I faced this one also.  The problem with towel rails is that they don't actually emit much heat, and if you then present them with a low flow temperature its almost nothing.

I considered standard rads with a hanging rail (several are available as add ons).  One supplier suggested a fancoil or an electric element in the radiator.  A couple of potential installers said - 'if I were you I'd just leave it as is and change it later if its a problem - most of the bathrooms we do are non-compliant'. 

In then end I reasoned that the bathroom door is usually left open, there is excess heat available from the adjacent hall, and if you are having a bath/shower there is excess heat available from the water in the bath, so I went with the 'leave it' approach.  That was the right decision for me.

Thanks - however the installation wouldn’t be compliant if I left it as is would it and therefore wouldn’t be signed off? That would be my preferred option as, at the moment, one is dual fuel and I can heat by electricity as well when needed. The other towel rail has never really been that effective but is in a small shower room which is heated as you say by basically leaving the door open. 

Posted by: @davesoa

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Thanks - however the installation wouldn’t be compliant if I left it as is would it and therefore wouldn’t be signed off? That would be my preferred option as, at the moment, one is dual fuel and I can heat by electricity as well when needed. The other towel rail has never really been that effective but is in a small shower room which is heated as you say by basically leaving the door open. 

There is compliant and compliant.

The bus rules say that the heat pump is compliant if it 

(c)it replaces the heat generating components of the original heating system installed in that property (where applicable), other than any—

(i)supplementary electric heater, including any immersion heater,

(ii)circulation pump, or

(iii)solar thermal collector.

I would suggest that a towel rail heater could be considered a 'supplementary electric heater' provided that the total emitter capacity (excluding the electric heater) is sufficient to heat the entire house.

I would have a quiet word with your prospective installer!

Technically correct. The best sort of correct. Thanks I’ll do exactly that.

@jamespa I wasn’t aware there might be a problem in a bathroom; we have always had a radiator in our < 4 sq.m. bathroom and I requested having a dual fuel towel rail placed above it. In fact I had requested the largest that could be fitted in the space and the electrical element is ~ 300 Watts - this dries the towels nicely and adds a little supplentary warmth to the bathroom - which we like.😉 Regards, Toodles.

I think the issue is that MCS recommends 23C for bathrooms, which requires large emitters, and as mentioned above most towel rails simply do not emit that much heat, so are unlikely to meet the design criteria for the room. For us, the solution was to install standard Stelrad radiators to meet the design criteria in addition to towel rails for drying towels (which emit even less heat when covered with wet towels). I appreciate not every bathroom will have physical space for two radiators, and I know our system designers highlighted it was a real problem for them getting large enough radiators into many bathrooms to meet the design temps. Challenging if they are required to have MCS sign off on the installation.

@old_scientist As I mentioned, our approx. 2 x 2 metre bathroom has a radiator and dual fuel towel rail and I requested an elevated temperature of 25 degrees - in fact we have 25-26 degrees which we enjoy and find very comfortable. Happy Toodles.

@toodles @jamespa. Your guidance is really helpful. I’ve found two towel rails at Victorian Plumbing that seem to meet the output requirements. One is rated at 1100 watts at 50C and the other 600 watts @50C. I’ll see if my installer will accept them. It may be I have to buy them and they give a credit for the two Stelrads. They are big beasts but they will fit.

Posted by: @old_scientist

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I think the issue is that MCS recommends 23C for bathrooms, which requires large emitters, and as mentioned above most towel rails simply do not emit that much heat, so are unlikely to meet the design criteria for the room.

Fair enough.  Although the requirement is in a paragraph which starts 

The following procedure shall be followed for systems where the heat pump is to supply 100% of the space heating load with or without a supplementary electric heater to ensure the correct sizing and selection of a heat pump and related components for each installation:

a) A heat load calculation should be performed on the building using internal temperatures not less than those specified in Table 1 and external temperatures
specified in Table 2 column A or B, according to the MCS Contractor's assessment of the building location. If column B is selected, no uplift factor for intermittent heating is required. Heat load calculations shall in other respects comply with BS EN 12831-12017"

and 'should' is defined as

" the term ‘should’ prescribes a requirement or procedure that is intended to be complied with unless reasonable justification can be given"

so there is arguably some limited wiggle room to provide a reasoned justification.   I have an understairs toilet without any heating, would any reasonable installer insist on putting in a radiator to guarantee its always at 18C?

@jamespa Beware; the use of the term ‘WC’ might lead to confusion!😉 Toodles.

@jamespa Solved thanks to your help. I found a ladder towel rail rated at 1500 watts @50C and a horizontal towel rail rated at 600 watts @50C. Therefore the overall heat requirement is met with  one rail higher than necessary and the other slightly under - the overall discrepancy is balanced out. The installer is happy with this.