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Zone Balancing, Boiler Protection, Buffer Piping/Control
Now entering our first heating season with our new gas-fired Peerless MI-03 installed last Spring, some observations:
2 CI rad zones, 1 baseboard. Heat Loss 38K, Boiler Net: 50K. DHW: Turbomax 45. Hydrostat 3200+. Piped in series. Longest zone 190ft (11-12ft head?) Total piping all zones, including boiler and near boiler return 1.25” piping about 400ft. which would be around 10 gallons plus the 6 CI rads. Say 20 gallons total volume? VT2218 Circ set for ∆T. Taco Sentry Zone Valves; Caleffi Circ Setters Cold start, hi limit 160º.
Concerned about low return temps.
1. Cycle Example: Daytime, outdoor temps high 30s. Call for heat from one CI rad zone. Any hot water in boiler left over from prior heating or DHW cycle is purged by circ. Boiler fires, circ pumps, boiler temps may go as high as 130+ then quickly go down to 115. Circ is then stopped by Hydrostat for 30-60 seconds on Circulator Hold until boiler temp reaches 125; circ starts, boiler temp again goes to 130s then within a minute falls to 115, etc. This can happen 6-7+ times in a 20-minute cycle. Not sure if the Hydrostat Circulator Hold is adequately protecting the boiler from the low temperatures. At the end of the 20 minute cycle boiler temp is usually mid-140s, sometimes higher.
2. In cycle the VT2218 is pumping anywhere from 3-12gpm. It usually settles on 3-6gpm. The circuit setter does not really have the power to throttle circ down completely to their settings. So if I test the flow of the setters when circ is pumping 9gpm, flow can show about 4-5gpm; if I test it when circ pumps around 3-4gpm, flow shows about 2.5-3. I understand that this throttling up and down is the nature of the ∆T circulator setting. Some Wallies had advised me that this circ would be fighting the circuit setters. Will it be helpful to slow down the flow to 2gpm so as to give the boiler more time to heat the cold return water? Perhaps I can try changing the mode to Fixed Speed? (One CI rad zone’s loss is 10K, the other 17K. We haven’t yet had the two CI zones call for heat at the same time but that would increase the cold return water issue.)
3. I thought perhaps we could switch out the VT2218 with the Turbomax DWH VR1816 ∆P circulator since that might be able to balance the flow on the fly with however many zones are calling.
Perhaps pros think the Hydrostat Circ Hold function is protection enough?