ASHP DHW seasonal auto changeover

Ashful · Sep 24, 2014

I currently run a BoilerMate off my oil-fired boiler, which is also heating parts of my house all winter. Without any real data to back it up, my gut feeling is that the BoilerMate is probably the most efficient means of heating water during the heating season, but of course it is horribly inefficient during the summer months. I say this because during winter, all residual heat loss beyond the label efficiency of the boiler is inside the house, adding to our net heating. Pulling heat from the air in the house (Nyle), and then having to replace it with increased boiler usage, means two conversions = less efficient. However, in summer, all residual heat loss must be overcome by our air conditioning, meaning we're paying for that inefficiency twice. We also run TWO dehumidifiers in our basement, so switching to a Nyle or similar HPWH tagged onto the BoilerMate would be doubly-efficient, in cooling our finished basement and dehumidification, during summer months.

The question is, change-over procedure. I think it might be most convenient and efficient to use a delay timer relay or similar to enable the Nyle and disable the boiler any time there is no call for heat in a period of 24 - 48 hours, and switch back to the boiler any time there is a call for heat in a 24 hour period. Thoughts on proposal and implementation?

STIHLY DAN · Sep 25, 2014

The residual heat from the boiler is going up the chimney not your space or hot water. Even tho your heat pump will add to the heat loss, its the same heat for hot water with no wasted chimney heat. So yes in winter it will still be cheaper.

woodgeek · Sep 26, 2014

Agree with Dan.

In winter, about half of the BTUs comes from kWh in the compressor and half is stolen from the space. If the boiler/DHW utility room is very well insulated and 100% conditioned, you could price it out that the 'stolen half' of the BTUs comes from your oil boiler. In reality, by lowering the temp in the boiler room, it reduces the loss of heat to the ground (uninsulated slab) and, as Dan said, up the flue (depending on the boiler venting). This means that the 'stolen BTU' fraction is <<50% of DHW BTUs even during the heating season.

So, to first approximation, just look at the EF number....if it is 2.2, figure it costs your electrical BTU cost (3414 BTU/kWh) / 2.2. The stealing will make it cost **slightly** more in the winter, and **slightly** less in the summer. In our climate in a semi-conditioned basement with a lot of waste heat, I think it is a wash, and you can just use the nominal EF. With our cheap kWh around here, I suspect the HPWH will beat the boiler even oin January, but you can pencil it out unambiguously.

Redbarn · Sep 28, 2014

Agree with Woodgeek.

We let our HPWH run until well into January and then it restarts again in late February.
Ran it year round one year but found that with a basement temperature in the low 50s,that it ran for long periods to "refill"the DHW tank heat. Looking at the efficiency charts, we were well into the less efficient zone and it showed in our electric bill.
We now have the HPWH on a thermostatt control that shuts it off when the basement temperature gets below 55 deg F and the same thermostatic control then turns on the oil water heater. This system is automatic and works well.

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ASHP DHW seasonal auto changeover

Ashful

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STIHLY DAN

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woodgeek

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Redbarn

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