Are buffer tanks really needed for renewables?

nande2000

Had a 'heated debate' with an installer about a new boiler. He was saying for an ASHP or Wood Pellet system anyone who tries to sell you a boiler withought a buffer tank is a Charlatan.
ASHP I can understand as a buffer tank will smooth out the cycling of the compressor and extend its life (only 5 years with a buffer tank the chap said!).

With a wood pellet boiler im a bit more skeptical; the reason he said was that when the water is up to temp there will still be pellets in the auger that continue burning and hot ash in the ashpan and the unit could overheat; but many of the smaller wood pellet boilers modulate down to a minimum output when they get up to temp and the pump keeps running. Isn't that just like an old solid fuel system and as long as you have a heat leak rad or the systems minimum output matches the house heat loss then the system will cope?

I like the idea of a buffer tank, but the extra expense (£2k) and the fact that HW in the summer will cost more to heat seem to negate the need for them with a small (15kw) wood pellet system....

paceinternet

Most leading brand ASHP's are inverter driven meaning they adjust their output to match the demand, so cycling is less of a problem. But they do like a minimum or optimum flow through them, so it depends on the heating and DHW circuits if this can be maintained without a buffer. A preferred solution is to have a Low Loss Header to decouple the flow through the ASHP and the HW circuits. Another consideration has to be the provision of enough heat for the reverse cycle defrosting in winter. Sometimes a buffer tank helps with this. But I agree that a buffer tank can also be wasteful, so try to get the system designed without one.

Wood burning boiler system designers advocate buffer tanks to allow the burning to operate at the best temperatures for maximum efficiency. This burning is unlikely to continually match the heat demand, so pumping that excess heat into a buffer tank is an answer. I was recently told that they are also concerned about metal corrosion and tar if the combustion is not optimised.

Not one answer for every application though.