First, what do you mean by "verified to be right-sized for my house?" If that means the unit sizes were selected based on a Manual J or similar calculation to calculate the expected maximum heating/cooling loads, then there may well be enough extra capacity in the existing system. If you mean that the system has been found to be running close to flat out to keep up with the heating or cooling demand at the design minimum outside temperature, then there is no extra capacity in what you have now.

Since you have two units, then even if the size of each was based on good calculations, those sizes were rounded up to whole tons for selecting the units. If each involved just a half ton round-up, you already have the extra ton you are assuming you need.

Adding 600 sqft of living area doesn't necessarily mean the extra load will be 18% of your present capacity. You need to calculate the difference based on the shell of the addition. Presumably you will have some added roof/attic area over the new space, but that could (probably ought to) be better insulated than over the existing house, so that the addition of that part of the new heat load may be minimal. Similarly, the addition will add some new exterior wall area, hopefully better insulated and with better windows than the existing structure has, and you need to subtract from the current maximum load for any existing walls that become interior walls.

You need good numbers, not guesswork and rules of thumb.

You may be able to calculate a decent approximation of the aggregate U*area of the existing house, then multiply by design maximum temperature difference to see how the result matches up with the current capacity, then correct for the proposed changes to get a properly calculated load for the new configuration. For this you need to have good information on power cost for just the heating system, from the electric bills over the heating system without the base load for hot water, appliances, lighting, etc (from the shoulder season bills). You also need a good estimate of overall COP (coefficient of performance) of the equipment, which ought to be available from the specs provided by the vendor at the measured EWT (entering water temperature) and air flow, with an allowance for pumping power. Finally, for the season for which you have the power cost you need the heating season total heating degree days for your location (assuming you are in a heating climate, zone 5 or higher). The calculations aren't rocket science, but if it is outside your comfort zone then you ought to have someone knowledgable do it for you.

By "verified", I simply mean both the installer and a second HVAC contractor have done calculations and declared it is the "right" capacity for the existing house. No one has ever done a measurement of how hard the system is working.

Your suggestions are all good and I will need to rely on an expert to run the calculations for me. Thanks!

From what you've detailed about you're exiting 2 systems, they seem to be well designed. Boreholes drilled to depths of 200 feet per ton are VERY GOOD!
I would suggest contacting this installer for a consultation. Perhaps the existing system can be modified with zoning so that additional HP capacity will not have to be added.

Post more details about your heat pumps (brand, model numbers) and ground loop configuration as this may useful. Are the ground loops for each system completely independent? Are the ground loops a series or parallel configuration? If the ground loops are in parallel do you know if the headers are located within the foundation or buried externally?

Where are you located? Does this 3300sq ft include a basement, if so how many sq ft? How hot and humid are your summers? That's the main concern; does the existing system have to capacity to provide adequate A/C and dehumidification?
If the heating capacity will be stretched only marginally, this can be made up with the backup or aux heaters. If the A/C capacity is too small you will have to add more capacity.

SR

The system is a Waterfurnace split system. Model numbers NDA026 pump with NAH026A handler and NDA036 pump with NAH036A air handler. There is one FC2-GL flow center tied serially through both pumps. This feeds the ground loop that splits in parallel to the 5 wells. The loop goes into the ground shortly after the pumps and splits off to the 5 wells underground.

Located in Seattle so weather is pretty temperate year round. No basement; first floor is 1690 sq ft and upper is 1590. No south-facing windows but lots to the east which actually has lots of tree-cover. Addition is to the south and will add no south-facing windows and just one large one to the east.

Both systems have backup heaters. Does this mean the backup heaters will just kick in more? I'm OK with that. The A/C is sad but I can do some heat management (whole house fan in the evenings) to make that better.

The installer seems pretty adamant that we need one 200' well per ton. Drilling another well and hooking into it is really more than I'm willing to deal with if this is just a matter of adding a small, separate system to the new areas. I'm happy to upgrade the pumps if that helps as that should be a much simpler prospect... I'd move the 3 to where the 2 is and then buy a 3.5 to replace the 3 giving me a total of 6.5. What happens if I put too much pump on the wells?

I would suggest speaking with WaterFurnace about your situation. It appears to me that you have a well-installed ground heat exchanger and that you may be able to condition the addition planned in perhaps 2 or more ways without adding to the ground loop system.

One method would be to use your existing GSHPs with zoning, no HP added. Another method would be to add an additional 2 stage GSHP with controls to coordinate all HPs. Your existing HPs may be 2-stage. You may be able to run 3 HPs simultaneously in 1st stage only and have enough ground heat exchanger. It may also be possible to configure 3 HPs so that no more than 2 may run at any one time with a priority sequence given for the zones that are most important to you. A variable speed flow center with motorized cut off valves to each HP should also be explored for greater efficiency and energy savings.

You may require more backup heating at times but you say you're OK with that. Why is you're A/C 'sad'?

Probable worst-case scenario might mean that 1 more borehole per ton capacity added would have to be drilled. If these could be connected to the existing headers, assuming they are buried outside and can easily be found, then this would mean less physical disruption. There would be the additional cost of course but it may not have to be done at the time of a 3rd HP installation, if at all.

Too may tons, not enough ground loop will mean that the system will not perform well, if at all; not a situation you want to be in. Managing the existing system with perhaps an additional HP might work. Consult local experienced & accredited professionals.

I hope this helps provide some new directions to explore. Please post your own findings and analysis.

SR

Thanks, SR.

I'm "sad" if I have to lose A/C (if the backup heat works but there is no "backup A/C"). :-)

All good suggestions. I'll talk w/my local installers, passing on some of the ideas here.

I'll let you know what ends up happening.

Unfortunately there is no way to compensate for inadequate A/C other than to add more A/C. That's why it's so important to get the A/C heat load and capacity right the first time, if the heating side comes up short you do have the option of a plenum heater.

SR

You might possibly be able to spend a little more in insulation/sealing techniques and only need a small amount of hvac to do the job. Spray foam insulation comes to mind or home wrap the addition and upgrade to 2x6 walls with R19 insulation. If you think outside of the box I think you can solve any heating and cooling issues without the need to drill any more wells or add any systems other than possibly zone the new addition which would be a huge cost savings.