Critique on hot water system design please!

mcpusc

Hello! I'm looking for advice/a critique on the hot water heating system I'm installing in my house. It's a 1920's bungalow in Seattle that originally had a gravity hot-air system. The previous owner replaced it, very poorly, with a forced-air furnace that kept the main room hot but nowhere else. We're doing a renovation & went with hot-water based on 1) our happy experience with hot-water heat in a rental a couple years back and 2) no bulkheads taking up our limited basement headroom!

I hired a local hydronic company to do a heat loss calculation for the house before we started. They included a proposal for a system but our budget required that I source & install the system myself (allowed by code here). I got a copy of MHH by Siegenthaler and worked through it 'til I felt comfortable with the concepts. I'd like to get some experienced eyes before I start soldering the mechanical room pipework together.

The basics:

2000 sq ft bungalow; equally split between conditioned basement (footing ~4 deep, cripple wall) with main living area above.
Single-pane windows
Blown-in cellulose insulation in 2x4 walls.

Design Temp 25F outside / 68F inside, 140F average water temp.
Heat loss @design temp 28700 BTUH

Lochinvar Knight WHN-055 boiler, 55/11 KBTUH
Lochinvar 30G buffer tank
Honeywell AQ200 Zone Control/ODR w/communicating thermostats
Runtal UF & UHX series radiators
all piping in copper
DHW as preheat through brazed plate HX

7 zones have been installed (Heat Loss/installed radiation@140F):
Open Plan Living/Dining/Kitchen 8400/10000
Shop 5200/6000
Bed(Master) 3600/4000
Bed(East) 2000/2100
Bed(West) 3800/4200
M. Bath 1000/1400
Bath 800/1300

I include a buffer tank because Seattle has very mild winters - we need heat 8 months or so a year, but only get below freezing a couple times. Since even at design load my zones are very small I wanted to avoid cycling. I've also included closely spaced tees and valves so the tank can be cut in and out of the system while maintaining P/S separation.

The feed system is unconventional, as backflow preventers require a permit & annual inspections here. My payback for using a feed tank, diaphragm pump, accumulator tank and check valve is 3 years over installing, permitting and inspecting a backwater preventer.

Thanks in advance for the help!

mcpusc

oops, the upload didn't go right the first time. Here's the schematic.

hot_rod

Consider a two pipe buffer tank. It cleans up all the P/S piping, and provides some additional benefits..

The loads could be pulled off with a mixing device with ODR to add some buffer tank draw if you run the tank at 140, or just reset the buffer with the boiler.

This Idronics explains the difference between 2 and 4 pipe buffer.

http://www.caleffi.com/sites/default/files/coll_attach_file/idronics_17_na.pdf

Solid_Fuel_Man

What are you using to control the DHW preheat circulator? Is it necessary? Will the Geosprings not keep up if in Electric mode? I'd consider a geyser add-on heat pump water heater tied into an indirect tank. That way you have dehumidification and a/c benefits when desired, but can heat water with the khn 100% as well.

Also what @hot rod said.

Taylor

hot_rod

Most of the radiant manufacturers offer this type of manifold. ECM circ, motorized mixer althea purge and balance devices.

Plug and play this into the buffer, it could be controlled by ODR.

Zman

Depending on your local energy costs, it is very likely that an indirect water heater will be far more efficient than the geospring.
Most people don't realize that the energy "saved" by the heat pump actually comes from the inside of the home and needs to be replaced by the boiler anyway.
If you post your costs, we could run the numbers.

Ironman

I would not recommend trying to initiate a domestic call with a flow sensor; you'll get very undesirable results. A heating boiler, like the WHN, is not designed or controlled to rapidly switch on domestic heating like an on-demand water heater.

I second the advice of others and would say invest in a good indirect.

Nice drawing.

mcpusc

hot rod said:

Consider a two pipe buffer tank. It cleans up all the P/S piping, and provides some additional benefits..

The loads could be pulled off with a mixing device with ODR to add some buffer tank draw if you run the tank at 140, or just reset the buffer with the boiler.

thank you for this suggestion. I hadn't come across a good description of why you'd want to choose between a 2-pipe and 4-pipe convfigurations. Lochinvar suggests the 4-pipe layout in the I&O for the buffer tank, so I went with it. Now that I've read up, 2-pipe sounds better for my use.

Is it useful to maintain the ability to cut in/out the buffer tank from the system? Or is this just overkill flexibility?

hot rod said:

Most of the radiant manufacturers offer this type of manifold. ECM circ, motorized mixer althea purge and balance devices.

Plug and play this into the buffer, it could be controlled by ODR.

I'm already committed to a site-built manifold from fittings, but could you tell me why you'd maintain the buffer at high temp. & mixing to a reset point vs letting the buffer reset along with the system? I figure that keeping tank temperture low reduces heat loss in the tank.

Solid_Fuel_Man said:

What are you using to control the DHW preheat circulator?

The zone controller (honeywell AQ250) has controls for a DHW demand and circulator.

Solid_Fuel_Man said:

Is it necessary? Will the Geosprings not keep up if in Electric mode? I'd consider a geyser add-on heat pump water heater tied into an indirect tank. That way you have dehumidification and a/c benefits when desired, but can heat water with the khn 100% as well.

Ironman said:

I would not recommend trying to initiate a domestic call with a flow sensor; you'll get very undesirable results. A heating boiler, like the WHN, is not designed or controlled to rapidly switch on domestic heating like an on-demand water heater.

I second the advice of others and would say invest in a good indirect.

The Geospring is existing and paid for with a very generous local rebate, before we settled on the hydronic heat. I'll probably replace it with an indirect when it dies but its staying until then. We've been happy with it, electricity usage is much lower than the electric it replaced. It takes most of the load off my dehumidifier too.

I want to install the DHW loop piping now as the mechanical room will be pretty snug. I got a deal on a surplus HX locally, so I figured I'd hook it up as a preheater and have the option to take the load of initially heating the water from electric and put it on gas.

Cooling is a plus in the summer so I'll disable the DHW loop then.

Ironman said:

I would not recommend trying to initiate a domestic call with a flow sensor; you'll get very undesirable results. A heating boiler, like the WHN, is not designed or controlled to rapidly switch on domestic heating like an on-demand water heater.

What are the problems with the flow sensor? is this a controls issue (debounce or something) or a water temperature quality issue you're talking about? The hot water produced won't be used directly, just fed to the heater.

Ironman said:

Nice drawing

Thank you!

Zman said:

Depending on your local energy costs, it is very likely that an indirect water heater will be far more efficient than the geospring.
Most people don't realize that the energy "saved" by the heat pump actually comes from the inside of the home and needs to be replaced by the boiler anyway.
If you post your costs, we could run the numbers.

Electricity is 6c/kWh for the first 10kWh/day, 12.5c/kWh after. Gas is $0.90/therm.


Thank you all for the feedback!

hot_rod

I'm already committed to a site-built manifold from fittings, but could you tell me why you'd maintain the buffer at high temp. & mixing to a reset point vs letting the buffer reset along with the system? I figure that keeping tank temperture low reduces heat loss in the tank.

Maintaining a high buffer tank is more common on wood or bio filed buffer tanks. The higher temperature allows for more draw down and stratification.

If you maintained a 140F tank and could pull down to say 100F for a radiant you leverage that wide delta and stretch out the boiler cycles.

Modulating the boiler to the load works well with mod cons, but the tank will not store as much for boiler off cycles.

I'm not sure how the efficiency would work between the two methods, modulating to the load, if you can run way down to say 100 would increase condensing. Running the higher buffer lessens the cycling.