Gas heating system recommendation

alerya · October 2019

Hi all!

I'm new here and by no means an expert. My spouse and I recently purchased a 70 year old 1282 sq ft 3 bed 1 bath home in the New England area with gas forced water baseboard heat . Upon inspection, the existing standard slant fin cast iron boiler was found to be at least 50 years old and possibly original to the home. There is a standard 40 gallon gas water heater, installed in May 2011. The current system was found to have poor draft and ventilation and does not have adequate clearance from the doors/walls of the utility closet where it is stored. Repair/replacement was strongly recommended, however we have had several professionals out to give estimates and recommendations, and they don't seem to agree on what the best system for our needs is.

We would also like to improve efficiency and hopefully reduce our heating bills with the system upgrade. The two dominant suggestions that are being made by the heating professionals are:

1) A wall hung 95%+ efficient boiler coupled with a 40 gallon indirect water heater
(Viessman 200 series, model #B2HB-35, Viessman 40 gallon Vito-Cell)
2) A wall hung 95%+ efficient combined boiler and on-demand water heater

One popular company in our area that a lot of the neighbors have used strongly dislikes combis and won't even install them. A couple of other contractors have said that a combination should be fine for our needs (it also costs several thousand dollars less than the separate system with the indirect).

We are fine with the added expense of the separate system with indirect if it is truly a great benefit and the efficiency of the two systems are comparable. We are a family of five, two adults and three children, and plan to be in this home for many years to come. Any experts or experienced folks care to weigh in?

Thanks so much!
alerya

Jamie Hall · October 2019

Got to admit that I'm not really keen on combis either. Not so much that they don't work; they do. The problem lies in getting them properly sized for the heating load and at the same time the domestic hot water load. They are rarely even similar -- and a unit big enough for the domestic hot water is often seriously oversized for the heating.

That said. In any event -- whatever you were to go for -- the very first step is to do, or have done, is a heating load calculation for the house. It's not that hard to do; there are several on-line calculators for that (I happen to like Slant/fin's -- https://www.slantfin.com/slantfin-heat-loss-calculator/ -- but there are others). Without that you simply have no idea what size heating load you are trying to meet -- and particularly with the newer mod/con boilers you can't even come close to the alleged 95% efficiency unless they are closely sized to the heating load.

Calculating domestic hot water load is actually more difficult, since the demand varies so much with the people in the house and how they use hot water, but for a combi or any other on-demand type of water heater you really have to do it, and be conservative about it, otherwise you will have cold showers and unhappy people (two showers at once, for instance, may require as much as 170,000 BTUh input to the heater!)(which illustrates the match problem -- the heating load isn't likely to be more than a third of that).

So... you need a contractor who is willing and able to do the heating and water load calculations, and recommend units (I'd go with the mod/con and an indirect) which will match -- and then will do a quality job of installing them. Particularly with the more modern units, the installation, and therefore your satisfaction, is more than half the battle. The best units in the world will perform poorly if not properly installed and set up.

Gas heating system recommendation

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