Sorry for the novel, but I want to provide as much info as would be helpful.
I have a 17 yr.old home which I have owned for 2 years. It is two-story with in-floor RH on both stories (in-slab on the first floor, gypcrete on the second). Avg outside winter temps are high/low 45/20. I am a DIYer, knowing just enough to get myself into trouble, an engineer by trade. The only work I have done on the RH system is to replace a Honeywell zone valve and a backflow preventer valve, both faulty when the home was purchased but discovered several months after purchase.
The boiler is a Weil Mclain CGa ('95, original to the house), 81% AFUE. The hot water outlet goes through a lever-style shut-off and directly spits into the 3 zone supplies, which disappear into the wall. On the wall adjacent to the 3 zone supplies are the 3 zone returns which each run through a zone valve and then union into a single supply feeding the TACO circulator via a lever shut-off valve, which then goes directly into the boiler supply. Tee-ing into the boiler supply after the circulator is the expansion tank with a spigot which receives cold water supply via a lever shut-off valve and the backflow preventer valve, which drains to the exterior of the home. I can take pictures and attach them this evening, if needed, but I don't have any right now. The system if visually pretty simple. All pipes are wrapped in foam.
There are three zones, one for the upstairs (8'-10' cathedral ceilings), one for the two downstairs bedrooms and bath (8' ceilings), and one for the main portion of the downstairs (ceiling and dining room with 8' ceilings and the family room with fireplace and vaulted ceilings to a 2nd-story loft).
Expensive $500/month propane bill ($2.50/gallon for propane). 4 space heaters typically only run up the electric bill $150/month.
Additionally, when we've run the RH, the home doesn't usually stay warmer than 65*F. We have had a home energy audit done, which indicated the slab was not insulated, but the rest of the house was well insulated and there were no leaks in the RH system. 3 or so local plumbers have been to look at the system and (after rather un-thorough exams) all concluded we should just replace with a new high-efficiency boiler (95-99% AFUE, $10k install).
Boiler manual indicates (many times in bold print) the minimum return water temp should be 130*F. My understanding is that RH is best at 90-120*F supply (70-100*F return), with 140*F max and exceeding that risks damage to flooring. The manual indicates for low-heat RH applications, the piping should feature a feedback path and mixing valve to ensure the return water temp is >130*F, but **our system has no such feedback**. Additionally, the temp. control for the boiler only goes down to 140*F (it has historically been set at 140, which the boiler temp. gauge confirms), so I can't even set it for optimal RH temperatures. When the boiler runs, it seems to run forever (like all night, I don't suffer from short cycling), and the PSI seems to maintain in the normal range (14 or so).
1. Is a new boiler really the solution or is the problem just RH. Space heaters work well and for 10k we might rather DIY install a solar water system large enough to run the main zone when solar production is normal and keep the heaters for the bedrooms (we live in the SW desert, a near-perfect solar environment).
2. Is our boiler piping correct for low-temperature RH w/ Weil Mclain CGa? Could fixing this, along with a new, efficient, low-temperature-friendly boiler produce substantial savings? We would switch back to the RH if we could get the cost down to avg. $250-300/month.
I'm not an expert, but I appreciate your constructive advice/expertise. I'm beginning to trust the opinion of contributors to this forum more than our local plumbers.