprehensive Range of Services: From HVAC to plumbing, GSMS offers a full spectrum of services to meet all your home comfort needs. Our skilled technicians are trained in the latest technologies and techniques, ensuring top-quality service for every job. Whether you need a quick repair, a new installation, or regular maintenance, we have the expertise to get the job done right.
Become a GSMS Home Club member today
Join the GSMS Home Club and enjoy exclusive benefits designed to keep your home’s HVAC and plumbing systems in top condition. Membership perks include:
- Priority Service
- No Diagnostic Fees
- 10% Off Repairs
- Home ID and Smart Monitoring
- Monthly Credits Towards Major Purchases
- Regular Maintenance
- Labor Warranty Upgrades
Call today for great service in Merrimack.
Trust the experts at GSMS to provide reliable, efficient, and top-quality services tailored to your needs. Whether you need a quick repair, a new installation, or regular maintenance, our dedicated team is here to ensure your Merrimack home remains comfortable and functional year-round.
Call 603-424-8258 today or fill out our online contact form to book an appointment!
Recent Jobs and Reviews in Merrimack, NH
Customers say: The recent reviews exhibit a predominantly positive sentiment, with recurring themes of satisfaction, professionalism, and expertise. Customers express gratitude, recommend the company, and acknowledge the quality of the work performed. Positive reviews frequently mention friendly and knowledgeable technicians, timely responses, and efficient service. Negative reviews are minimal and do not represent a clear, recurring trend. Overall, the reviews suggest a positive experience with the company, with many customers expressing satisfaction with their HVAC and plumbing needs.
Our Local Reviews by City
- Amherst, NH (9)
- Atkinson, NH (2)
- Auburn, NH (1)
- Barrington, NH (20)
- Bedford, NH (26)
- Candia, NH (2)
- Chester, NH (2)
- Concord, NH (6)
- Deerfield, NH (1)
- Derry, NH (2)
- Dover, NH (1)
- Durham, NH (2)
- Epping, NH (1)
- Gilford, NH (1)
- Gilmanton, NH (1)
- Goffstown, NH (7)
- Greenland, NH (2)
- Hampstead, NH (2)
- Henniker, NH (3)
- Hillsborough, NH (1)
- Hollis, NH (11)
- Hooksett, NH (11)
- Hudson, NH (6)
- Lee, NH (1)
- Litchfield, NH (2)
- Londonderry, NH (12)
- Manchester, NH (45)
- Merrimack, NH (28)
- Milford, NH (6)
- Moultonborough, NH (1)
- Nashua, NH (38)
- New Boston, NH (3)
- Newbury, NH (1)
- Newmarket, NH (2)
- Northfield, NH (1)
- Northwood, NH (1)
- Pembroke, NH (1)
- Raymond, NH (1)
- Rochester, NH (2)
- Salem, NH (3)
- Sandown, NH (2)
- Stratham, NH (3)
- Tilton, NH (1)
- Weare, NH (5)
- Windham, NH (7)
- York, ME (1)
Job Locations and Reviews

Replace 40 gallon natural gas atmospheric water heater.



I performed a furnace maintenance. I started off with checking all the wiring and tubing connections and everything looked good. I checked the inducer motor capacitor, and that was inspect. I replaced the air filter that the client supplies. I replaced the humidifier pad. I flushed the trap and the condensate pump with cleaner. I ran the unit and checked the gas pressures and they were inspect. I did a combustion analyzer test for CO,CO2 and oxygen levels and they were within manufacture specifications. Everything looks and runs great!

Found a loose wire for the thermostat and found that the door switch is very sensitive and wasn’t pushed in all the way.

Replace S30 with S40. Configure new control. Troubleshoot zone stat with communication error. Thank you!
Excellent in all areas of service. Punctual, courteous, kept us well informed of service progress. Upon completion of the furnace installation Adam explained all features of the new system , and addressed questions . The above statement can be said of all GSMS employees as we have used them over the years for several different systems installation. Excellent in all areas.

Replacement of the existing failed gas furnace with a new Lennox high-efficiency gas furnace. The existing Trion air filtration system, Reme Halo UV light, and bypass humidifier will remain in place and be reconnected to the new furnace. The existing A/C coil will be held in place, and the new furnace will slide into position. All necessary supply and return plenum modifications will be made to allow for a seamless installation of the new system. Due to the height limitations of the new furnace, furnace "footies" (elevating risers) most likely will not be able to be installed; while not a code requirement, these are a beneficial feature we typically prefer for basement installations. Gas and electrical connections will be re-made. System venting will remain in place and be modified to follow manufacturer standards. The existing neutralizing condensate pump will remain in place; however, the drain at the PVC will be changed to meet current code requirements, including proper trapping
Excellent service

I started off with cleaning the outdoor units coil with water. I checked electrical connections and everything looked good. Next I went inside and I pulled out the air filters and I cleaned them with soap and water. I cleaned both of the indoor coils with coil cleaner. Lastly I ran the units in cooling mode and checked the temperature coming out and everything looked great.

