air source heat pump with undersized radiators

[AJH]

This is the second new build block of flats with a green heating system that doesn't perform well. Previously it was a solar thermal roof mounted array backed up by a pellet boiler heating 12 key worker flats.

Now my sister in law is in a joint ownership block of 10 apartments converted from a detached Victorian town house, each apartment has a heat meter to proportion costs. There is a communal air source heat pump sitting in the back garden but instead of using under floor loops the architrave has specified radiators as if for a conventional system running at ~80C, apparently the calcs show this and there was little liaison with the company supplying the heat pump. Litigation is on the cards but I'm only pondering a solution. There is little chance of retrofitting radiators or floor loops.

I haven't seen it yet so I don't know the power requirement but my first thought is for a combi gas boiler in the feed from the heat pump, one that can accept a 50C feed.

Mostly I'm wondering about the control algorithm as, presumably, the heat pump will meet demand even with the lower flow temperatures and undersized radiators. The simplest would be to fire the combi whenever the feed water fell below 50C but with the limited heat exchange it would mean the return temperature to the heat pump would be too high so I'm trying to think of some sort of injection circuit that would still allow the heat pump to provide energy much of the time. Any thoughts?

AJH

[Onetap]

Wow; why wasn't there someone with some engineering nous involved?

80 degC isn't used much for conventional heating systems now, they're usu= ally designed for lower temperatures and 20 degC dT, to get the low return = temperatures required by condensing boilers. Which doesn't help you much.

I would suggest a weather compensated control system, if you haven't alre= ady got one. You should have, but if they've cocked up the rad sizing, who = knows? This which runs the heating at the lowest temperature needed to give= the required heat output, according to the outside temperature. You can us= e your heat pump at low temperatures for most of the heating season. It wor= ks as the 'lead boiler', when it can. It won't work in the coldest weather,= when you'll need higher flow temperatures.

It is technically fairly easy to link in multiple heat sources as a prima= ry/secondary system. The heating distribution circuit is the primary circui= t. You can have gas boilers, ASHPs, pellet boilers, whatever, linked to thi= s as secondary sub-circuits via injection pumps.

The problem is that, once you have to fire up a gas boiler, the return te= mperature starts to rise and at some point, the efficient heaters, requirin= g low return temperatures will lock out. Not ideal, but that's the best you= 'll get by working only in the plant areas.

A combi is NOT what you want; I assume you mean condensing?

How is DHW provided?

You'd be sensible to get someone competent to have a look at the plant, t= he heating debacle has made itself known, but there are probably other prob= lems that haven't been noticed.

I was involved in a similar c*ck-up involving excessive return temperatur= es and plant lock-out (wasn't me, Guv, honest), twenty+ years ago. I worked= for the client, the c*ck-up was by the consulting engineers employed by th= e client. I did come up with a propoal that could have fixed it, but the cl= ient's budget had been spent with no visible return and they'd gone off the= high efficiency plant altogether. One of the big regrets of my career.

What area is this in?

[dennis]

*If* its just the rads that are too small you could just fit fans to increase the heat output. A DIY job if you want, just fit radiator cabinets and some low voltage fans like those used in PCs.

[The Natural Philosopher]

I looked into retrofitting a heat pump here. Totally impossible. Heat pumps provide water in the 30C-50C range and need direct immersion heating to get up to 60C, let alone 80C.

I was faced with totally replumbing the house: needed a differnt water tank with larget primary coil, larger bore CH plumbing and a complete change of all radiators. ONLY the UFH would have been suitable

In short if yopu are doing a heatpump the WHOLE plumbing system needs to be swapped out.

[Dave Liquorice]

Architrave? Do you mean arcihtect?

Architect doesn't know what he is doing. Sack 'em and find some one who does understand heat pumps and associated systems.

Though you said it had conventional sized radiators.

Why a combi? I think you'd be better of with a heat store with a condensing system boiler as back up. You might need two heat stores one to take the output of the air source heat pump and another to provide conventional temperature haeting circulation water and DHW via a heat exchanger. The system boiler keeps this store at >60C. Between the two stores you have another heat pump to transfer the energy from the air source pump into the system where it can be used.

It might be possible that air source heatpump can be retrofitted with a 2nd stage to get it's output suitable for feeding the main store directly or you could change it for a two stage device.

Thinking through this a bit more I'd expect to find a suitably large (low temp...) heat store anyway to take the output of the heat pump. For 10 flats one would be looking at quite a heat demand so quite a large heat pump and associated demand on the power supply.

