Singular or plural of 'infrastructure'

Solution 1:

Note that using the singular form infrastructure in the way you do here is not using the 'singular noun', but rather the non-count usage (which is identical in form to the singular count usage). This is deducible from the fact that no article is used.

The choice between using the singular arguably count ('the existing infrastructure') or unarguably non-count ('existing infrastructure') (anarthrous, ie no article) of the lexeme is yours, though these Google n-grams ('using existing infrastructure', 'using the existing infrastructure', 'using existing infrastructures') would seem to support my feeling that the non-count usage is more common.

The plural usage would be chosen to highlight (/ encourage people to consider) different possible set-ups / systems. While certainly not ungrammatical, it's less usual, and I'd say unnecessary. And a hot water tank is hardly an infrastructure (the central heating system would qualify). Again, note that the definite article in 'the central heating system', 'the existing infrastructure' is licensed even if not mentioned previously provided that you've mentioned say 'domestic heating' (perhaps only in your title).

Collins CoBuild correctly identifies that infrastructure may be used in both count and non-count usages:

infrastructure Word forms: infrastructures

(variable noun) [ie occurs in both count and non-count usages] ...

[Some] other dictionaries are inaccurate here. CED, for instance, has

infrastructure noun (C usually singular)

............

Actually, I'd use the non-count usage, but switch to

• '... can make use of existing infrastructure, perhaps using a hot water tank or the building mass...'

This is because a hot water tank (etc) is part of the/an infrastructure, and not a complete infrastructure.

Solution 2:

In the previous line/paragraph, if you've already written about the infrastructure that you're trying to point to in this sentence, then the following would work:

Electric heating devices can use the existing infrastructure, such as the hot water tank or the building mass, for thermal storage.