I posted this in another forum and members suggested I post here as well for the expertise this forum has. Apologies if you're seeing this twice.
This past summer we bought a 1933 house in metro Boston with a forced hot water hydronic system complete with cast iron radiators and a 40+ year old peerless gas boiler. Against my better judgement, we proceeded to remove all the radiators, replace the boiler with a Bosch Greenstar 100k (95% efficient) with indirect DHW, and an Unico high velocity system for heating and cooling on both floors, zoned separately. So hydro-air heating. The air handler is a Unico M3642 (3 ton) unit and the condenser is a Bosch BOVA-060 5 ton unit with heat pump. I'll note that I just had a blower door test which came back at 16 ACH50 for the 2000 sf balloon framed house, and we're trying to bring that down with air sealing.
Fast forward to the heating season and the system is pretty unbearable. The heat pump is set as primary heat down to 40 degrees F, and then the hydroair system takes over. The blower speed is turned down as low as it can go, and it still sounds like a windstorm outside everytime it turns on. The high velocity system just makes it feel drafty with the cool air temp from the heat pump, though the boiler is not much better. At just below 40 degrees F, the boiler heats supply water to cose to 160 degrees F, which I assume means it's never able to condense because I suspect the return water temperature isn't more than 20 degrees lower. The system does modulate temperature, but I've never observed it below 140-something, and I wonder if it's just cooling down then. The system also cycles multiple times per hour as it reaches temperature, stops and the indoor temp immediately drops. I don't know if this means the boiler is close to short cycling. When it was 25 degrees outside last week, the hydroair just never turned off for hours continuously, but at least the exiting air temperature was warm enough to not be uncomfortable. Even when the outdoor temperature was only 35 degrees, it took 10 hours for the indoor temperature to increase from 64 to 70 degrees (setback for a vacation).
In a nutshell, all of my fears about the new system compared to the old have come true. The one thing the system does well is maintain even temperature everywhere, and I suspect that some of it's poor heating attributes will serve it well for cooling.
Am I crazy to want to put CI radiators back in and use the new boiler for what it should be good at doing? We gutted the low height cast iron piping in the basement for future head height, but all the CI piping to the second floor is still in place. In an ideal world, I'd hope the system could be repiped with oxygen barrier pex for home runs and zoning by floor. I'm not sure if these two different hydronic systems would be compatible, or if we'd just have to abandon the Unico heating coil and redo the boiler for radiators.
I did a load calculation usig the Slant/Fin app which suggests 70k btu/h @ +5 degree outdoor temp, 70 degree indoor temp. It's also been suggested this is overestimating -- it's ~35btu/sf. The Bosch 100k boiler feels oversized considering it only has a 5:1 turndown. I calculated the CI radiator output and it came to 118k btu/h @ 180 degree water temp. So definitely oversized to be able to lower water temp into the condensing region for a lot of the heating season. Any way to make the current boiler work for higher outdoor temperatures without short cycling?
This is definitely a costly mistake, but I'm taking the long term view for comfort. Thanks for your thoughts, very much appreciated.