HW cylinder -- surprising numbers

[Steve S]

Now that my heating controller is nearly done I've been using it to collect data. Some of it is quite surprising, at least to me.

We have a ~25 year old 120L HW cylinder with foam insulation. Over a period of 6 hours with no heat input (and just a couple of handwashes) the temperature dropped from 61.5 C to 56.5 C. Measurements have a resolution of

0.1C. According to my calculations that represents a loss of ~2.5 megajoules, which over 6 hours is an average of ~115W.

I daresay modern cylinders could do a lot better than this, but somehow I was expecting the losses to be higher.

Steve S

[Ian Stirling]

If you've got the space, an extra wrap of loft-insulation can be a good plan.

[Rob Morley]

I expect there are regions of colder water that you're not measuring.

[Calvin]

That fits well with my observation that I can heat our DHW all summer and barely be able to measure the oil used (hand washing, washing machine and dishwasher). Come autumn and the start of the CH season we use about 1cm a day. It really makes me wonder how these solar systems which basically just do DHW can ever be justified.

[Steve S]

I don't think so. Sensor is 3/4 of the way down the cylinder.

Steve S

[Ed Sirett]

I beleive that a modern Part-L compliant cylinder would do a bit better but these numbers are not very far out.

To improve this, make sure that all the hot pipework is insulated the losses from 1m of HW pipework is about 20W

The hand basin uses may have taken out a fair bit from the cylinder even if you did not wait for the WH to come through.