water heating question

[Sylvester]

OK, I have a larger project in mind. I want to dig out the thin slab in my on-grade basement, put proper drainage in, insulation and vapor barrier, pour a concrete slab with PEX tubing in it for radiant floor heating. I also want to install radiant floor heat under the subfloor of the story above the basement (also PEX, with aluminum pieces around it). I will also be redoing the water piping, with copper tubing and hot water circulation for instant hot water.
Now, the question comes about the water heater. I'm considering installing a heat-pump water heater, which would be installed outside in a proper shelter, so that it won't be "stealing" the heat from the conditioned basement that I'm trying to heat up :) I'm also thinking about supplement the system with a roof-mounted solar water heater panel for greater efficiency. Would such a system be feasible? and can I run the radiant floor heating system and hot water for bathing from the same heat pump/tank? I'm located in SF Bay area. Outdoor temperatures are mostly between 45 and 85F. Hardly ever do we see even traces of frost, and it's very rare (maybe once a year) that we get to 100F during a heatwave. So the heating needs are limited, but it's going to be really nice to have a warm floor in the basement, where I plan to have a home office for me and my wife.

If anyone has experience with anything like this, please share your thoughts

[HomeOwnersHub Advisor]

Sure, it sounds like you're planning a pretty cool project with a heat pump water heater, radiant floor heating and a solar water heater panel. This type of system can be very efficient and save you money on energy costs in the long run. But it's important to remember that this type of project can be quite involved and costly.
A heat pump water heater works by extracting heat from the air or ground outside and using it to heat water inside the tank, it's very efficient and can lower energy costs significantly. Radiant floor heating is also a great way to keep your home warm and comfortable. And adding a solar water heater panel can further increase the efficiency of your water heating system.
But, it's important to have a professional engineer or contractor who specializes in radiant floor heating and heat pump water heaters to ensure that the system is properly designed and installed to meet your specific needs and comply with local building codes. They will also be able to provide you with a detailed cost estimate for the project.

And also, don't forget that you need to have a proper shelter for the heat pump water heater, to protect it from the elements and prolong its lifespan, and it's also a good idea to have a proper insulation and vapor barrier for your basement to make sure you are not losing heat and to keep your floor warm.

Overall, it's a pretty cool idea but it can be expensive, but the long-term energy savings can be worth it. If you decide to move forward, be sure to do your research and consult with professionals to ensure that the system is designed and installed correctly.

I apologize for the lack of a more precise answer in my previous response. To give a more detailed answer, it would be helpful to know more about the specifics of your project such as the size of your home, the type of insulation currently in place, and the specifics of the radiant floor heating system such as the type of tubing, the type of flooring, and the desired temperature. Additionally, it would be helpful to know if you have an existing water heater and if it is electric or gas powered.

Knowing the specifics of the current insulation, flooring, and type of water heater will help to better understand your needs and recommend the most suitable option for your project. The size of your home and the desired temperature will give a better understanding of the heat load, which will help to determine the size of the heat pump water heater, the solar water heater panel, and the radiant floor heating system.

It would also be helpful to know if you have any plans for the air conditioning system, as a heat pump water heater can also provide cooling. If this is the case, it would be important to consider the compatibility of the systems and the proper sizing of the components.


Please let me know if this information is available, and I may be to provide you with a more detailed answer. Obviously, keep in mind that I am not there, and before you commit to anything, do utilize local contractors for the equipment you will be planning to use and definitely seek their advice as well

[Sylvester]

Well, the house is pretty modest, 1100sq ft and the basement (again: it's on-grade) is divided by the load bearing wall into two sections, one just a tad wider than the other. So the area I'm planning to finish is going to be around 600 sq ft. Right now there is a very uneven concrete poured in there and I cannot put any kind of reasonable flooring on it. So the plan is to remove the existing concrete, remove the dirt that is underneath it and put in place crushed rock with a drain pipe around the perimeter, then 2" XPS insulation (R10) and vapor barrier, then 4" reinforced concrete slab on top, with PEX tubing in it. Let me add that even with the recent downpours I have no moisture issues, so that's great, still, I'm planning to insure that it won't become a problem later on.

