Establish the new conditions
Tell the provider exactly what changed: furnace removal, closed vents, insulation, air sealing, or other work. Ask how representative cold-weather temperatures will be measured where the heater would draw air and where water and drain lines run.
A summer reading or one warm afternoon is not a winter assessment. If installation cannot wait for winter, request the professional's conservative design basis and what remains uncertain. Do not use an improvised heater in the crawl space as proof that the location is suitable.
Compare the equipment limits
Ask the installer to identify the selected model's allowed location, air-temperature range, airflow requirements, and behavior outside its heat-pump range. Separate efficient heat-pump operation from backup heating and from freeze protection for the whole water system.
Have qualified providers evaluate the tank, supply pipes, condensate route, access, moisture, and existing crawl-space condition. Do not assume a backup element protects every nearby pipe or drain.
Request alternatives if the space is unsuitable: a better location, a properly designed enclosure or building correction, or another equipment choice. Any ventilation or ducting must follow the actual model instructions and applicable building requirements.
Name the related work
The heater installer should identify the equipment requirements. Appropriate building, insulation, moisture, plumbing, and electrical providers should own their related work, with coordination and cost included in the plan.
Your next step is to request a winter-site evaluation that reflects the home after furnace removal. Choose equipment only after the proposed location, freeze concerns, and related work have a supported answer.