AC maintenance went great. We swapped the zone stat under warranty. Thank you!

Combi boiler maintenance. Everything went great. No repairs necessary. Thank you!

I found that the outdoor inverter board is improperly communicating with the EEV valve “A” for the kitchen head.

Clean drier vent up to 25'. Tape termination vent to prevent clumping up of lint. Thank you.

Installed and set up smart AC.

Clean flame sensor.Will set date to do complete pm

Installed trap,backflow valve,atm vent to furnace drain in basement

Warranty replace TXV. All set thank you!
Friendly and knowledgeable

Thank you for signing up for GSMS HomeClub! I entered you into out system and completed an AC maintenance. Please see the AC Maintenance form for details of today's services. Thank you!
Everything is working great!

I replaced the flow control valve on the RHEEM tankless water heater. The old one was leaking from the top of the valve. The client also hears a swishing banging sound when toilets are flushed and the washer machine starts. Unfortunately I couldn’t replicate it and get it on video so the homeowner said that he will try and get a video of it when it happens next time.
GSMS helped us out after I discovered water surrounding the furnace. Qwentin quickly found the issue, explained what happened and what needed to be fixed. Great communication, excellent service, and he left the area cleaner than he found it. GSMS is always a pleasure to work with.

First thing I did was locate where the leak was coming from. The drain line coming out of the evaporator coil was clogged up. I snaked the line and then ran a brush through it with cleaner. I also blew out the line with a CO2 cartridge. The client wanted me to perform an AC maintenance while I was there, so I started with cleaning the evaporator coil with coil cleaner. Next I checked the electrical connections and the capacitor and everything looked good. I replaced the air filter with a 20 x 25 x 5 air bear filter. Next I went outside and I cleaned the outdoor units coil with water and coil cleaner. I checked electrical connections and the capacitor and those were good as well. Lastly, I performed a temperature split and connected my gauges to the unit. I ran the unit for about 20 minutes and all my numbers looked great.

Installation of Wi-Fi modules for existing single zone mini split and new mini split system.

Replace water control valve that is leaking.
Five Stars

Replaced bath rm faucet
Couldn’t have asked for a better installation of dishwasher/toilets. Mark was very personable and professional. Arrived on time and finished on time. Excellent experience

Install toilets and diswasher

Install amtrol diatrol

AC preventive maintenance today. I flushed the condensate drain with cleaner and ensured it was not clogged. Please see mainteance form for more detailed results. I found a hole in the heat exchanger, I shut the gas off. The furnace is not to be operated until the heat exchnager is replaced.
Very professional in all respects.

Installation of a three-zone mini-split system to provide the home with conditioned air and supplemental heating. The outdoor unit will be placed on the back side of the home as discussed. First Floor: One indoor unit will be installed on the first floor, positioned above the fireplace. While we will aim to center the unit over the mantle, refrigerant lines will exit the home on the left side to avoid the chimney. Second Floor: Two indoor units will provide coverage for the second floor. One unit will be installed in the master bedroom. Refrigerant lines for this unit will run into the attic and then to the outdoor unit on the other side of the home. The condensate line will drain onto the garage roof and into the gutter system. The second unit will be installed in the guest bedroom. Refrigerant lines for this unit will also run into the attic to maintain a clean exterior aesthetic and avoid running lines over the chimney. The condensate line will drain down the exterior of the hom
I have been working with GSMS for close to 8 or 9 years now. Had them replace my entire HVAC system and on demand hot water heater installed. Today just had my annual service for my HVAC unit done. They really do great work and all their technicians are very skilled and knowledgeable. I highly recommend them for anybody for your HVAC and plumbing needs!

AC mainteance went great today. Please see form for details! Thank you
Great overall service, always has done a great job for me!
Mark and his entire team do a wonderful job. Show up on time and treat your home with respect. Recommend them without reservations.
Outstanding service
Granite State Mechanical Services has been serving our home for 30 years. They show up on time, complete the work and treat your home with respect. I recommend them to all our neighbors.
Excellent service
We count ourselves lucky for customers like you and look forward to working with you again in the future!
Great service as always. Turned out grandchildren tired off the emergency gas shutoff when they were playing some game
Thank you for taking the time to leave such a positive review! We're glad to hear you had a great experience with us.
Your GSMS team set the standard in your business area for integrity, honesty, performance and quality. They are courteous and we trust them being in our home.
High recommend. Called GSMS because of a leak. Jack arrive and within minutes diagnosed the problem - and fixed it. Thank you
Always a great experience!
Very polite technicians. Worked diligently all day. Cleaned up everything before leaving.
GSMS is a great trustworthy company to work with. All the staff members and service technicians are always great and very professional!
Excellent service
Mark as always did a great job
We are so grateful for your kind words.
Very professional and courteous. Arrived at the time when he said he would and took the time to what might be causing my leaking issue
Thank you for the wonderful review and for taking the time to share your feedback.
a pleasure to work with
We thank you for sharing your positive experience. It made our day.
Mark and the team always do a great job solving our issues in a timely manner!
Thank you so much for taking the time to leave us this amazing review!