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single boiler for rad heat & hot water

New poster here from the "handy man with engineering background" category. I've been refurbishing our 100 year old 3 decker and have mastered basic plumbing and electrical work in the process.

the problem .... Our first floor is divided into 2 units which share a gas fired steam heating system in good repair, one zone and one thermostat. We have a single hot water boiler for the house. I'm considering an upgrade that would make the tenants more comfortable and to allow me to turn over the gas bill to them. ($2500+ per year). The joists in basement are exposed. I'm considering a gas fired combination radiant heat and hot water unit for each apartment that mixes heat transfer water and hot potable water. I've seen many good posts on this subject but none has addressed my specific question. Here it is:

Can the boiler be one of those single pass hot water heaters with piping that mixes heating water and hot potable water?

Before you all yell NO YOU IDIOT, you can't mix hydronic heating with potable water! .... I've gotten a schematic from a vermont firm specializing in supplying DIYers which shows such a system. Would this pass code in Mass? Are there important design considerations which would make this sort of system operate poorly or not at all? The obvious benefit is the lower cost of a single heating unit with no storage losses from hot water tank.

Re: single boiler for rad heat & hot water


Very few states these days allow the combination heat & HW that you describe----you can Google the Mass. Plumbing & Heating codes for your state.

What you've got there is a BUSINESS (read semi-commercial) rental operation---I encourage you to think more along the lines of zoning off each of the apts so that they have their own t-stat---if you want, there are ways to determine which apts use the most/least heat by monitoring the zone valves & determining the rent on that basis---hot water heating is EXTREMELY CONDUCIVE to very efficient zone valves--the flexible PEX plastic piping can be threaded up thru walls & floors without too much trouble & the baseboard units go in very quickly.

If you discover inadequate insulation, don't ignore the blown-in cellulose insulation for the exterior walls (R19) and the attic (R40)---this is done from the outside for a few hundred $$$; the windows must be tight double pane or storms.

You didn't post the sq. footage of the apts or the total sq.footage of the entire house.

If the present HW boiler is in good shape, you can simply zone off the 2nd & 3rd floors, then install a 2nd HW boiler with zones & separate t-stats for the 2 apts on the 1st floor----but first calculate the total sq. footage of the entire house, multiply by an arbitrary # such as 40 to get a rough estimate of the total btu/hour needed for the entire house & check the btu output of the present HW boiler.

It may be possible to heat the entire house with the present HW boiler, if it's big enough---it's very common in these setups to see a copper manifold attached to the hw SUPPLY pipe coming out of the top of the boiler having 5 zone valves in a row (or circulators) and then the same # of zone valves or circs on the RETURN line going back into the boiler (4 ZVs for each of the apts & the 5th for the indirect hwh)---this setup is usually the most cost-effective, not only in installation, but in ongoing operating costs.

Thus, if, say you have 800 sq.ft. on each floor:

800 X 3 = 2400 sq.ft. x 40 (heat factor) = 96,000 btu/hr needed to heat the house---check the label on the boiler---it will give the OUTPUT in BTUs/hr so you can compare the two numbers---in any event you should consider in addition, going with an INDIRECT HOT WATER HEATER for all the HW for the house.

An IHWH can provide as much domestic hw as the apts need & it needs no separate flue---the boiler hw is used as a heat exchanger to heat the domestic supply---the IHWH would also be put on a manifold/piping for each apt---this type of hw heater has been found to be the most efficient & low-cost way of producing domestic hw---the well-insulated tank will last for decades without problems.

Also Google "rental unit" "heating systems" hydronic; or "multi-family" "heating systems" hydronic.

Contact local heating contractors who specialize in this type of install for further info--also contact local real estate agents who own rental units & deal with this issue on a daily basis---they can probably steer you to heating contractors who do this type of install---don't ignore oil dealers, who are also licensed to install gas equipment & often charge less.

I would encourage you to first consider baseboard over radiant, because it is so easy to install at a cost much lower than radiant.

Since this is income-producing property, the IRS via Schedule E allows you to write off any retrofit expenses you incur for insulation, windows, boilers, etc. for that percentage of the new item tjat cam be applied to the 3 rental units.

Re: single boiler for rad heat & hot water

Thanks Nashua .... lots of good information. but we have a communication gap. I meant to say one "hot water heater" for the whole house and not "hot water boiler." sorry for the slip.

The first floor has two apts (800sq ft & 400 sq ft, good windows, no insulation) heated by a single gas fired STEAM boiler in good working order but difficult or impossible to zone yes? steam hammer in return line etc etc.

I was hoping the expense of radiant floor heating would be mitigated by my ability to run pex and the reduced expense of combining heat and hot water in single heating unit. I'd like to pass on heat and hot water costs to first floor tenants if possible. Any information you might have concerning the legality or viability of such a system would be especially appreciated. I'll poke around in mass code to see if I can find my own answer but it's a big code for an amateur.

I need to defray new install costs and/or pass on heating costs to tenant to encourage frugality on their part. I am interested in creating a single system with 2 zones and logging valve positions etc in order to divide gas bill fairly. Then perhaps I could add a second heating unit in the future. Links to information on what sort of equipment to buy in order to do this would be great.

Brian Shriver

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