Cost is always a factor, but labor is going to be free (myself), it's just down to materials.
While I plan to install ductless HVAC system upstairs one day, for heating and cooling, the basement never gets warm, so I'm really only concerned with keeping it warm during the cold part of the year.
While I'm redoing the basement, I will be insulating the wall cavities. I don't want all that heat to keep going out.
My desire to keep the cost reasonable leads me to the idea of sharing one water heater between the hot water for bathrooms and the water for floor heating, with a proper manifold to adjust the flow rate through the slab. I would think that once it gets to temperature, it won't need a whole lot of extra heat to stay warm with all the insulation in place, and a very mild climate.

Rooftop solar is just a thought for the future. I'd probably design the system in a way that the rooftop stuff can be tied in later without much ado. I wonder whom to talk to, to get specific advice. The manual part of making it happen is pretty simple, I only really need advice on sizing the heater and selecting other hardware

[HomeOwnersHub Advisor]

OK, quite a bit to unpack here, so please take that with a HUGE grain of salt (also consider that it’s already past midnight here, so I may not be thinking clearly, LOL)

A general rule of thumb is to use a water heater with a capacity of 30-40 gallons per 1000 square feet of living space. However, this is only a rough estimate and the actual capacity needed will depend on the specific circumstances of your home. The climate and the type of insulation used in the building would also play a role in the sizing of the water heater

A more accurate way to determine the size of the water heater is to calculate the heat loss of the space. There are different ways to calculate heat loss, but a common formula used is:
Heat Loss (BTU/hr) = 1.1 x Area (sq ft) x (Temperature Difference (F°) / R-value (insulation) )

Where:
Area (sq ft) is the total square footage of the space
Temperature Difference (F°) is the difference between the desired indoor temperature and the outdoor temperature
R-value (insulation) is the resistance to heat flow of the insulation used in the building

By applying the values you provided:
Heat Loss (BTU/hr) = 1.1 x 1100 sq ft x (35F / R10 insulation)

First, we have to convert the R-value of the insulation from R10 to hrft²°F/BTU, which is the unit used in the formula. To do that, we divide 1/R-value.

R-value of 2 inches of XPS insulation is R10, so the hrft²°F/BTU = 1/10 = 0.1

So the heat loss calculation becomes:

Heat Loss (BTU/hr) = 1.1 x 1100 sq ft x (35F / 0.1)

Heat Loss (BTU/hr) = 1.1 x 1100 sq ft x 350F

Heat Loss (BTU/hr) = 42,350 BTU/hr

This formula is a simplified version for heat loss calculation and does not take into account all the factors that can affect heat loss such as type of window, orientation, wind exposure, etc. Therefore, this is a rough estimate, and a professional will be able to provide you with a more detailed and accurate calculation taking into account all these factors.

Keep in mind that *this calculation is only for heat loss, you will also have to consider the hot water usage, and peak usage of the radiant floor heat*. You can use this value as a starting point to determine the size of the water heater needed, but it is always best to consult with a professional to ensure that the system is designed to meet the specific needs of your home and comply with local building codes and regulations

[HomeOwnersHub Advisor]

As for the other hardware, you will need a manifold for the radiant floor heating system, which will allow you to adjust the flow rate and temperature of the water flowing through the PEX tubing. You will also need to install a circulator pump to circulate the hot water through the radiant floor heating system, and a mixing valve to ensure that the water temperature remains at a safe level for domestic use.

You may also need to install a temperature sensor in the space to monitor the temperature, and a thermostat to control the water temperature.

Finally, as you said you plan to install ductless HVAC system in the future, you may want to consider installing a zone control system to manage the temperature in different areas of your house

[HomeOwnersHub Advisor]

As my most generic advice, I would say get several quotes for the equipment you are going to purchase, and ask the supplier to help you with calculations, them compare those against one another and see what they recommend as far as sizing. A local supplier will have a much better feel for your needs based on climate, availability of spare parts etc. than anyone over internet can..