[AJH]

That's not a bad idea, I wonder how much draught would be required and how obtrusive it might be.

AJH

[AJH]

Pass, it looks like it was specified for the planning permission and the architect was depending on the heat pump supplier to work with the builder. The supplier's web site plainly favours underfloor heating for the pump.

If it's anything like the relationship that developed between Sandler, in Germany, and the contract engineers for the solar system in Brixton, the equipment was about right but the plumbers did not understand the niceties, Sandler's engineers just gave up when faced with the poor implementaion and I think this is true with Nu-Heat. The contracts were convoluted between the architect and the builder who dealt with the suppliers. In between all knowledge of the system has been lost, they cannot even get a response from the suppliers on how access the heat meters via a network even though the hardware interface was supplied. quired by condensing boilers. Which doesn't help you much.

Okay

temperature starts to rise and at some point, the efficient heaters, requiring low return temperatures will lock out. Not ideal, but that's the best you'll get by working only in the plant areas.

This was my thought and I was just wondering how to minimise use of gas. the weather compensating controller looks good.

Yes probably but I said combi because of the need for DHW which may be affected ( see answer to your question below. They have crammed so much living space into the structure to maximise the number of appartments there's precious little space for anything else, like individual combis or a heat store.

A second heat pump which also can back up the central heating. There is an immersion in the system but its main function is to get the system temperature up enough to sterilise ( think legionella bacteria) once a month.

and plant lock-out (wasn't me, Guv, honest), twenty+ years ago. I worked for the client, the c*ck-up was by the consulting engineers employed by the client. I did come up with a propoal that could have fixed it, but the client's budget had been spent with no visible return and they'd gone off the high efficiency plant altogether. One of the big regrets of my career.

I had similar at Brixton, the power requirement calculated was marginal and DHW depended on good startification of the heat store, both DHW and underfloor heating were drawn from the same level. Heating was not prioritised between flats and neither was it charged through the heat meter. A few flats simply left the roomstat high and controlled temeprature by opening windows. The pumps for the plate heat exchangers were inhibited below 50C so in cold weather minimal heating and no DHW.

Woking, probably the sustainable energy capital of England ;-)

AJH

[Alan Braggins]

[...]

If he could retrofit radiators twice the size (or twice as many), there might be a chance that running them at 50 would manage.

[Onetap]

Yes, this one;

Heating to 60degC for one hour per day is recommended. You really don't want to bugger around with low temperatures in a centralised DHW system. The legislation will ensure the building managers get thoroughly shafted if anyone gets an infection from it.

[Onetap]

Was that the first of these problematic blocks of flats? Who was responsible for sorting that out?

Other thoughts;

Did the pellet boiler job qualify for the RHI payments or has anyone looked into this?

Are you (plural) aware of the building managers' responsibilities under the L8 Legionella Code of Practice?

The heat meter is possibly a BMS link, usually an light pulse input, one pulse per kJ or something.

E-mail if you want any more ideas.

[AJH]

hand of the man behind his throne, who I suspect wished for the royal accolade.

AJH

[AJH]

to bugger around with low temperatures

get thoroughly shafted

I'll have to look at that later, thanks.

AJH

[AJH]

for sorting that out?

Yes, all commissioned about 5 years ago when I did snagging for the scottish company which installed the pellet boiler.

It pre dated this.The engineering consultants didn't understand that integrating the solar panels which might just make 85C at the top of the tank on a sunny day, with a pellet boiler that was set to 75C in a header system that combined the two flows before the heat store meant that the pellet boiler stayed on demand all of the day, a small draw off at night would have fired it anyway, so it would always have started the day with a hot top 1/3 of the tank, inhibiting any solar input until a few hundred litres had been drawn off. This had the effect of keeping the boiler on 24/7 in the winter with no maintenance contract, eventually some internal baffles burned through, writing the boiler off.

At this stage they should have taken stock, stuck a condensing boiler in and bought a larger, different boiler under RHI but they decided to replace like with like and that wasn't RHI compliant.

Not really, been out of it for 3 years since the firm went bust but still dabble with my own bits and pieces.

pulse per kJ or something.

I doubt it, it will probably be one landis-gyr ultra sonic thing in each flat with a temperature probe on the inlet and return of the flat heating and one on the input DHW.

I've passed some of the suggestions on, Thanks

AJH