Recommendations for Closed Loop Hydronic Heat and DHW system

Hello,

I’m looking for some recommendations for which concept I should go with to supply both hydronic heat and DHW in a house with a small heat loss and a fairly large DHW demand. My house currently has an open loop hydronic system using a natural gas hot water tank that is about 11 years old. I very much want to switch this to a close loop system and change the tank soon as it is getting old. Preferably I would also like the new system to put out a little more heat as the current one can’t hold the set point on the colder nights.

I have a fairly low design day heat loss of about 22,000 btu/h since my house is small, not too old, and in a pacific north west climate. The house has 4 separate zones using Turbonics wall mounted fan coils which I do not plan on changing. There are 5 separate wall fans, one zone feeds 2 fans in parallel, the other zones feed 1 wall fan each. I have a fairly large Jacuzzi tub that needs approximately 50 gal of 110F water and we often need to run two showers at once, although not at the same time as the tub. The current hot water tank is 65 gal and I set it to about 125-130F in the summer and to its max temperature (about 140-150F) on the coldest days. At 140-150F the systems can only produce about 17,500 btu/h of heat so that is why I think it struggles to hold the set point on really cold nights. My current gas line can support up to about 100,000 btu/h otherwise I would need to replace it with a larger diameter which is possible, but adds cost. I also do not have a lot of space so the foot print / wall space of the new system can’t be too much bigger than what the current tank takes up.

Most contractors are recommending a 200,000 btu/h combi unit with a 10:1 turndown. That seems very oversized for my heating, especially considering the small zones (about 3,500 but/h each), but I imagine it would do ok with DHW although min flow rates, cold water sandwich might be a small annoyance. Based on my research I have come up with the following ideas, but I am looking for some advice on which path to choose. Please let me know what you think.

Options (in no order):

Combi (20,000 to 200,000 btu/h) + 20gal buffer tank for heat side:
This will work, but it appears to be a compromise on both the heat and DHW side. Reading other posts on this forum gives me the impression that several professionals are not fans of combi systems which makes me not favor this option.

HTP Phoenix LD (60gal) + Flat Plate Heat Exchanger for heat:
This is probably the simplest and most cost effective solution. I would expect great DHW performance, but it will only get to about 150F on the heat side once going across the heat exchanger. Although 150F works for the heat, it is what I can get now so it still might be lacking output on the cold nights. Perhaps something I could live with given the cost and simplicity of this system.

Boiler (aprox 17,500 to 175,000 btu/h) + 30 gal buffer tank + Flat Plate Heat Exchanger for DHW:
This is an interesting approach, but I suspect it isn't common, and likely costly. In this system the boiler only runs to maintain the buffer tank temp based on an aquastat (perhaps with a reset curve). Both the DHW FPHE and the heat system draw off the buffer tank in either a 4-pipe or 2-pipe (preferred) configuration. The DHW FPHE uses a circulator pump switched from a low flow rate (0.2gpm) sensor. There is no DHW storage, a timer could be used to pre-heat the FPHE occasionally during the day if necessary. This system doesn’t appear to be common but I have seen some sketches of it. I really like that the small tank is used to buffer both the DHW and heat side. It is like a combi except the buffer tank addresses a lot of the short comings on the DHW side. It will fit in the current space too but be more costly then the two options above since it needs an additional pump and a fairly large FPHE.

Small Boiler (8,000 to 80,000 btu/h) + 60 gal indirect:
This is a more traditional likely a very good plan, but requires me to relocate my existing manifold (and the lines to it) to make room for the tank and boiler. I also wonder if the 8,000btu/h min is low enough to prevent short cycling when only 1 (~3,500 btu/h) zone is calling heat, which often happens in the shoulder months. Also, tank recovery time is a small concern too since most manufactures recommend larger boilers for an indirect tank this size.